The plant kingdom thus comprises individuals which are structurally and functionally unlike one another. Subsequently several systems of classification of plant were proposed by taxonomists.

The branch of botany dealing with the study of algae is called as phycology or algology. It is derived from the Greek word Phykos which means 'alga' or 'sea weed'. They are simple, thallose, autotrophic non-vascular plants having unicelled sex organs and no embryo formation. According to Fritsch, (1935) the designation alga must include all holophytic organisms, as well as their numerous colourless derivatives, that fail to reach the level of differentiation characteristic of archegoniatae plants.

# OCCURANCE : The algae occur in a variety of habitats which are summarised here under :

(i) Fresh water forms : They occurs in rivers, ponds, pools, lakes and ditches. Some forms e.g., diatoms occur as passively floating and drifting flora. They are called as phytoplanktons. Those forms which remain attached to bottom soil are called as (epipelic). Several forms remain attached to bottom or at the bank or to submerged objects. They are described as (benthos). Many forms remain attached to rocks or stones. They are described as (epilithic or lithophytic). Some blue-green algae form water blooms on the surface of water.

(ii) Marine forms : Most of the members of brown algae, red algae some green and blue-green algae occur in sea. While some occur as phytoplanktons and benthos, others occur as lithophytes. The giant forms like Macrocystis (60 meters) and Nereocystis (50 meters) are also marine.

(iii) Terrestrial forms : Several members of green and blue-green algae and a few others occur on damps oil. While forms like Oscillatoria and Nostoc occur on alkaline and calcareous soil, Fritschiella grown on acidic soil. Xanthophyceae members like Vaucheria and Botrydium growing on damp shady soil or on shady walls, are often described as Sapophytes.

# SPECIALISED HABITS
(a) Cryophytes : Plants growing on snow or ice are called as cryophytes. Different algal forms produce a specific colour effect while growing as cryophyte e.g., yellow-green by Chlamydomonas yellowstonensis, red by C. nivalis, black by Scotiella nivalis and purple-brown by Ancylonema nordenskioldii.

(b) Thermophytes : Plants growing in hot water are called as thermophytes. Some blue-green algae grow in hot water springs at about 70°C e.g., Oscillatoria brevis.

(c) Epiphytes : Several algal forms grow on other plants (algae, angiosperms) as epiphytes. e.g., Oedogonium, Cladophora, Vaucheria etc.

(d) Endophytes : Some blue-green algae grows as endophytes inside other plants e.g., Anabaena growing inside the leaf of Azolla (fern), Nostoc inside the thallus of Anthoceros (hornwort) and Anabaena, Nostoc, Oscillatoria inside the coralloid roots of Cycas.

(e) Epizoic : Algae growing on the bodies of animals are described as epizoic. e.g., Cladophora crispata grows on snail shell, Characium grows on the antennae of mosquito larvae, Cyanoderma (red alga) and Trichophilus (green alga) are grow on scales of sloth.

(f) Endozoic : Algae growing inside the body of animals. e.g., Chlorella grow with in the tissue of Hydra. Some blue-green algae also grow in the respiratory tracts of animals. The blue-green algae which grow endozoically inside the protozoans are called as cyanallae.

(g) Symbiotic forms : Some algae like Chlorella, Nostoc etc. growing in symbiotic relationship with members of Ascomycetes and Basidiomycetes (Fungi) constitute the lichen.

(h) Parasites : The alga Cephaleuros virescens grows a parasite on the tea leaves. In addition, Rhodochytrium, Phyllosiphon are other parasitic algal forms.

The algae show a considerable variation in the organization of the thallus :
(i) Unicellular forms : Several members of algae are unicelled. They may be motile (Chlamydomonas) or non-motile (diatoms). Forms showing movement by pseudopodia are called as rhizopodial. Some forms have a thick wall and become sedentary for certain duration in their life history. They are called as coccoid e.g., Chlorella, Chroococcus.

(ii) Multicellular forms : The multicelled algae show a considerable range in their organization.
(a) Colonial : A colony consists of independent organisms. While the colony of Volvox is motile, that of Hydrodictyon is fixed. A colony having fixed number of cells and division of labour is called as coenobium e.g., Volvox.
(b) Palmelloid : Here the vegetative cells of the alga get surrounded by a mucilagenous matrix e.g., Tetraspora.
(c) Dendroid : Here the colony appears like a microscopic tree. There is secretion of mucilage from the polar end e.g., Ecballocystis.
(d) Filamentous : Most of the algal forms are filamentous. The filaments may be uniseriate or multiseriate, free floating or attached, unbranched (Ulothrix) or branched (Cladophora). The branches may be monomorphic (Cladophora) or dimorphic (Batrachospermum). The branching may be lateral or dichotomous, true (Ectocarpus) or false (Scytonema). The filaments may be monosiphonous (Batrachospermum) or polysiphonous (Polysiphonia). In some filamentous forms there is distinction of a prostrate system and an erect system, thus constituting the heterotrichous habit. e.g., Stigeoclonium.
(e) Siphonous : An aseptate, multinucleate (coenocytic) condition of a filament or thallus constitutes the siphonous habit e.g., Vaucheria.
(f) Parenchymatous : Parenchymatous organization of the thallus has been observed in many members of brown algae (Sargassum, Laminaria), red algae (Gracillaria, Porphyra) and a few green algae (Chara, Ulva) etc.

The algae show three types of cellular organization, prokaryotic, mesokaryotic and eukaryotic. The mesokaryotic organization is seen in Dinophyceae where a nuclear membrane and chromosomes are present but the basic proteins are absent. Here we shall describe only prokaryotic and eukaryotic organization in some details.

(i) Prokaryotic : The blue-green algae, also called as Cyanobacteria, show prokaryotic cell structure. The cell wall may have a mucilagenous sheath. The inner wall layer is made up of mucopeptide. It is a polymer of two aminosugars. The cell wall is followed by a plasma membrane. Light microscope, the cell is distinguished into a central colourless centroplasm and a peripheral coloured chromoplasm. The cell lack a well organised nucleus, mitochondria, E.R., and Golgi. However, 70 s ribosomes are present. There are no plastids either but the cell does possess photosynthetic lamellae which are comparable to thylakoids. The cells have gas vacuoles and granules of several types such as alpha, beta, polyhedral and structured granules. The alpha–granules are now called as polyglucoside granules whereas the structured granules are cyanophycin granules.
In some filamentous forms like Nostoc, Anabaena, certain large specialized cells also occur. They are called as heterocysts. They form polar nodules, they may be intercalary or terminal in position.

(ii) Eukaryotic : Most of the algal groups (except blue-green and dinoflagellates) show eukaryotic cell structure. The cell wall is made up of cellulose. However, there may be additional layers of pectin and chitin. While silica is also present in the cell wall of diatoms whereas some red algae (Corallina) have inpregnation of CaCO3. The cells possess a well organised nucleus. The minimum chromosome number in algae is n = 2 (Porphyra linearis) and the maximum number is n = 592 (Netrium digitalis). The cells possess distinct mitochondria, plastids, E.R., ribosomes and Golgi. In Micromonas, there is only one mitochondrion in a cell. The shape of chloroplasts is variable. It may be cup-shaped (Chlamydomonas, Volvox), spiral (Spirogyra), stellate (Zygnema), girdle-shaped (Ulothrix) reticulate (Oedogonium) but generally discoid. There may be a single thylakoid in the granum of Rhodophyceae, two in Cryptophyceae, three in Phaeophyceae and Bacillariophyceae but generally many. The cells may also possess simple or contractile vacuoles.
The motile forms and male reproductive cells also possess a photosensitive eye spot or stigma. In dinoflagellate the eye spot is distinguishable in a lens, photosensitive region and a pigmented cup.
The motile forms also possess flagella. They show the usual 9 + 2 structure. They are of two types – acronematic (whiplash type) and pleuronematic (tinsel type).

The grouping of algae based on pigments. Algae possess three classes of pigments namely chlorophylls, carotenoids and biliproteins.

(i) Chlorophylls : A chlorophyll molecule consists of a head and a tail. The head is made up of porphyrin, a tetrapyrrole closed ring compound with a central Mg atom; the tail is made up of phytol. Some five types of chlorophylls occur in algae namely chl. a, b, c, d and e. The chlorophyll a occurs in the all algal groups, b in Chlorophyceae, c in Phaeophyceae and Bacillariophyceae, d in Rhodophyceae and e in Xanthophyceae.

