`color{Blue} ul(mathtt ("THALLUS"))` : Derived from the greek word (thallos), means "a green shoot" or "twig". It is the undifferentiated vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts and lichens.


●Algae are chlorophyll-bearing, simple, `color{green}"thalloid"` , autotrophic organisms.

`color{Brown} "Habitat"` :
●They are largely aquatic (both fresh water and marine) organisms.
●They occur in a variety of other habitats: moist stones, soils and wood.
●Some of them also occur in association with fungi (lichen) and animals (e.g., on sloth bear).

`color{Brown} "Size and Structure"` :
●The form and size of algae is highly variable.
●The size ranges from the microscopic unicellular forms like 𝘊𝘩𝘭𝘢𝘮𝘺𝘥𝘰𝘮𝘰𝘯𝘢𝘴, to colonial forms like Volvox and to the filamentous forms like 𝘜𝘭𝘰𝘵𝘩𝘳𝘪𝘹 and 𝘚𝘱𝘪𝘳𝘰𝘨𝘺𝘳𝘢.
● A few of the marine forms such as kelps, form massive plant bodies.

`color{blue} ul(mathtt ("Zoospores"))` : Zoospores arc the motile spores which are thin-walled and flagellated and produced for asexual reproduction by some aquatic fungi and algae. These are produced in the zoosporangium.

`color{blue} ul(mathtt ("Aplanospores"))` : Aplanospores are the thin walled, non-motile spores, used for asexual reproduction in many terrestrial fungal species.

`color{blue} ul(mathtt ("Isogamy"))` : Isogamy is the process in which two physiologically and morphologically similar gametes fuse with each other.

`color{blue} ul(mathtt ("Anisogamy"))` : Anisogamy is the process in which two physiologically and/or morphologically dissimilar gametes fuse with each other.

`color{blue} ul(mathtt ("Oogamy"))` : It is the process which leads to the fusion between one large, non-motile (static) female gamete and a smaller, motile male.


● Reproduction in algae can take place by different means. It Includes the vegetative means, asexual reproduction and sexual reproduction.

`star` `color{Violet}"Vegetative Reproduction"`: Vegetative Reproduction occurs by means of:

● `color{Brown}"Fragmentation"` : In this, each fragment develops into a thallus.


`star` `color{Violet}"Asexual reproduction"` Asexual reproduction is by the production of different types of spores.

● `color{Brown}"Zoospores"` : They are the most common type of spores in algae. They are flagellated (motile) and on germination gives rise to new plants.
● `color{Brown}"Aplanospores"`: In lesser classes, aplanospores (non motile) are present.


`star` `color{Violet}"Sexual Reproduction"`: Sexual reproduction takes place through fusion of two gametes. These gametes which fuse could be.

●`color{brown}"Isogamous"` : Flagellated and similar in size e.g. 𝘊𝘩𝘭𝘢𝘮𝘺𝘥𝘰𝘮𝘰𝘯𝘢𝘴 or Non-flagellated but similar in size e.g. 𝘚𝘱𝘪𝘳𝘰𝘨𝘺𝘳𝘢.

●`color{brown}"Anisogamous"`: Dissimilar in size e.g. some species of 𝘊𝘩𝘭𝘢𝘮𝘺𝘥𝘰𝘮𝘰𝘯𝘢𝘴
●`color{brown}"Oogamous"` : Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete e.g., 𝘝𝘰𝘭𝘷𝘰𝘹, 𝘍𝘶𝘤𝘶𝘴.

● At least a half of the `color{Green}"total carbon dioxide fixation"` on earth is carried out by algae through photosynthesis. Being photosynthetic, they increase the level of dissolved oxygen in their immediate environment.

● They are the primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals.

●Many species of 𝘗𝘰𝘳𝘱𝘩𝘺𝘳𝘢, 𝘓𝘢𝘮𝘪𝘯𝘢𝘳𝘪𝘢 and 𝘚𝘢𝘳𝘨𝘢𝘴𝘴𝘶𝘮 are among the 70 species of marine algae used as food.

●Certain marine brown and red algae produce large amounts of `color{green}"hydrocolloids"` (water holding substances), e.g., `color{Green}"algin"` (brown algae) and `color{Green}"carrageen"` (red algae) are used commercially.

●`color{Green}"Agar"`, one of the commercial products obtained from 𝘎𝘦𝘭𝘪𝘥𝘪𝘶𝘮 𝘢𝘯𝘥 𝘎𝘳𝘢𝘤𝘪𝘭𝘢𝘳𝘪𝘢 are used to grow microbes and in preparations of ice-creams and jellies.

●𝘊𝘩𝘭𝘰𝘳𝘦𝘭𝘭𝘢 and 𝘚𝘱𝘪𝘳𝘶𝘭𝘪𝘯𝘢 are unicellular algae, rich in proteins and are used as `color{Green}"food supplements"` even by space travellers.

The algae are divided into three main classes: Chlorophyceae, Phaeophyceae and Rhodophyceae.

The classification is carried out on the basis of these criteria:

●The type of pigments present within
●The nature of reserve food
●The nature of Cell Wall
●Flagellar Number an Position of Insertions
●Habitat

●`color{Maroon}"Members"` : The members of chlorophyceae are commonly called `color{Green}"green algae"`.

●`color{Maroon}"Habitat"`: Occur both in marine and fresh water.

●`color{Maroon}"Structure of plant body"`: The plant body may be unicellular, colonial or filamentous.

●`color{Maroon}"Pigments and its location"`:
`star` The pigments (chl a and b) are localized in definite chloroplasts.
`star` The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species.

●`color{Maroon}"Colour"`: They are usually grass green due to dominance of pigments `color{Green}"chlorophyll a and b"`.

