Lesson 1 of 12 · 7 min

How plant classification changed

NCERT §3 (introduction)

Before the trip, the club borrows an old herbarium from the school library. Its plants are filed by the number of stamens in the flower, so a grass and a lily sit in the same folder while two close cousins end up in different ones.

The story this chapter follows: Meera's coast-to-hills field trip

Meera's science club goes on a week-long field trip. It starts on a rocky seashore with seaweed piled on the tideline, climbs through a damp hill forest where moss covers every boulder and ferns line the streams, and ends at a hill station planted with pines, a Cycas in the rest-house garden and orchards full of fruit. Every specimen they press into their herbarium has to be placed in one of the plant groups.

The lesson in notes

In short

Kingdom Plantae today holds algae, bryophytes, pteridophytes, gymnosperms and angiosperms. Fungi, and the walled members of Monera and Protista, were once counted as plants but have now been removed from the kingdom.

So cyanobacteria, still often called blue-green algae, are no longer treated as algae at all; they are prokaryotes in Monera.

The earliest schemes used only gross external features such as habit, colour, and the number and shape of leaves. They leaned mainly on vegetative characters, or on the structure of the androecium, which was the basis of Linnaeus's system.

Such systems were artificial. Because they rested on a handful of characters, they could split species that are closely related. They also weighted vegetative and sexual characters equally, although vegetative characters are the ones the environment changes most easily.

Natural systems followed. They grouped plants by natural affinities, using internal features such as ultrastructure, anatomy, embryology and phytochemistry as well as external form. George Bentham and Joseph Dalton Hooker gave such a system for flowering plants.

Phylogenetic systems, built on evolutionary relationships, are the ones accepted now. They assume that members of one taxon descend from a common ancestor.

Numerical taxonomy, easily run on computers, uses every observable character. Each character is given a number or code and the data are processed, so all characters count equally and hundreds can be handled at once.

Cytotaxonomy uses cell information such as chromosome number, structure and behaviour. Chemotaxonomy uses the chemical constituents of a plant to settle confusing cases. Both are especially useful where fossil evidence is missing.