Systems of plant classification

Why classify plants?

There are more than three lakh known species of plants. Nobody can learn them one by one. Botanists group them, so that knowing a group tells you a lot about every member. A good system should put together the plants that are truly related.

In the chapter Biological Classification, you met Whittaker's five kingdoms. This chapter looks inside kingdom Plantae. It holds five big groups: the algae, the bryophytes, the pteridophytes, the gymnosperms and the angiosperms. (Today the fungi and the blue-green algae are kept out of it.)

Artificial systems

The earliest systems were artificial. They used only a few characters that are easy to see, mostly the form of the plant body: its habit, its colour, the number and shape of the leaves.

Linnaeus classified flowering plants mainly by the androecium, the stamens of a flower. Plants with the same number of stamens went into the same group, even if they were quite unlike in every other way.

Artificial systems have two weaknesses.

• They give the same weight to vegetative characters and to sexual characters. Vegetative characters, such as leaf shape, are easily changed by the surroundings.
• So they often separate close relatives, and put together plants that are not related at all.

Natural systems

Natural systems try to show the natural affinities between plants. They look not only at the outside form, but also at internal features:

• the ultrastructure of cells;
• anatomy, the arrangement of tissues;
• embryology, how the embryo develops;
• phytochemistry, the chemicals that a plant makes.

George Bentham and Joseph Dalton Hooker gave such a system for flowering plants.

Phylogenetic systems

After Darwin, biologists wanted groups that follow evolution. Phylogenetic systems are based on evolutionary relationships. They assume that the members of one taxon share a common ancestor. Many botanists today use them.

Fossils are rare for many plants, so the family tree is often not known. Then other kinds of evidence help.

Three new kinds of evidence

• Numerical taxonomy uses computers. It scores all the observable characters, codes them as numbers, and gives each the same weight. Hundreds of characters can be compared at once.
• Cytotaxonomy uses the cells: the number, structure and behaviour of chromosomes.
• Chemotaxonomy uses the chemical constituents of the plant to settle doubtful cases.

The picture shows four algae that you will meet in the next lessons. Even these simple plants look very different from one another.

You should find that the score depends on the share of characters alike, not on the number alone. $18$ alike out of $24$ gives $75\%$, but $18$ out of $40$ gives only $45\%$.

Try it in Python

The program scores five plants on eight characters and compares every pair. This is a tiny piece of numerical taxonomy.

Practise with new numbers

Common mistakes

• Calling Linnaeus's system natural. It used only the stamens, so it is artificial.
• Thinking numerical taxonomy weighs important characters more. It gives every character the same weight.
• Mixing up cytotaxonomy and chemotaxonomy. Cyto- means the cell and its chromosomes. Chemo- means the chemicals.
• Putting the fungi in kingdom Plantae. They have their own kingdom.

Remember these

• Artificial systems used a few superficial characters, such as the number of stamens (Linnaeus).
• Natural systems used many features, inside and out (Bentham and Hooker).
• Phylogenetic systems follow evolution: a taxon shares a common ancestor.
• Numerical taxonomy: all characters, coded as numbers, of equal weight.
• Cytotaxonomy: chromosomes. Chemotaxonomy: chemical constituents.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is a system based on the number of stamens called artificial?

2. Name two internal features that natural systems use.

3. What does a phylogenetic system assume about the members of one taxon?

4. Which kind of taxonomy counts and studies chromosomes?

5. In numerical taxonomy, is a flower character worth more than a leaf character?