(ii) Carotenoids : These pigments are isoprene (C5H8) derivatives. They are of two types namely carotenes and xanthophylls. Some six types of carotenes have been located in algae which have been designated as α, β, γ, ε carotenes, lycopene and flavicine. Of these, β–carotene is found in all the algal group, α–carotene in Chlorophyceae and Rhodophyceae, γ–carotene and lycopene in Chlorophyceae, ε–carotene in Cryptophyceae and Bacillariophyceae and flavicine in Cyanophyceae. Some 20 xanthophylls have been identified from different algal groups such as lutein, fucoxanthin, violaxanthin, astaxanthin, zeaxanthin, myxoxanthin etc. Of these, lutein occurs in Chlorophyceae, Phaeophyceae, Rhodophyceae and Cyanophyceae, fucoxanthin in Phaeophyceae, Rhodophyceae and Bacillariophyceae. Myxoxanthin occurs in Cyanophyceae and astaxanthin in Chlorophyceae.

(iii) Biliproteins or Phycobilins : This comprise a bile pigment conjugated to a protein moiety. They are of three types namely phycocyanin, phycoerythrin and allophycocyanin. While c–phycocyanin and c–phycoerythrin occurs in Cyanophyceae, r–phycoerythrin and r–phycocyanin are found in Rhodophyceae. The allophycocyanin is found in both the groups.

The food is reserved in different forms in various algal groups, some of which are summarised here under :
(i) Chlorophyceae : Starch and Oils.
(ii) Xanthophyceae : Chrysolaminarin (carbohydrate) and Oils.
(iii) Bacillariophyceae : Chrysolaminarin and Oils.
(iv) Phaeophyceae : Laminarin (carbohydrate), Mannitol (alcohol) and Oils.
(v) Rhodophyceae : Floridean Starch and Galactan –SO4 polymers.
(vi) Cyanophyceae : Cyanophycean Starch (glycogen) and Cyanophycin (protein).

The algae reproduce vegetatively, asexually and sexually. Various method involved in reproduction are discussed in the following account.
# (i) Vegetative reproduction : It occurs by following types.
(a) Fragmentation : It occurs due to breakage of filament or thallus into fragments, each of which behaves as an independent organism e.g., Ulothrix, Spirogyra etc.
(b) Fission : The unicelled forms like diatoms, desmids multiply by fission i.e., simple cell division.
(c) Budding : A bud arises as a papilla on the parent cell. It enlarges and finally separates e.g., Protosiphon.
(d) Propagules or Gemmae : They arise as modified branches. They are rich in food and germinate into new plant on detachment e.g., Sphacelaria.
(e) Hormogonia : In certain blue-green algae e.g., Oscillatoria, Nostoc, the filament breaks up into hormogonia due to the formation of separation disc (necridia) or at the junction of intercalary heterocysts.
(f) Tubers : In certain forms like Chara, tubers are formed on the lower nodes as also on the rhizoids. They germinate into new plants.
(g) Akinetes : Due to deposition of food material followed by thickening of the parent wall, a cell is transformed into an akinete. They may be formed in a chain. On the arrival of favourable conditions, they germinate to forms a new plant e.g., Cladophora, Ulothrix, Nostoc etc.

# (ii) Asexual reproduction : It occurs by the formation of various types of spores in sporangia. Except the zoospores, all other types of spores are non-motile.
(a) Zoospores : These are thin walled motile spores. They are anteriorly biflagellate and the two flagella are similar in Cladophora. In Vaucheria and Ectocarpus they are laterally biflagellate and the two flagella are dissimilar. Multiflagellate zoospores are formed in Oedogonium and Vaucheria. In Vaucheria the flagella are present all over the surface in pairs and hence it is called as synzoospore.
(b) Aplanospores : They are thin walled and non-motile spores commonly formed in Chlamydomonas, Ulothrix etc.
(c) Autospores : They are also thin walled, non-motile spores which resemble the parent cell e.g., Chlorella.
(d) Hypnospores : These are thic walled non-motile spores formed to tide over unfavourable condition. They germinate on the arrival of favourable conditions e.g., Chlamydomonas, Ulothrix.
(e) Exospores : These are specialised type of aplanospores formed externally by pinching off the exposed protoplast e.g., Chamaesiphon.
(f) Endospores : In many blue-green algae, one or more endospores are formed inside an endosporangium by fragmentation of the protoplast e.g., Dermocapsa, Pleurocapsa.
(g) Carpospores : In red algae, carposporangia are formed at the tip of gonimoblast filaments which produce a single haploid or diploid carpospore e.g., Batrachospermum, Polysiphonia.
(h) Tetraspores : Four non-motile tetraspores are formed inside a tetrasporangium as a result of mitosis in brown algae (e.g., Dictyota) or by meiosis in red algae (e.g., Polysiphonia).
(i) Monospore : The juvenile stage of Batrachospermum, a red alga, multiplies by forming a single monospore formed in side a monosporangium.

# (iii) Sexual reproduction : The sexual reproduction in algae is broadly of three types as under :
(a) Isogamy : It involves fusion of gametes which are morphologically and physiologically similar. They are called as isogametes e.g., Chlamydomonas eugametos. In diatoms, there is simplification of isogamous reproduction. Here two nuclei of opposite strains (+ and –) fuse and the phenomenon is called as autogamy. In Chlamydomonas sp. two vegetative cells may fuse to form a zygospore and the phenomenon is called as hologamy. As a result of fusion of two gametes, the zygospore is formed.
(b) Anisogamy : It involves fusion of two gametes which are dissimilar e.g., Chlamydomonas, Ectocarpus, Pandorina etc. When the two gametes are morphologically dissimilar, the anisogamy is said to be morphological e.g., Chlamydomonas braunii, Ectocarpus secundus. Here the smaller gamete may be called as male and the large one as female. When the two gametes are morphologically similar but differ in their behaviour, the anisogamy is said to be physiological e.g., Spirogyra, Ectocarpus siliculosus.
(c) Oogamy : In this process there is formation of unicelled sex organs. The male sex organ is called as antheridium and the female as oogonium. The antheridium forms the male gametes called antherozoids which are generally flagellate. The oogonium forms a non-motile female gamete called egg. The oogamy involves fusion of antherozoids with egg. The simplest type of oogamy is seen in Chlamydomonas coccifera.
In Sargassum the sex organs are formed in special pitcher shaped depressions called conceptacles formed on receptacles. In red algae (Polysiphonia) the male gametes called spermatia are non-motile. The female sex organ called carpogonium is formed on a specialized filament. The highest degree of specialization is seen in Chara where the antheridia and oogonia appear to be surrounded by sterile cells. The structures so formed are called as globule and nucule, respectively. As a result of fertilization, the zygote is formed which secretes 1 – 2 thick walls and undergoes a period of rest. On the arrival of favourable conditions, it germinates. Usually it undergoes meiosis to forms meiospores. In some forms, it forms the diploid plant e.g., Cladophora, Ectocarpus.

Linnaeus (1754) differentiated a group of plant and called as 'algae' where he included lichens and liver worts also. This group was delimited to its present form by de Jussieu (1789). Endlicher (1836) proposed the term Thallophyta where he included algae along with fungi. Fritsch (1935) classified algae into the eleven classes as under :
(i) Chlorophyceae (Green algae)
(ii) Xanthophyceae (Yellow-green algae)
(iii) Chrysophyceae
(iv) Bacillariophyceae (Diatoms)
(v) Cryptophyceae
(vi) Dinophyceae (Dinoflagellates)
(vii) Chloromonadineae
(viii) Euglenineae (Euglenoids)
(ix) Phaeophyceae (Brown algae)
(x) Rhodophyceae (Red algae)
(xi) Myxophyceae or Cyanophyceae (Blue green algae)

# (i) Chlorophyceae
(a) Plants fresh water or marine.
(b) Forms unicelled to parenchymatous.
(c) Cells showing eukaryotic orgnaization.
(d) Chief pigments – Chlorophyll a, b; alpha , gamma, beta– carotenes, lycopene; lutein, violaxanthin.
(e) Reserve food – Starch and oils.
(f) Zoospore formation occurs.
(g) Male gametes flagellate.
(h) Flagella identical.
(i) Sexual reproduction – Isogamous, anisogamous or oogamous.

