How do we classify the living world?

Life used to be simple! Early scientists classified organisms as either Animal or Plant. Animals moved, had nervous systems and showed heterotrophic nutrition, among other features. Plants, in contrast, were photosynthetic with a cell wall enclosing the cytoplasm. Bacteria and fungi aren't usually photosynthetic but as they have cell walls they were looked upon as being plant-like. (We now know their cell walls are really quite different.) In this simple system of plants and animals, "plants" which had no recognisable shoot/root/leaf regions were said to have a body termed a thallus and were termed Thallophytes. In this earliest classification system bacteria, fungi and algae were all put into this grouping.

There is, however, a more fundamental divide than all that. We now know that based on cell structure we can divide organisms into prokaryotes and eukaryotes. The eukaryotic cells (of plants, animals and fungi) have a nucleus, internal membrane systems (i.e. organelles like mitochondria) and distinctive 80S ribosomes. The bacteria show the radically different prokaryotic organisation without internal membranes, with naked DNA in place of a nucleus, with 70S ribosomes and with different flagellar structure.

Today we recognise;-

	Prokaryotes - with their radically different cell structure, namely the bacteria and more specifically of relevance to this course the blue-green bacteria (once called blue-green algae).
	Plants - eukaryotic, photosynthetic organisms with cell walls
	Animals - motile, heterotrophic eukaryotic organisms, the cells of which are not surrounded by cell walls
	Fungi - non-motile, cell wall-bound, spore-bearing eukaryotes with a saprophytic or parasitic mode of heterotrophic nutrition

A fifth kingdom is also recognised by most scientists ;-

Protoctista - defined really as none of the above! It comprises eukaryotic microorganisms and their immediate descendants, viz. protozoa, slime molds, water molds, algae.

Most botanists have a problem with the idea that algae are not plants.
In this course we will certainly be treating algae as members of the Plant Kingdom.

And just to complicate matters further...

An international research project, dubbed Deep Green adds a new twist to our quest to understand the true evolutionary relationships among plants. Their work suggests that green plants including green algae belong in one kingdom, red algae in another and brown algae in yet a third with, of course, animals and fungi in their own separate kingdoms. Stay tuned for more...

What is a plant?

All the features of plants can readily be understood in the context of their autotrophic mode of nutrition. If plants simply transform light energy into chemical energy then all that is required is for the plant to be anchored firmly in one place with a maximum surface area to capture sunlight. There is no need for plants to move or to have sophisticated nervous systems such as animals need to enable them to find food. (By analogy, have you ever seen a solar water heater that moved? Its not an attribute that householders feel to be necessary!)

Far from being boring and just sitting there and doing nothing plants by their subtlety are quite intriguing.

Because they cannot move, plants must be extremely responsive to environmental cues and have their life cycle in perfect synchrony with the seasons. Plants must tide over inhospitable dry seasons or winters as seeds, bulbs etc or in the case of some trees in a "bare-bones" leafless state. A plant cannot simply pick up itself like an animal and get out of the rain!

Because they cannot move and because it is their lot in the web of life to be eaten by animals, plants have a unique mode of development. This involves plants growing by meristems at their tips. Plants also have remarkable powers of regeneration resulting from the fact that plant cells are totipotent. Any single plant cell can regenerate an entire plant with all its cell types - a feat no animal can match. A limb lopped off an animal is certain disaster. For a plant it just regenerates the missing portion!

Plants use the cheapest material around to generate size - water and carbohydrates (Wiebe, 1978). They have a unique means of attaining stature with the most basic of skeletal materials - no bones, no expensive proteins!

Each plant cell comprises a protoplast with a central vacuole which makes the cell much bigger than it would otherwise be. The protoplast is enclosed by a cell wall based on polysaccharides. The tendency of the protoplast to take in water (by osmosis) is counteracted by the cell wall which restricts this expansion. This mutual antagonism between protoplast and cell wall makes each cell a turgid little box. It is this simple construction principle that allows herbaceous plants with little or no strengthening material to rise several feet into the air relying on this system of one turgid box on top of the next!

The earth today is dominated by the flowering plants. Our purpose in this course is to understand how these flowering plants came into being. We can only understand this by studying the plant groups that came before. This course is, therefore, a brief survey of the plant kingdom from an evolutionary perspective.

PLANT CLASSIFICATION SYSTEMS

There are several plant classification systems and little agreement, e.g. some botanists believe the ferns are a class within the Division Pteridophyta, others consider them a division of their own. In this course, rather than use a formal classification system we will simply refer to these groups by their informal names;-

Algae
Bryophytes
Pteridophytes
Gymnosperms
Angiosperms

See the handout for more details.

There are a few other higher categories of classification you may come across in your reading.

Lower plants usually include algae and Bryophytes, while higher plants refer to Pteridophytes, Gymnosperms and Angiosperms.

The word Cryptogams literally means "hidden wedding" and alludes to the fact that the sex life of these plants (algae, Bryophytes and Pteridophytes) was once not understood. Phanerogams ("open wedding") are the seed plants - the gymnosperms and angiosperms.

Thallophytes are plants whose body is not differentiated into root/stem/leaves but is termed a thallus. Algae fall into this category (and fungi did too when they were considered to be plants).

Vascular plants are those with vascular tissue (xylem & phloem). Embryophytes (all but algae) are plants that bear an embryo and are synonymous with land plants.

Let's make a start by looking at the Algae. Click the button at left.

� C. M. Sean Carrington 1997

updated 15 October, 1999