Plants are subdivided into two major groups, the prokaryotes and the eukaryotes; the latter is by far the larger of the two groups.

Prior to the colonization of the land by plants the Earth was only inhabited by members of these groups which lived in fresh or salt water or perhaps on continually moistened rock surfaces.

This section is devoted to a more detailed treatment of three very primitive groups of plants, bacteria, algae and fungi.

At the ends of the sub-sections examples are given of the fossil representatives.

PROKARYOTES

These are the most primitive plants. All of their cellular contents are distributed within the general protoplasm. Activities such as aerobic respiration and photosynthesis are not confined to separate structures cut off from the rest of the cytoplasm by membranes. There is no nuclear membrane.

(a) BACTERIA

It may seem surprising that bacteria are classified as plants. However they have much more in common with plants than any other group of living things.

• They do not ingest solid food as animals do (e.g. Amoeba).
• They have cell walls which are structures lacking in most animals.
• There is an enormous number of species and each species may have an astronomical number of individuals.

Some species can carry out photosynthesis whilst others carry out activities that release energy from simple chemical molecules.

Diseases may be caused by pathogenic bacteria whilst others may be entirely harmless. It is thought that the first organisms on Earth might well have been bacteria that lived in the chemical conditions 3.5 billion years ago. They are unicellular; some of the species have minute hair-like structures which they can use to propel themselves around.

FOSSIL EXAMPLES

• Spherical: Archaeosphaeroides, Huroniospora;
• Rod-like: Eobacterium;
• Stellate: Eoasterion.

(b) CYANOBACTERIA

This name is now used for those organisms that were previously known as blue-green algae. The change brings together the prokaryotic plants into one coherent group. They occur principally, but not entirely, as single-celled species or ones that exist in long filaments. To judge by the forms of those known as fossils, their shapes do not appear to have changed much over billions of years. Their reproduction takes place entirely asexually by simple fission of the cells.

FOSSIL EXAMPLES

• Spherical single cells: Eosphaera;
• Irregular cells: Kakabekia, Palaeolyngbya;
• Filamentous: Animikia, Gunflintia.