#(ii) Xanthophyceae
(a) Plants generally fresh water.
(b) Forms unicelled to siphonous.
(c) Cells showing eukaryotic organization.
(d) Chief pigments – Chlorophyll a, e; beta–carotene, violaxanthin, neoxanthin.
(e) Reserve food – Chrysolaminarin and oils.
(f) Zoospore formation occurs.
(g) Male gametes flagellate.
(h) Flagella non-identical (unequal).
(i) Sexual reproduction – Isogamous, anisogamous or oogamous.

# (iii) Phaeophyceae
(a) Plants marine.
(b) Forms unicelled to parenchymatous.
(c) Cells showing eukaryotic organization.
(d) Chief pigments – Chlorophyll a, c; beta–carotene, fucoxanthin, lutein, violaxanthin, diatoxanthin.
(e) Reserve food – Laminarin, mannitol and oils.
(f) Zoospore formation occurs.
(g) Male gametes flagellate.
(h) Flagella unequal.
(i) Sexual reproduction – Isogamous , anisogamous or oogamous.

# (iv) Rhodophyceae
(a) Plants generally marine.
(b) Forms filamentous to parenchymatous.
(c) Cells showing eukaryotic organization.
(d) Chief pigments – Chlorophyll a, d is present but chlorophyll c is absent; , beta–carotene, lutein, violaxanthin, fucoxanthin, myxoxanthin, phycoerythrin, phycocyanin and allophycocyanin.
(e) Reserve food – Floridean starch, galactan –SO4 polymers.
(f) No zoospore formation.
(g) Male gametes non-flagellate.
(h) Sexual reproduction by specialized type of oogamy.
(i) Life cycle haplobiontic or diplobiontic.

# (v) Myxophyceae (Cyanophyceae)
(a) Plants generally fresh water, a few forms marine.
(b) Forms unicelled to filamentous.
(c) Cells showing prokaryotic organization.
(d) Chief pigments – Chlorophyll a; beta-carotene; luteins, myxoxanthin, oscillaxanthin, c-phycocyanin, c-phycoerythrin, allophycocyanin.
(e) Reserve food – Cyanophycean starch (glycogen) and cyanophycin (protein).
(f) No zoospore formation.
(g) No flagellate bodies.
(h) No sexual reproduction.

There are sufficient evidences which suggest that the land plants have originated and evolved from algae. Morphological, cytological and biochemical similarities between the green algae and the land plants suggest that the ancestral algae was green, flat and fleshy with heterotrichous filamentous habit similar to Fritschiella (Smith). The various evidences which suggest algal origin of land plants are :
(i) Presence of similar type of photosynthetic pigments – Chlorophyll a, Chlorophyll b and carotenoids in both green algae and land plants.
(ii) Presence of cellulose and pectose as constituents of cell wall in both the groups.
(iii) Accumulation of starch as reserve food material.
(iv) Similar structure of flagella in green algae and land plants.

(i) Useful aspects

(a) Nitrogen fixation : Some fifty species of blue-green algae are capable of fixing atmospheric nitrogen in the soil e.g., Anabaena, Aulosira, Cylindrospermum, Calothrix, Gleotrichia, Nostoc, Scytonema, Stigonema, Tolypothrix etc. Under aerobic conditions, nitrogen is fixed by heterocysts only. The fixation is brought about by the enzyme nitrogenase. Under anaerobic condition the vegetative cells also show nitrogenase activity.

(b) Algae as food : Many green algae such as Chlorella, Ulva, Caulerpa, Enteromorpha, etc. are used as food. Chlorella, a unicellular green alga, possesses a high quality of food value. It has about 50% protein and 20% of lipid and carbohydrates. The Chlorella protein contains all the amino acids essential for human nutrition. Besides, it contains vitamins A, B, C, K and various other essential elements. Ulva is collected and processed as food product. Ulva lactuca has formerly used in salad and soup in Scotland.

(c) Green algae in space research : In recent years biologists have realized that unicellular green algae (e.g., Chlorella) could be used to provide O2 during space flight trips. The alga can reuse CO2 during the process of photosynthesis and release O2 for the use by Astronauts.

(d) Antibiotics : The genus Chlorella yields an antibiotic chlorellin, which is used against Gram +ve and Gram –ve bacteria, especially Escherichia coli, Shigella dysenteriae and Staphylococcus aureus. The genus Caulerpa also yields antibiotics.

(e) Alginates : Alginic acid is a polymer of carbohydrate. It occurs in the cell wall and middle lamella. The alginates particularly ammonium, Fe, Na and K, salts are water soluble. They are obtained from Laminaria, Ascophyllum, Fucuc, Nereocystis, Turbinaria etc. They are viscous, gel-forming and non-toxic. Hence they are used in pharmaceuticals as emulsifiers and stabilizers as well as for making pills, antibiotic capsules etc. They are also used in the preparation of soups, jellies, cosmetics, toothpastes, polishes, hair dyes, compact powders, lotions, shampoos etc.

(f) Carrageenin : It is a polysaccharide colloid (phycocolloid) obtained from the red algae Chondrus crispus and Gigartinia stellata. It is widely used in soups, sauces, milk shakes, cheese, jellies, cream and fruit juices. It is also used in painting and printing.
(g) Kieselguhr or Diatomite : The fossil deposits of unicelled alga, diatoms are formed due to their highly siliceous cell wall (frustules). This is called as diatomite or diatomaceous earth. It is used in making sound proof buildings, lining furnaces and boilers, as insulating material and also as a filter.

(h) Agar-agar : It is a non-nitrogenous carbohydrate consisting of two polysaccharides namely agarose and agaropectin. It is obtained from several red algae e.g., Gracilaria, Gelidium, Gigartinia, Pterocladia, Chondrus, Furcellaria, Phyllophora etc. It is insoluble in cold water but soluble in hot. It is used as a base for a variety of culture media.

(i) Source of minerals and elements : The members of brown algae called 'kelps' have been the source for obtaining iodine e.g., Laminaria, Macrocystis, Fucus. About 25% of total iodine is extracted from kelps. Besides iodine, the kelp also contain Bromine, Boron, Copper, Cobalt, Chromium, Iron, Manganese, Molybdenum and Zinc.

(j) Sewage disposal : Green unicellular algae such as Chlorella and Chlamydomonas are used in sewage disposal ponds. They remove CO2 and restore O2 by the process of photosynthesis and makes the sewage water habitable for many fishes and aerobic bacteria.


(ii) Harmful aspects

(a) Algal toxicity : Some dinoflagellates like Prymnesium, Gymnodinium are extremely poisonous to fishes. The blue-green alga Microcystis secretes hydroxylamine which not only kills aquatic life but also the birds and cattles who care to drink that water. While Lyngbya and Chlorella may cause skin allergies in human beings.

(b) Algal parasitism : The red alga Cephaleuros virescens causes red rust of tea thus destroying the tea leaves. Similar disease are caused by the species of Cephaleuros to coffee plant, Piper and Citrus sp.

(c) Fouling of marine vessels : Some brown and red algae grow on the metallic and wooden submerged parts of naval vessels. As a result, their surfaces are corroded. This creates problems in their navigation.

(d) Spoilage of drinking water : Forms like Anabaena, Microcystis not only spoil the taste of drinking water but also produces toxic effect. The water filters are blocked due to growth of diatoms, Spirogyra, Oscillatoria etc. Forms like Chaetophora, Anacystis grow inside the water pipes and boilers, and thus corrode their surface by their secretion. The growth of algae is controlled by using algicides such as dichlorophen, sodium perborate, phygon XI, exalgae, delrad, cuson etc. Besides, cyanophages (LPP-1) are also used for the destruction of Lyngbya, Phormidium and Plectonema.

(e) Water blooms : Algae grow abundantly in water reservoirs where excess of nutrients are available to them. This algal growth floats on the water surface and look like foam or soap lather. It is called water bloom. e.g., Member of cyanophyceae (Microcystis, Anabaena, Oscillatoria etc.) are common. Water bloom deplete oxygen of water reservoirs and therefore, aquatic animals die of deoxygenation.