●`color{Maroon}"The nature of reserve food"`:
`star` Chloroplasts may have one or more storage bodies in some members called as `color{Green}"pyrenoids"` which contain starch and proteins
`star` Food may also be stored in the form of oil droplets.

●`color{Maroon}"The nature of cell wall"`: Cell wall is rigid made of an inner layer of cellulose and an outer layer of `color{Green}"pectose"`.

●`color{Maroon}"Mode of Vegetative Reproduction"`: Vegetative reproduction usually takes place by fragmentation or by formation of different types of spores.

●`color{Maroon}"Mode of Asexual Reproduction"`: Asexual reproduction is by flagellated zoospores produced in zoosporangia.

●`color{Maroon}"Mode of Sexual Reproduction"`:
`star` It shows considerable variation in the type and formation of sex cells
`star` It may be isogamous, anisogamous or oogamous.

●`color{Maroon}"Example"`: 𝘊𝘩𝘭𝘢𝘮𝘺𝘥𝘰𝘮𝘰𝘯𝘢𝘴, 𝘝𝘰𝘭𝘷𝘰𝘹, 𝘜𝘭𝘰𝘵𝘩𝘳𝘪𝘹, 𝘚𝘱𝘪𝘳𝘰𝘨𝘺𝘳𝘢 𝘢𝘯𝘥 𝘊𝘩𝘢𝘳𝘢

●`color{Maroon}"Members"`: The members of phaeophyceae are commonly called `color{Green}"brown algae"`.

●`color{Maroon}"Habitat"`: They are mostly marine found in warm seas/oceans.

●`color{Maroon}"Structure of plant body"`:
`star` They range from simple branched, filamentous forms (Ectocarpus) to profusely branched forms as represented by kelps, which may reach a height of 100 metres.
`star` The plant body is attached to the substratum by a `color{Green}"hold fast"`, has a `color{Green}"stalk"`, called `color{Green}"stipe"` and leaflike photosynthetic organ, called `color{Green}"frond"`.

●`color{Maroon}"Pigments and its location"`:
`star` They possess `color{Green}"chlorophyll a, c, carotenoids and xanthophylls"`.

●`color{Maroon}"Colour"`: They vary in colour from olive green to various shades of brown depending upon the amount of the xanthophyll pigment, `color{green}"fucoxanthin"` present in them.

●`color{Maroon}"The nature of reserve food"`: Food is stored as complex carbohydrates, which may be in the form of `color{Green}"laminarin or mannitol"`.

●`color{Maroon}"The nature of cell wall"` : The vegetative cells have a cellulosic wall usually covered on the outside by a `color{Green}"gelatinous coating of algin"`.

●`color{Maroon}"Contents of the cell"` : The protoplast contains, in addition to plastids, a centrally located vacuole and nucleus.

●`color{Maroon}"Mode of Vegetative Reproduction"`: Vegetative reproduction takes place by fragmentation.

●`color{Maroon}"Mode of Asexual Reproduction"`: In most brown algae, it is by `color{Green}"biflagellate zoospores that are pear-shaped"` and have two unequal laterally attached flagella.

●`color{Maroon}"Mode of Sexual Reproduction"`:
`star` It may be isogamous, anisogamous or oogamous.
`star` Union of gametes may take place in water or within the oogonium (oogamous species).
`star` The gametes are `color{Green}"pyriform"` (pear-shaped) and bear two laterally attached flagella.

●`color{Maroon}"Example"`: 𝘌𝘤𝘵𝘰𝘤𝘢𝘳𝘱𝘶𝘴, 𝘋𝘪𝘤𝘵𝘺𝘰𝘵𝘢,𝘓𝘢𝘮𝘪𝘯𝘢𝘳𝘪𝘢, 𝘚𝘢𝘳𝘨𝘢𝘴𝘴𝘶𝘮 𝘢𝘯𝘥 𝘍𝘶𝘤𝘶𝘴

●`color{Maroon}"Members"` : The members of Rhodophyceae are commonly called `color{Green}"red algae"`.

●`color{Maroon}"Habitat"` :
`star` Majority of the red algae are marine with greater concentrations found in the warmer areas.
`star` They occur in both well-lighted regions close to the surface of water and also at great depths in oceans where relatively little light penetrates.

●`color{Maroon}"Structure of plant body"` : The red thalli of most of the red algae are multicellular. Some of them have complex body organisation.

●`color{Maroon}"Pigments and its location"` : The characteristic red colour is due to the pigment, `color{Green}"r- phycoerythrin"` and the other pigments arc `color{Green}"chloropllylls a and d"`.

●`color{Maroon}"The nature of reserve food"` : The food is stored as `color{Green}"floridean starch"` which is very similar to amylopectin and glycogen in structure.

●`color{Maroon}"The nature of cell wall"` : Cell wall is rigid made of cellulose

●`color{Maroon}"Mode of Vegetative Reproduction"` : Vegetative reproduction usually takes place by fragmentation.

●`color{Maroon}"Mode of Asexual Reproduction"` : Asexual reproduction is by non-motile spores.

●`color{Maroon}"Mode of Sexual Reproduction"` : Sexual reproduction is oogamous and the gametes are non-motile.

●`color{Maroon}"Example" `: 𝘗𝘰𝘭𝘺𝘴𝘪𝘱𝘩𝘰𝘯𝘪𝘢, 𝘗𝘰𝘳𝘱𝘩𝘺𝘳𝘢 , 𝘎𝘳𝘢𝘤𝘪𝘭𝘢𝘳𝘪𝘢 𝘢𝘯𝘥 𝘎𝘦𝘭𝘪𝘥𝘪𝘶𝘮.