●Our understanding of the plant kingdom has changed over time.

● Fungi and members of the Monera and Protista having cell walls (which were peviously included in Kingdom Plantae in two kingdom classification system) have now been excluded from Plantae.

`star` So, the cyanobacteria that are also referred to as blue green algae are not ‘algae’ any more.

●According to Whittaker’s Five kingdom Classification, Plantae includes following five divisions:
`color{Green} "Algae (Thallophyta), Bryophytes, Pteridophytes, Gymnosperms and Angiosperms"`.

Classifications use orderly ways to express information and, depending upon their objectives, can be:
`color{Green}"Artificial, Natural, or Phylogenetic (phyletic)"`.

Thus the three major classification systems within angiosperms which have been proposed are as follows.

● Used only `color{Green}"gross superficial morphological characters"` such as habit, colour, number and shape of leaves, etc.

● They were based mainly on vegetative characters or on the androecium structure (system given by `color{Green} "Linnaeus"`).

● All the plants in a single group share the same characteristics, but they are not closely related to one another genetically.

● E.g : Important floras sometimes group plants using color of their flowers.


`color{Maroon}"Advantages"`:
●It is `color{Green}"easy to remember"` as only a few characters are used.

●The traits considered were practically important to humans.

`color{Maroon}"Disadvantages"`:
● This system use only a few `color{Green}"superficial characters"` which leads to many different organisms being grouped together.

● It does not represent natural and phylogenetic relationships.

●They separated only the `color{Green}"closely related species"` since they were based on a few characteristics.

● Also this system was not acceptable as it gave `color{Green}"equal weightage"` to vegetative and sexual characteristics inspite of the fact that often the vegetative characters are more easily affected by environment.

● They provide only a few clues about relationships among the taxa and hence are not predictive.

● It is also called as `color{Green}"phenetic system of classification"`.

● It is based on the `color{Green}"natural affinities"` among the organisms and consider the external features as well as internal features, like ultrastructure, anatomy, embryology and phytochemistry.

● This classification for flowering plants was given by `color{Green}"George Bentham and Joseph Dalton Hooker"` and is widely popular even today.

`color{Brown}"Advantages"` :
● Only related organisms are placed in the same group whereas unrelated organisms are kept in separate groups.

● It demonstrates natural relationships among the organisms.

●Plants which are grouped together have many of the same characteristics and thus,are `color{Green}"highly predictive"`. That is, by listing the characteristics of a plant, one can predict the natural group to which it belongs.


`color{Brown}"Disadvantages"`:

●More emphasis is given on the natural characters.

●Also several related families get placed separately whereas unrelated families get placed together.

●It does not represent phylogenetic relationships among organisms

● Based on the `color{Green}"evolutionary"` as well as `color{Green}"genetic relationships"` between the various organisms and is completely acceptable.

● This assumes that organisms belonging to the same taxa are related to one another by a `color{Green}"common ancestor"`.

● Now information from many other sources is also used to help resolve difficulties in classification, especially when there is no supporting fossil evidence.

● Previous classifications were primarily based upon plant anatomy and morphology with great emphasis upon reproductive morphology.
Today, additionally the techniques of `color{Green}"biochemistry and molecular biology"` are used by taxonomists to add details of internal organization

For the suitability in studies various categories have branched out in taxonomy. These are as follows.

● It is based on all observable characteristics and is now carried out using `color{Green}"computers"`.

● `color{Green}"Quantitative assessments"` are made based upon the similarities and differences between various taxa.

● `color{Green}"Number and codes"` are assigned to all the characters and the data are then processed.

● In this way each character is given `color{Green}"equal importance"` and at the same time hundreds of characters can be considered.

● Based on `color{Green}"cytological information"` like chromosome number, structure and behaviour, especially during cell division (mitosis).

● Chromosome studies are used to trace lineages.

● Example
`star` `color{Green}"Chromosome number"` is constant for a species, e.g., 48 in apes, 46 in man, 20 in maize and 16 in onion. So, information on the chromosome number can indicate the species being studied.

`star` The `color{Green}"pairing and banding patterns"` of the chromosomes are used to understand the relationships between different species e.g., evolution of humans from apes shows a particular difference in banding pattern.

●It is the system based upon the presence and absence of various `color{Green}"chemical constituents"` (proteins, amino acids, enzymes, hormones, etc.), specific chemicals (secondary metabolites) which is used to demonstrate relationships between various organisms.

● Mainly `color{Green}"chemical and protein tests"` are performed to understand relationships between different organisms via the chemicals they produce.

● For example, 35 families of plants been characterized by the presence of chemical compound –`color{Green}"raphide"` within them.