Bryophyta (Gk : Bryon = moss ; phyton = plants) includes the simplest and primitive land plants. De Jussieu (1789) placed mosses etc. under acotyledons along with algae and fungi. De Candolle (1813) placed liverworts and mosses in the class Aetheogamous of the division Cellulare. Robert Broun include algae, fungi, lichen and mosses under bryophyta. It occupies a position intermediate between algae and pteridophyta. Due to peculiar type of their habitats, they are regarded as 'the amphibians of the plant kingdom'.

Habitat : Bryophytes usually grow in moist and shady places. The plants grow densely together and form green carpets or mats on damp soil, rock, walls, barks of trees and on decaying logs in forests, especially during the rainy season.

Some bryophytes grow in diverse habitats such as – aquatic (e.g., Riccia fluitans, Ricciocarpus natans, Riella), epiphytes (e.g., Dendroceros, Radula protensa and many mosses), saprophytes (e.g., Buxbaumia aphylla, Cryptothallus mirabilis), and in dry habitats such as dry heaths (e.g., Polytrichum juniperinum), deserts (e.g., Tortula desertorum) and dry rocks (e.g., Porella platyphylla

(i) The life cycle of bryophytes consists of two distinct phases – the gametophytic phase and the sporophytic phase. The haploid gametophyte is dominant, long lived, green and independent whereas the diploid sporophyte is short lived and dependent upon the gametophyte. The two phases come one after the other in alternating manner and both are morphologically distinct.
(ii) The plants are small, range from few millimetres (e.g., Zoopsis) to 30–40 centimetres. The tallest species may reach upto 70 cm in length (e.g., Dawsonia).
(iii) The gametophytes are either thalloid (i.e., not differentiated into true roots, true stem and true leaves) or leafy shoot having stem-like central axis and leaf-like appendages.
(iv) The roots are completely absent and they are replaced by unicellular or multicellular thread like rhizoids. In some higher forms the multicellular rhizoids form cords.
(v) The vascular tissue (i.e., xylem and phloem) are completely absent. In few mosses (Polytricum) the xylem like hydroids, which conduct water and phloem like leptoids, which conduct the assimilates, have been reported.

The apical growth in bryophytes take place by a single apical cell or a group of meristematic cells arranged in a transverse row. In Riccia, Marchantia and many jungermanniales the apical growth takes place by a transverse row of apical cell. In mosses, it occurs by single pyramidate apical cell. In Anthoceros, on the other hand, there may be a single apical cell or a transverse row of such meristematic cells.

The bryophytes reproduce vegetatively, asexually and sexually. Various methods involve in reproduction are discussed in the following account.

# (i) Vegetative reproduction : The bryophytes reproduce vegetatively by following methods :
(a) Death and Decay : Most of these plants reproduce vegetatively by gradual death and decay of the older part of the plant body.
(b) Adventitious branches : Many plants like Riccia fluitans, Reboulia, Asterella, Pellia etc. reproduce by adventitious branches. They separate and produce new plants.
(c) Tubers : Several species of Riccia, Anthoceros, Sewardiella, Asterella etc. produce tubers which give rise to new plants on the arrival of favourable conditions.
(d) Gemmae : Several members, reproduce vegetatively by forming multicelled gemmae. In Marchantia, Lunularia, the gemmae are produced in gemma cups. In some liverworts, 1–3 celled gemmae are prodcued on the axis or on the 'leaves' or on thalli. Gemmae are also produced on the thallus of Anthoceros. Several mosses also produce gemmae on the 'leaves' (Bryum), or axis or rhizoids or on the protonema (Funaria).
(e) Leafy propagules : Some liverworts also reproduce vegetatively by forming leafy propagules.
(f) Primary protonema : The mosses generally reproduce vegetatively by breaking of the primary protonema. New gametophores now arise from the buds differentiated on it.
(g) Secondary protonema : In several mossess a secondary protonema may arise from the rhizoids or primary protonema or even from the injured sporophyte. It may produce buds which give rise to new gametophores.
(h) Rhizoids : Mosses may also reproduce vegetatively from the rhizoids e.g., Leucobryum.

# (ii) Sexual reproduction : The male sex organs is called as antheridium and the female as archegonium. The antheridial stalk is very distinct whereas the archegonial stalk is generally short. They may be of embedded type e.g., Riccia, Anthoceros or of projecting type e.g., Marchantia, mosses.
(a) Antheridia : They are generally borne on the dorsal surface of the thallus. While in Riccia the antheridial chambers are open, in Anthoceros they are closed. The antheridia lie embedded within the thallus in both the plants. In Jungermanniales the antheridia are borne in one or more rows. In Marchantiaceae they are present on a special branches, the antheridiophores or the male receptacles which may be stalked or sessile. While in most of the members the antheridia are superficial in origin, in Anthoceros they are endogenous. Each antheridium is distinguishable into a stalk and the body. The antheridial body consists of a mass of androgonial cells covered by a 1-cell thick sterile jacket. The terminal cell of the jacket, when distinct, is called as operculum. Each androgonial cell finally behaves as androcyte mother cell. The androcyte mother cell then forms two androcytes (antherozoid mother cell), each of which is metamorphosed into a biflagellate antherozoid.
(b) Archegonia : These are also borne on the dorsal surface of the thallus. In Riccia and Anthoceros they lie embedded in the thallus. In many members of Marchantiaceae they are borne on special branches called archegoniophores or the female receptacles, that may be stalked or sessile. The archegoniophore or carpocephalum has rows of archegonia protected by involucre or perichaetium. The archegonia are flask shaped structures distinguishable into a long neck and a globular, swollen venter. A multicelled stalk is also present in mosses but in others it is very short. The neck is one-cell thick. It is generally made up of six vertical rows of cells but in Jungermamnniales it is compose of 4 or 5 vertical rows only. The neck is capped by four cover cells and contain varying number of neck canal cells inside. While in Riccia there are only 4 neck canal cells, the mosses however, possess more than six of them. The venter is also 1-cell thick in most of the plant but in Jungermanniales it is 2–3 layered. In mosses it is double layered. The venter contains an egg and a ventral canal cell.

The bryophytes are fundamentally terrestrial plants but require presence of water to complete their life cycle. The water is needed for dehiscence of antheridia, liberation of antherozoids, transfer of antherozoids from antheridia to archegonia, opening of archegonial neck, and the movement of antherozoids into the archegonial neck.

Before fertilization the walls of androgonial cells disorganise to form a mucilagenous mass. The opercular cell is removed and the antherozoids are liberated. The neck canal cells and the ventral canal cell also disorganise. The cover cells split apart giving a free passage to incoming antherozoids. The antherozoids are attracted towards the egg by chemotactic stimulus, which in bryophytes, is provided in the form of sugars. Antherozoids enter in to archegonia and fertilized the egg.

(i) The diploid fertilized egg (zygote) is the first cell of sporophytic generation. It divides and develops into a sporophytic plant body, called sporogonium.
(ii) The wall of venter forms calyptra, which provides a protective covering to the developing sporogonium.
(iii) The sporogonium, in most of the cases, is differentiated into foot, seta and capsule.
(iv) The sporogonium is completely dependent on the gametophyte for water and mineral supply and, in most of the cases, partly or wholly for organic nutrition. The sporogonium remains attached to the gametophytic plant body throughout its life.
(v) The sporogonium is mainly concerned with the production of asexually formed haploid spores (or meiospores). The spores are produced inside the capsule of sporogonia as a result of meiosis in the spore mother cells.
(vi) The spores are the first cells of gametophytic generation. They germinate to produce the gametophytic plant body either directly or through a juvenile filamentous stage, called protonema.

Bryophytes exhibit a distinct and heteromorophic alternation of generations in which two phase gametophytic and sporophytic follow each other in regular sequence. The sporophytic plant body (2 N) of bryophytes is dependent on the gametophyte (N).

Eichler (1883), Engler (1892), Bower (1935) divided Bryophyta into two classes – Hepaticae and Musci. On the other hand, Campbell (1940), Smith (1955), Takhtajan (1953) divided into three classes namely Hepaticae, Anthocerotae and Musci. Proskauer (1957) changed the names of these classes in accordance with the recommendations of the code, into Hepaticopsida, Anthocerotopsida and Bryopsida.

