Plants face quite severe problems when growing on land as compared with growth in water.
In the latter situation, the plant is partly supported by the water according to Archimedes’ Principle, in the manner commonly experienced by humans swimming. On land this measure of support is lost and must be supplied by some sort of rigid load-bearing cells.
This can be achieved in two ways:
- Cells have the capacity of absorbing water by a process called osmosis until they become so full that they are like balloons full of air. In this turgid state they become rigid and self-supporting provided that water is available to maintain the situation. However, in a drought they lose water and the plant wilts;
- Some cells have strong walls and in large numbers they form wood which is self-supporting without the need for water.
By a combination of these two types of cells land plants are able to overcome the problem of support on land.
Another problem encountered by land plants is that of transport of substances from one part to another.
In water the entire plant is surrounded by a solution of the substances it requires. However, on land the roots are in the soil solution, but the leaves may be a considerable distance from them so that a system of tubes is necessary to conduct solutions from the one to the other.
A highly successful system of tubes has evolved which can conduct solutions and at the same time can also support the plant because the walls are hard and rigid. Pipes with rigid walls are often used by modern architects in the design of buildings (e.g. the Pompidou Centre in Paris) as they are capable of providing considerable support.
Probably the most serious problem faced by land plants is that of preventing the loss of prohibitive quantities of water vapour.
Covering the external surfaces of land plants with either a waterproof wax layer, the cuticle, or cork on the older woody parts, largely solves the problem of water loss. But the plant cannot cut itself off from the atmosphere altogether as it needs to exchange gases (i.e. breathe) in order to live. Minute pores on the leaf surfaces which can be opened or closed according to the immediate water supply situation enable a land plant to conserve water and at the same time keep some contact with the air. Cork also has fine pores which serve the same purpose as those on the leaves. Before the land could be colonized by plants these structures had to be evolved so that plants could take up carbon dioxide from the air and release oxygen while at the same time conserving water.
Methods of reproduction also require much modification in order that aquatic plants may successfully multiply their numbers on land.
This is partly because successful reproduction usually involves not only multiplication but dispersal from the parent plant as well. Whereas in water individual cells, small detached fragments and fertilised eggs may be widely distributed by currents, river flow and tides, these modes of transport are not available to land plants. Even the necessary movements of spermatozoa to fertilize the eggs could present insuperable difficulties on dry land. It is very significant that in the highest evolved land plants – those that flower – spermatozoa are never released into the environment at all. Indeed, the mechanism of fertilization in those plants has become so modified that spermatozoa as such are never involved.
Air currents, however, are available to land plants and these are used in a variety of ways.
Reproduction in land plants almost always involves the dispersal into the air of microscopic bodies called spores or pollen, as hay fever sufferers know to their cost. These single-celled structures must of course be covered by a waterproof layer so that they do not dry out in the air. In fact, this waterproof wax layer is so resistant to decay that un-germinated spores and pollen may remain in rock for hundreds of millions of years. Furthermore, the rock may be broken down by a very corrosive substance such as hydrofluoric acid without damaging the contained spores. The spores or pollen of each plant species have a highly individual pattern of ornamentation on their surfaces which can be said to be like the fingerprints of the species. Thus the parent plants can be identified from the surface pattern on their spores. Fossil spores are of vital importance in oil exploration since much useful information can be deduced from the species present in rock samples and also from their state of preservation.
We are now in a position to visualize what is the likely form of a typical representative of the earliest land plants which colonized land surfaces about 400 million years ago.
On a human historical timescale, it was as if American history had gone from the Mayflower to the Atom Bomb in only about five years. In vertebrate animal evolution it was over 200 million years after the first amphibians walked on the land before the really large dinosaurs appeared.
It would have needed a vertical stem to bear leaves exposed to sunlight and also to bear the reproductive structures so that spores could be released into the air. The stem would need some hard, woody, and tubular cells to maintain rigidity and to transport solutions. There would also be a requirement for some kind of root system to anchor the plant and absorb substances from the surface on which it grew.
It is interesting to see, therefore, in the earliest land plants the various features without which they could not pursue a successful life on land [Fig. 1].
The stem contained, centrally placed, some strengthening woody cells that enabled it to remain upright without the support it would have had in water. These supporting cells also transported solutions which the upper parts needed as they were situated some distance away from the ground. Projections from the outside of the stem increased the surface area for the interception of light by the green colouring substance so that food could be manufactured. The outside was also covered with a wax layer to minimize water loss. However, the wax layer was perforated with a number of holes so that gases, especially carbon dioxide, could diffuse into the plant and be used for making food and releasing energy when necessary.
Since the holes would be a serious disadvantage in very dry weather, they had a mechanism whereby they could be closed when the plant was losing too much water vapour. Although by inference these early plants must have had some kind of root system for anchorage and absorption of substances from the ground, such systems tend to be rather delicate and do not preserve well as fossils, hence not much is known about them. Also seen on this early land plant are the reproductive structures borne in such a way that they could release spores to be dispersed by passing air currents.
What is quite extraordinary about these early stages of land colonization by plants is the relative rapidity with which the land flora developed.
Only 60 million years after the first colonizers got a grip on the land there were forests of huge trees which at the bases of their trunks were up to 8 feet in diameter. A reconstruction of one is shown in Fig. 2. An incomplete fossil trunk of another species (Pitus primaeva) was found that was 59 feet long. Although 60 million years seems a long time in human terms, it has to be seen against the entire 3.5 billion years from the earliest plant cells to the present day of which 60 million is only 1.7%.