# (i) Hepaticopsida : The latin word Hepatica means liver. Thus the members of hepticopsida are popularly known as liverworts. The important characters of hepaticopsida are :
(a) The gametophytic plant body is small, dorsiventral, thallose or leaf axis (foliose).
(b) Chlorophyllous cells contain many chloroplasts and one to several oil bodies.
(c) Pyrenoids are absent.
(d) Rhizoids are unicellular.
(e) Sex organs develop from single superficial cells.
(f) Sporogonium has little or no chlorophyllous tissue and stomata.
(g) The capsule is not linear. It lacks columella and intercalary meristem.
(h) Capsule dehisces by drying of capsule-wall, usually by more than two valves.

# (ii) Anthocerotopsida : This class is characterised by the following characters –
(a) Gametophyte is thalloid. Thalli are lobed, dorsiventral, internally homogenous without any differentiation of tissues.
(b) Air chambers and air pores are absent but mucilage cavities may be present.
(c) Rhizoids are only smooth walled.
(d) Scales are absent.
(e) Each cell possesses single (some times more) large chloroplast with central pyrenoid.
(f) Oil bodies are absent.
(g) Antheridia are endogenous in origin, borne singly or in groups inside the closed cavities.
(h) Sporogonium is differentiated into foot, meristematic zone and capsule (the seta is absent).
(i) Capsule has central sterile columella.
(j) The capsule dehisces basipetally by two valves and shows hygroscopic twisting.

# (iii) Bryopsida : The members of bryopsida are commonly known as mosses. The class is characterised by the following characters –
(a) Gametophyte is differentiated into two stages – prostrate protonema and erect radial leafy shoot.
(b) Leaf-like appandages are spirally arranged on stem – like axis.
(c) Rhizoids are multicellular with oblique septa.
(d) Sex organs develop from superficial cells.
(e) Sporogonium is differentiated into foot, seta and capsule.
(f) Wall of capsule is several layered with stomata on epidermis.
(g) The capsule has central columella.
(h) Elaters are absent.

(i) Soil conservation : Mosses grow in dense mats over the soil surface. They bind the soil particles and prevent soil erosion by running water.

(ii) Formation of soil : Mosses along with lichens play a very important role in the formation of soil over the bare rocky surface. They grow on rocks and add organic matter to the substratum after their death. It makes the rock surface suitable for the growth of higher plants.

(iii) Use in nursery : The Sphagnum plants have magnificent property of retaining water. They can with hold water two hundred times more than their own weight. Hence they are widely used by gardeners to keep cut plant parts moist during transportation and propagation.

(iv) Peat : Sphagnum plants grow as semiaquatic or submerged in acidic marshes. The older portions of plants die but do not decay due to peculier germicidal properties. Constantly increasing mass of dead remains accumulate year after year. These dead remains are slowly compressed and become hardened due to weight and forms a compact dark coloured peat rich in carbon. The peat is dried, cut into pieces and used as fuel. It is mixed in clayey soil to keep it porous and mixed in sandy soil to improve its water holding capacity.

(v) Other uses : Certain bryophytes are used to obtain a number of antibiotic substances. Some bryophytes grow in specialized areas and, therefore, used as indicator plants. Some bryophytes have important medicinal uses. For example – The tea prepared from Polytrichum commune is used to dissolve kidney and gall bladder stones.

The pteridophytes (Gk. Pteron = feather and phyton = plants ; means plants with feather like fronds or ferns). They are flowerless, seedless, spore producing vascular plant which have successfully invaded the land. Pteridophytes represent an intermediate position between bryophytes and spermatophytes (Gymnosperm and Angiosperm). It is classified by Carolus Linnaeus (1754) under the class cryptogamia. They are also called vascular cryptogams. The group has a long fossil history. It is the most primitive group that flourished in Devonian and Carboniferous periods of Palaeozoic Era.

Habitat : The plants of pteridophytes are mostly terrestrial. They prefer shady habitats. Some species of Selaginella and Adiantum are xerophytes. A fern, Acrostichum aureum is a halophyte. Some species e.g., Selaginella oregana, Psilotum flacidum, Lycopodium squarrosum and ferns like Asplenium nidus, Pleopeltis sp. are epiphytes. Marsilea occurs as a terrestrial, amphibious as well as an aquatic plant. There are true aquatics ferns like Salvinia, Azolla and Ceratopteris.

(i) The main independent plant body of pteridophytes is sporophyte. It is differentiated into true roots, true stem and true leaves.
(ii) The primary root is short lived. It is replaced by adventitious roots. The root has a permanent growing apex.
(iii) The stems are usually herbaceous (except in some woody ferns) and branched monopodially or dichotomously.
(iv) The leaves may be small microphyllous (e.g., Lycopodium, Equisetum) or large macrophyllous (e.g., Pteridium, Pteris and other ferns).
(v) All the vegetative parts possess vascular tissues (i.e., xylem and phloem) organized in definite groups or steles. Secondary growth does not occur in most of the living pteridophytes (except in Isoetes).

Apical growth : The pteridophyte generally possess a single apical cell with three cutting faces in the shoot apex. The root tip also has a single apical cell but with four cutting faces.

Spore producing organs : The plants of pteridophytes are sporophytes. They reproduce asexually by forming spores in sporangia. They are homosporous but a few plants are heterosporous also e.g., Isoetes, Selaginella, Marsilea, Regnellidium, Pilularia, Azolla and Salvinia. In Selaginella the sporangia are borne in relation to sporophylls which constitute a strobilus. In Equisetum they are borne on sporangiophores which constitute a cone. In ferns the sporangia are borne in sori on the sporophylls. The sori are of three types –
(i) Simple sorus : Here all the sporangia mature at the same time.
(ii) Gradate sorus : Here the oldest sporangium lies in the centre and the sporangia on either side show successively younger stages.
(iii) Mixed sorus : It shows mixed arrangement of younger and older sporangia.
In Marsilea, Azolla, Salvinia etc. the sori are present in a box like structure called sporocarp. The sorus may be naked or covered by an inducium. The inducia may be true or false. A true inducium is a specially developed structure whereas a false inducium is formed by curving of the sporophyll margin.

The sporangia are generally stalked structures. Each sporangium is distinguishable into a jacket enclosing a mass of sporogenous tissue. The sporangial jacket may be 2-4 layered. The innermost wall layer is the tapetum. No tapetum is formed in Psilotum and Tmesipteris. It is a nutritive layer which degenerates at maturity of the sporangium. The sporangial jacket in some ferns shows distinctions of annulus and stomium. On the basis of development the sporangia have been classified by Goebel, 1881 into two categories as under :

(i) Eusporangiate type : Such a sporangium develops from a group of superficial initials. They divide periclinally into outer and inner components. The outer cells form the wall whereas the inner cells give rise to sporogenous tissue.

(ii) Leptosporangiate type : Such a sporangium arises from a single superficial initial. It divides periclinally into outer and inner components. While the inner cell forms the stalk, the outer gives rise to sporagnium proper. In Marattia alata, the sporangia in a sorus may fuse to form a synangium.

The plants may be homosporous, i.e., produce only one type of spores (e.g., Lycopodium, Pteridium) or heterosporous i.e., produce two different types of spores, smaller microspores and larger – megaspores (e.g., Selaginella, Marsilea etc.). The spore germination is homosporous pteridophytes may be bipolar (e.g., Lycopodium, Equisetum) or tripolar (e.g., Hymenophyllum) or amorphous (e.g., Angiopteris).
The spores germinate to produce haploid gametophyte, called prothallus. The homosporous pteridophytes produce bisexual (monoecious) gametophytes whereas heterosporous one produce unisexual (dioecious) gametophytes.

The archegonia and antheridia are generally of embedded type. The archegonium consists of neck which usually projects from the surface of the prothallus. It contains 1-2 neck canal cells. There is no venter. The egg and the ventral canal cell remain surrounded by the cells of prothallus. The antheridia are generally sessile. They have a 1-cell thick jacket enclosing a mass of androgonial cells. They form the androcytes which are metamorphosed into biflagellate (Lycopodium, Selaginella) or multiflagellate (Equisetum, ferns) antherozoids.

Before fertilization the walls of androgonial cells get dissolved and antherozoids liberated. The neck canal cells and the ventral canal cell disorganise. The neck cells/cover cells separate to give a free passage to incoming antherozoids. Fertilization is affected by water medium (zooidogamous). The antherozoids are attracted towards the egg by a chemotactic stimulus provided by the degeneration of neck canal cell and ventral canal cell, in the form of malic acid.

As a result of fertilization the zygote is formed. It divides into an upper or anterior epibasal cell and a lower or posterior hypobasal cell. In Selaginella, Lycopodium the epibasal cell forms the suspensor and the hypobasal gives rise to embryo proper. By further segmentation a quadrant and then an octant is formed. Usually the epibasal quadrant forms the stem and root and the hypobasal gives rise to foot and root. The young sporophyte of pteridophytes is dependent upon the gametophyte for food which is drawn with the help of its foot. Like bryophytes, the pteridophytes also show heteromorphic alternation of generations.

The stelar theory was proposed by Van Tiegham and Douliot (1886). Some important types of steles found in pteridophytes are :

# (i) Protostele : Solid core of xylem surrounded by phloem, pericycle and endodermis. The types of protosteles are :
(a) Haplostele : A protostele having a central smooth core of xylem surrounded by phloem, pericycle and endodermis e.g., Selaginella sp., Lygodium, etc.
(b) Actinostele : A protostele having star-shaped xylem core with radiating ribs e.g., Psilotum, Lycopodium serratum, etc.
(c) Plectostele : A protostele having xylem broken into parallel plates alternating with parallel phloem plates e.g., Lycopodium clavatum.
(d) Mixed protostele : A protostele having several xylem groups scattered and embedded in phloem e.g., Lycopodium cernuum.

# (ii) Siphonostele : A stele having central pith. It is formed by medullation (or appearance of pith) in the protostele. The types of siphonosteles are :
(a) Ectophloic siphonostele : The central pith is surrounded by xylem, phloem, pericycle and endodermis. The phloem occurs only outside the xylem e.g., Osmunda.
(b) Amphiphloic siphonostele : The ring of xylem is surrounded on both outer and inner sides by phloem, pericycle and endodermis e.g., Marsilea rhizome, Adiantum pedatum rhizome.

Modification of siphonostele
(i) Cladosiphonic siphonostele : A siphonostele not perforated by leaf gaps e.g., a few species of Selaginella.
(ii) Phyllosiphonic siphonostele : A siphonostele perforated by leaf gaps – e.g., Nephrolepis.
(iii) Solenostele : A siphonostele perforated by leaf gaps which are scattered but not overlapping e.g., Ferns.
(iv) Dictyostele : A siphonostele perforated by several overlapping leaf gaps. Each separate strand is called meristele. e.g., Dryopteris, Pteridium, Pteris, etc.
(v) Polycyclic dictyostele : A dictyostele consisting of two or more concentric rings of meristeles e.g., Pteridium aquilinum.
(vi) Eustele : Much dissected siphonostele having vascular strands separated apart by parenchyma e.g., Equisetum.
Polysetelic condition : Presence of more than one stele e.g., Selaginella kraussiana.

The sporophytes reproduce asexually producing spores in sporangia. When all the spores are alike i.e., almost of the same size, the phenomenon is called homospory. However, in some pteridophytes, two types of spores are formed which differ significantly in their size as also in function. This phenomenon is called as heterospory. It is seen in pteridophytes like Selaginella, Isoetes, Stylites, Marsilea, Regnellidium, Pilularia, Azolla, Salvinia and Platyzoma.
It is believed that during the course of evolution, heterothallism was followed by heterospory. Some homosporous ferns e.g., Equisetum, Ceratopteris produced two types of gametophyte, thus representing incepient heterospory.

The terms apogamy was coined by de Bary (1878). It is defined as formation of sporophyte from a gametophytic cell other than egg without fertilization. It was first observed by Farlow (1874) in Pteris cretica. Thereafter it was observed in several other plants e.g., Lycopodium, Selaginella, Nephrodium, Lastrea, Marsilea etc. The apogamy is of two types, obligate and facultative. If either both of the sex organs are nonfunctional or absent, the apogamy occurring on account of this is said to be obligate. But if a gametophytic cell is induced to form the sporophyte without fertilization, the apogamy is called as facultative or induced.

The formation of gametophyte from a sporophytic cell without meiosis is called as apospory. This phenomenon was first observed by Druery (1884) in Athyrium filix-femina. Thereafter it has been established in several pteridophytes. e.g., Pteridium aquilinum, Asplenium, Trichomanes etc. Induced apospory was seen in Pteris species.

Formation of sporophyte from egg without fertilization is called as parthenogenesis. Farmer and Digby (1907) observed that in homosporous, leptosporangiate ferns, apospory was always followed by parthenogenesis. This phenomenon has been observed in several species of Selaginella and Marsilea.

The pteridophytes occupy an intermediate place between bryophytes and spermatophytes. They represent affinities with both the groups.
(i) Similarities with bryophytes
(a) Both have terrestrial mode of life.
(b) Water is indispensable for the process of fertilization.
(c) Male gametes are flagellated.
(d) The structure and ontogeny of sex organs i.e. antheridium and archegonium is based on similar pattern.
(e) Both the groups have definite alternation of sporophytic and gametophytic generations.
(f) Sexual reproduction is of oogamous type. Zygote is retained within the venter of archegonium to form embryo.
(g) Sex organs are surrounded by sterile jacket.
(h) The young sporophyte is partially or wholly dependent on the gametophyte for nourishment.
(ii) Similarities with spermatophytes
(a) In both the cases, sporophytic plant body is large, independently existing and dominant phase of life cycle.
(b) The plant body is differentiated into true stem, leaves and roots.
(c) Vascular tissue is present.
(d) Spores are produced inside the sporangia.
(e) Presence of distinct alternation of generations.
(f) Process of photosynthesis is mainly confined to leaves. Stomata are present on the leaves.

# Fossil history of pteridophytes : The pteridophyta have a long fossil history. Their first traces were identified in the silurian period of paleozoic age (about 400 million years age). Pteridophytes flourished well during devonian, mississipian and pensylvanian periods of late paleozoic age. This period can be well recognised as "age of pteridophyta".

(i) Ornamental value : Many ferns are grown as ornamental plants in gardens for their large, Show and graceful foliage. e.g., species of Lycopodium, Nephrolepis, Selaginella, Lygodium, Anemia, Cyathea etc.
(ii) Medicines : An anthelmintic drug is obtained from the rhizomes and petioles of the fern Dryopteris. Lycopodium clavatum is used in skin diseases. Equisetum arvense has diuretic properties.
(iii) Food : The sporocarps of Marsilea are rich source of starch and used by tribals for their nutritive value.
(iv) Soil conservation : Plants like Selaginella are useful in soil conservation.

Gymnosperm (Gk. Gymnos = naked ; sperma = seed) are the plants with exposed or naked seeds or ovules. These plants represent the most ancient group of seed plants. They have been generally placed in the division spermatophyta (seed bearing plants) along with angiosperms. They were not grouped separately as gymnosperms. But Robert Brown (1827) separated them from angiosperms and placed under a distinct group due to presence of unprotected ovules in them. The gymnosperm originated much earlier then angiosperms. However, most of the members of this group have now become extinct and only few living forms are known today. The living gymnosperm are generally grouped under four orders (Cycadales, Ginkgoales, Coniferales and Gnetales).

(i) The plant body is sporophyte and differentiated into root, stem and leaves.
(ii) The plant possess well developed tap root system. In some cases the roots are symbiotically associated with algae (e.g., Coralloid roots of Cycas) or with fungi (e.g., Mycorrhizal roots of Pinus).
(iii) The stem is erect, aerial, solid, woody and branched (unbranched in Cycadales) but almost tuberous in Zamia.
(iv) The leaves may be microphyllous or megaphyllous.

(i) Manoxylic wood : Cambial activity is short lived, cortex and pith are broad, parenchymatous rays are broad, wood is soft and commercially useless. e.g., Cycas.
(ii) Pycnoxylic wood : Cambial activity is long lived, cortex and pith are reduced, parenchymatous rays are few, wood is hard and compact, wood is commercially most important and used as good quality timber. e.g., Pinus.

Gymnosperms are heterosporous, i.e., produce two different kinds of spores – the male microspores and the female megaspores. The spores are borne inside the sporangia. The two types of sporangia are borne on special leaf-like structures, called sporophylls. The microsporangia (pollen sacs) are born on microsporophylls (= stamens) and the megasporangia (ovules) are borne on megasporophylls (= carpels).
The sporophylls are usually aggregated in the form of compact structures called cones or strobili. The cones are generally unisexual, i.e., the male cones are microsporangiate (pollen cones) and the female cones are megasporangiate (seed cones). The male cones are short lived whereas the female cones are long lived. The female cones remain attached on the plants for several years till the maturity or ripening of the seeds.

The microsporangium (Pollen sac) produces numerous light pollen grain. Pollination is anemophilous (wind pollination). The ovules are orthotropous and remain exposed on the megasporophyll. Each ovule surrounded by integuments. It incloses the nucellus and a female gametophyte formed from the haploid megaspore. The female gametophyte contains archegonia. The pollen grains are captured by the pollination drop secreted by the micropyle of the ovule. When it dries, the grains are sucked in the pollen chamber. The pollen grains now germinate. A pollen tube is formed due to elongation of the tube cell. In Cycas and Ginkgo the pollen tube is haustorial in nature. The lower end of the tube bursts and releases the male gametes which fuse with the egg to form the zygote.

Fertilization occurs by siphonogamy, i.e. the male gametes are carried to the archegonia through pollen tube (except in Cycas where pollen tube functions as haustorium and fertilization occurs by zoodiogamy). Fertilization thus takes place in the absence of external water.

The zygote undergoes free-nuclear divisions in Cycas followed by wall formation. There are no free-nuclear divisions in Sequoia and Gnetum. The embryo is soon differentiated into an upper haustorial, middle suspensor and lower embryonal regions. In Pinus, on the other hand, the zygote gets differentiated into four tiers of four cells each, designated as open tier, rosette tier, suspensor tier and embryonal tier. Cleavage polyembryony is seen in Pinus. The embryonal part shows differentiation of radicle, hypocotyl, cotyledons and plumule.

As a result of fertilization the ovule develops into a seed. The integument forms the seed coat. The outer fleshy layer of the integument forms the testa whereas the middle stony layer gives rise to tegmen. The nucellus persists as a cap like perisperm. In Taxus a fleshy aril is also present at the base as a cup like structure. The seeds of gymnosperms comprise tissue of three generations namely parent sporophytic (integument and nucellus), gametophytic (endosperm) and second sporophytic (embryo).

When a group of plants is represented by a single genus or species while rest of the other representatives of the group have become extinct and fossilized the long surviving individual is called a living fossil e.g., Ginkgo biloba. However, Cycas is also regarded as a living fossil because most of the cycad species are confined to tropical and subtropical region and the group is becoming endangered. Therefore, cycads have been referred as reptiles of plant kingdom or panda of vegetable kingdom.

Robert Brown (1827) recognised the gymnosperms as a group distinct from Angiosperms. However, Bentham and Hooker (1862-83) in their 'Genera Plantarum' placed them between Dicotyledonae and Monocotyledonae, Chamberlain (1934) divided gymnosperms into following two sub-classes.
(i) Cycadophyta
(ii) Coniferophyta

(i) Sub-class I. Cycadophyta : These are characterised by the presence of unbranched stem and large foliage leaves. Internally, the stem has large pith and cortex but the wood is relatively small. It includes following 3 orders.
# Order 1. Cycadofilicales : It is a group of fossil plants. These plants resembled with ferns, hence they were given the name Pteridospermae (i.e., seed bearing ferns). e.g., Lyginopteris, Medullosa.
# Order 2. Bennettitales or Cycadeoidales : It is also a group of fossil forms. These plants resembled with modern cycads. e.g., Cycadeoidea, Williamsonia.
# Order 3. Cycadales : It includes both living and fossil forms. e.g., Cycas, Nilssonia, Zamia.

(ii) Sub-class II. Coniferophyta : These are characterised by long profusely branched stem and simple small leaves. In stem the amount of wood is much more than cortex and pith. It includes following four orders.
# Order 1. Cordaitales : All the members of this order are extinct. e.g. Cordiates, Dadoxylon.
# Order 2. Ginkgoales : All the members of this order, except for Ginkgo biloba are extinct. Ginkgo biloba is a medium sized tree with branched stem and bilobed leaves. Because of the resemblance of the leaves of this plant with those of Adiantum (maiden hair fern), the name Maiden hair tree has been given.
# Order 3. Coniferals : The order includes both fossils and present day forms. e.g., Pinus, Cedrus, Sequoia.
# Order 4. Gnetales : Gnetales are modern group consisting of living forms. The order differs from other gymnosperms in the presence of vessels in the xylem. e.g., Ephedra, Gnetum, Welwitschia.

(i) Ornamentals : Some of the gymnosperms are grown in the gardens in different parts of the world e.g., Cycas revoluta, Ginkgo biloba, Auraucaria cookii, A. bidwilli, Biota orientalis, Cupressus sp., Juniperus sp., Thuja sp., Taxus baccata, Cryptomeria japonica etc.

(ii) Wood : Several plants of this group yield useful timber. The wood of Cedrus deodara is used for making railway sleepers. It is also used as a structural timber and making bridges. The wood of Callitris verrucosa, Pinus roxburghii, P. wallichiana, P. pinaster, P. lambertiana etc. is used for making furniture. Juniperus virginiana wood is used for making pencils. The gymnosperm Agathis australis is perhaps the largest timber producing tree of the world. Soft wood of many gymnosperms is used for making toys.

(iii) Resins : Several conifers yield resin which is obtained by tapping. By distilling the oleoresin obtained from pines. The resins are of three types namely hard resins, oleoresins and gum-resins. Several hard resins are obtained from living and fossil conifers which are as under :
(a) Copals : Kauri copal is obtained from Agathis australis and manila copal from Agathis alba.
(b) Amber : It is obtained from the fossil conifer Pinites succinifera.
(c) Sondarac : This pale-yellow resin comes from Callitris quadrivalis and Tetraclinis articulata. The turpentines are oleoresin which are also contributed by conifers. An important source of turpentine is Pinus australis, P. ponderosa and P. caribeae. Besides, some of the following products of turpentine nature are also obtained from conifers :
 Canada balsam : It is obtained from Abies balsamaea.
 Spruce gum : It is obtained from Picea rubens.
 Bordeaux turpentine : It is obtained from Pinus pinaster.
 Venetian turpentine : It is obtained from Larix decidua.

(iv) Essential oils : They are obtained from several plants. These oils are used mainly in perfumery, soap industry etc. The important oil yielding plants are Tsuga canadensis, Picea glauca, Abies siberica and Cedrus deodera. The oil obtained from Juniperus virginiana (cedar wood oil) is also used in microscopic work.

(v) Paper industry : The wood of several gymnosperms, particularly those of conifers is used in paper industry e.g., Abies pindrow, Picea smithiana, Cryptomeria japonica, Pinus roxburghii, Tsuga canadensis etc.

(vi) Edible : The seeds of Pinus geradiana (chilgosa) and P. roxburghii are edible. Sago is obtained from Cycas revoluta. The seeds of Cycas sp. are ground and used in the preparation of many edible products.

(vii) Medicinal use : Species of Ephedra yield an alkaloid called ephedrine. It is used in the preparation of medicines for the treatment of cough, asthma and bronchitis.

The angiosperms, or flowering plants, constitute the most dominant and ubiquitous vascular plants of present day flora which changed the green and yellow melancholy of the earth's vegetation by the colourful brightness and fragrance of their flower. The term angiosperm means 'enclosed seed' because the ovules or potential seeds are enclosed within a hollow ovary. In this respect they are considered most highly evolved and advanced as compared with the naked seeded gymnosperms.

# (i) Angiospermous plants grow in almost every kind of habitats. In the deserts, these plants grow, flower, shed seeds and complete their life cycle in a few weeks of rainy season. Some flowering plants like Zostera, occur in shallow seas. A small orchid even lives underground. It survives as a saprophyte on decaying organic matter because of the mycorrhizal association which helps to obtain nourishment. In rain forests, some plants grow on the branches of other plants but do not obtain water or food from them. They are called epiphytes (e.g., Vanda).

# (ii) The angiospermous leaves show reticulate or parallel venation forming areoles. The libriform fibres are present in the xylem and the companion cells are present in the phloem. The true vessels are present in the xylem of angiosperms.

# (iii) The angiosperms produce flowers which normally consist of 4 whorls of appandages – the two outer accessory and reproductive structure such as sepals and petals and the two inner essential parts – stamens and carpels.

# (iv) The stamens (microsporophylls) are bilaterally symmetrical. Each stamen consists of a filament and an anther.

# (v) The anthers produce tectate pollen grains with exine differentiated into rod-like columellae covered by a tectum.

# (vi) In angiosperms, the insects and animals also act as pollinating agents. For this purpose the flowers possess bright and showy petals, edible pollen and nectar.

# (vii) The carpels (= megasporophylls) are rolled and partly sterile so that they enclose the ovules within a hollow ovary that is connected with the stigma and style.

# (viii) The female gametophyte is highly reduced and consists of single egg cell, two synergids, three antipodals and two polar nuclei. The archegonia are absent.

# (ix) The most characteristic feature of angiosperms is double fertilization.
- (a) The male gamete fuses with the egg producing diploid zygote that develops into embryo or new sporophyte.
- (b) Another male gamete fuses with the polar nuclei (triple fusion) resulting in the formation of triploid endosperm.

# (x) After fertilization, the ovules ripens into seeds and ovary ripens into fruits.

(i) The smallest angiosperm is Wolffia. The plant body of Wolffia consists of tiny flat oval green stem (phylloclade) having a few small roots. The plants are about 1 mm in diameter and found free floating in aquatic habitats like ponds, etc.
(ii) The tallest angiosperm is Eucalyptus. Their trees may attain a height upto 100 meters or more.
(iii) Banyan (Ficus bengalensis) tree covers a large area. It's slanting aerial branches spread in all directions. The tree spreads with the help of prop or pillar roots.

Based on the duration of life, the plants are divided into following 4 categories :
(i) Ephemerals : This category includes the plants which live only for a few weeks because of a very short growing season. Such plants are found near deserts or in very cold countries. For example, Arabidopsis species have a life span of 20–28 days.
(ii) Annuals : The plants of this category live and complete their life-cycle in a single favourable season. During this period, they grow in size, produce flowers, shed their seeds, undergo senescence and die. They pass the unfavourable period in the form of seeds. Many crop plants (e.g., wheat, rice, maize, etc.) are annuals. The smallest angiosperm – Wolffia is an aquatic annual.
(iii) Biennials : The plants of this category complete their life-cycle in two favourable seasons (i.e., in two years). They grow vegetatively in the first season and produce flowers and set seeds in the next. Often they produce some storage organs, as in the sugar beet, where food is stored in their swollen roots.
(iv) Perennials : Plants of this category live for more than two years. Generally they live for many years and bear the flowers and fruits during specific seasons. Some perennials continue their vegetative growth for several years and produce fruits and seeds only once in their life time, e.g., Agave, Bamboos, etc. They are called monocarpic. Others produce flowers and fruits every year after attaining a definite stage of maturity, e.g., Mango, Lemon, Apple, etc. Such plants are called polycarpic.

Depending upon the habit of plants, the angiosperms belong to following categories –
(i) Herb : These are small, soft, non-woody plants without persistent parts aboveground. The height of plants usually reaches upto 1 m. The plants may be annual (Brassica), biennial (Sugar beet) or perennial (Canna). The perennial herbs usually possess underground rhizomes which form the new aerial shoots every year. The plants of banana are perennial herbs.
(ii) Shrubs : These are woody plants of relatively low height (1-4 m). They typically branch at or near the base and do not have a main trunk, e.g., Rose. They are mostly perennial.
(iii) Trees : These are perennial woody plants with one main trunk. The trunk may or may not be branched. These are of the following types :
(a) Caudex : The stem is unbranched and usually bears a crown of leaves at the apex. e.g., Date-palm.
(b) Excurrent : The lower part of stem is thicker which gradually tapers above. Branches arise from the main stem in acropetal succession and plant appears conical e.g., Pinus.
(c) Deliquescent : The apical bud of the main stem dies after some time and branches and sub-branches spread in different directions. e.g., Tamarindus, Ficus.
(iv) Culms : In these plants, nodes and internodes are extremely clear. Internodes of such plants are usually hollow. These plants are grasses but cannot be considered as herb or shrub or tree. e.g., Bambusa (Bans).

Warming (1895) divided the plants, on the basis of their adaptation to water, into four major groups – hydrophytes, mesophytes, xerophytes and halophytes. A fifth group epiphytes can also be included.
(i) Hydrophytes : The plants which grow in aquatic habitats are called hydrophytes. They are further grouped as –
(a) Submerged (e.g., Hydrilla)
(b) Attached floating (e.g., Nymphaea)
(c) Free-floating (e.g., Eichhornia, Wolffa)
(d) Amphibious or partly emerged hydrophytes (e.g., Sagittaria).
(ii) Mesophytes : These are the plants which grow under moderate moisture and temperature conditions. They have no special adaptations to grow either in very dry or in very wet conditions (e.g., Sun flower, Brassica). These plants do not possess special adaptations to reduce transpiration.
(iii) Xerophytes : The plants which grow in dry or xeric habitats (i.e., under deficient supply to available water) are called xerophytes. These plants face acute shortage of water and therefore, develop morphological, structural and physiological adaptations in order to survive under such habitats. The adaptations in plants are mainly to check the transpiration and survive under acute shortage of water. e.g., Cynodon (Doob grass), Casuarina, Euphorbia tirucalli, Asparagus, etc.
(iv) Halophytes : Halophytes are those plants which grow in saline habitats, i.e., in salt marshes, alkaline soils, river estuaries, saline ponds near seashore or sandy and heavy soils having excess of salts. In such habitats, the water is present in sufficient amount but due to high osmotic concentrations it is physiologically not available to normal plants. Such conditions are said to be physiologically dry. e.g., Spartina, Atriplex, Portulaca etc.
(v) Epiphytes : These are the plants which grow on other plants for space only. The plants are autotrophic and occur both in aquatic and terrestrial habitats. e.g., Vanda (an orchid).

On the basis of modes of nutrition plants are classified as follows –
(i) Autotrophs : These plants manufacture their organic matter from inorganic matter.
(a) Photoutotrophs : These are green coloured due to the presence of chlorophyll. In the presence of light they are capable of synthesizing their food from CO2 and H2O. e.g., Mango, Mustard etc.
(b) Chemoautotrophs : Those plants which synthesize their food from CO2 and H2O by using energy product in the chemical reactions. e.g., Many bacteria.
(ii) Heterotrophs : They are either unable to photosynthesize their food or are unable to take their water and minerals directly from the soil or unable to synthesize protein. They are classified as follows :
(a) Parasites
(b) Saprophytes
(c) Symbionts
(d) Insectivorous plants
A detailed discussion of these group is given in chapter "Plant nutrition".

The plants of Angiosperms divided into two major groups as – Dicotyledons and Monocotyledons.
(i) Dicotyledons : They are show following distinguished characteristics.
(a) Tap roots found in the members of this group.
(b) The leaves in members of these class exihibit reticulate (net like) venation.
(c) The flowers are tetramerous or pentamerous having four or five members in the various floral whorls, respectively.
(d) The vascular bundles arranged in a ring, numbering 2–6, open and with cambium.
(e) The seeds of dicotyledons are with two cotyledons as the name indicate.
(ii) Monocotyledons : They are show following distinguished characteristics :
(a) Adventitious roots found in the members of this group.
(b) The leaves are simple with parallel venation.
(c) The flowers are trimerous having three members in each floral whorl.
(d) The vascular bundles scattered in the ground tissue, many in number, closed and without cambium.
(e) The seeds of monocotyledons are with one cotyledons as the name indicate. e.g., Cereals, bamboos, sugarcane, palms, banana, lilies and orchids.