In spite of the huge diversity of plant life, there are really relatively few major groups into which it can be subdivided.
Some of these groups, as already indicated, are little known to the general public because they are the minor, surviving remnants of the earliest land plants dating back hundreds of millions of years. The so-called clubmosses are a good example. Their name derives from the club-shaped groups of sporangia that they produce on plants that superficially resemble mosses. They are now very insignificant and tend to pass unnoticed among the large numbers of conspicuous flowering plants. However, selaginella may sometimes be seen as a soil-covering plant on pots or trays in greenhouses. Three hundred million years ago their ancestors attained the stature of major forest trees.
Another group which does not today compete very strongly with the flowering plants is that of the horsetails. They are, however, more widely known than the clubmosses and the larger species may attain the height of four or five feet in very marshy situations in temperate climates and up to thirty feet in the tropics. These very tall stems, though, are only about an inch in diameter and require support from the surrounding vegetation.
In temperate regions the species are plants usually only about a foot high, often seen on railway embankments or other waste ground. They have a highly characteristic appearance with branches emerging in circles, called ‘whorls’, at regular intervals up the stem rather in the way that those of Christmas trees do. Their entire structure, main stem, and branches alike, is made up of a considerable number of sections arranged end-on-end. Another curious feature is that they contain a great deal of silica, sand in other words, in some of their cells. So much, in fact, that in colonial times in North America these plants were called ‘scouring rushes’ and were used for cleaning domestic utensils. As was the case with the clubmosses, the horsetails once formed a major part of the land flora, attaining the size of large trees.
Unlike the last two groups, the ferns have retained a grasp on the land surface which has enabled them to form a noticeable part of the present-day flora. This is partly due to the fact that they have exploited the possibilities of the perennial mode of life which has enabled them to spread invasively into many plant communities without depending on the need to fertilize ova. Thus, the spore-producing generation has long-lived underground parts that grow larger year by year and, intemperate regions, produce an annual crop of leaves.
Bracken presents the supreme example of success in this life-style. Its underground stems may grow long distances from the original parent plant. The difficulties experienced by agriculturists trying to eradicate bracken are a measure of the plant’s successful mode of life. However, it has to be pointed out that there is a price to be paid for the propagation of only one of the two generations. The species loses the opportunity of exploiting genetic variability which fertilization would make possible. In an area colonized by bracken all of the plants are genetically identical and may thus be subject to widespread outbreaks of disease. There will be also a deficiency of variants capable of exploiting subtle changes in environmental conditions. Nevertheless, these disadvantages seem not as yet to have restricted the widespread growth of ferns such as bracken.
Of the non-flowering seed-plants the most familiar at the present day are undoubtedly the conifers. Like the three preceding groups already described, the conifers have a long ancestry. They are descended from plant types discernible among the early land flora. They developed the highly successful mode of reproduction involving seeds and, gradually, through the hundreds of millions of years after the first colonization of the land they ousted the clubmosses, horsetails, and ferns from many plant communities. Eventually, of course, they themselves were forced to take second place to the flowering plants.
Again, as was the case with the development of large trees, the emergence of the ancestors of these major plant groups took place in about 40 million years after the first colonization of the land. To take a different analogy this time, if the 3500 million years of plant evolution on Earth were to be equated with a 24-hour day, the appearance of the major plant group ancestors would appear to occupy only 15 minutes after the first colonization of the land.
At this point it will be convenient to deal with two other major plant groups. One, the bryophytes, is subdivided into the mosses and the liverworts. The latter are less commonly seen by the general public although boat owners who travel frequently on canals and rivers may often see them shrouding lock gates and walls where they are kept constantly moist by spray from leaks between the timbers. They look like green seaweed spread over the wood and brick and the fact that they need a constant supply of moisture for successful growth gives a clue to their mode of life. In fact, the obvious generation in the lives of mosses and liverworts is the one that is not well adapted to land life. It is rather a harsh commentary on the life-style of mosses and liverworts to say that they appear to represent a sort of evolutionary experiment that simply did not lead to success. It is nevertheless true that they never appear to have made any great contribution to the land flora at any time either in the present or in geological times. Because of their delicate structure they do not make good fossils and they are consequently not well represented in rock formations but the fact that they have survived to the present day indicates that they were probably as common in the past as they are now.
Moss and liverwort plants have no protection against desiccation and must therefore live in two possible ways:
- The liverworts pursue a mode of life that requires them to be more or less constantly moist. Hence, they live usually in constant proximity to waterways of various kinds or very damp woodlands. In tropical rain forests, of course, the climate ensures a constant supply of moisture and they can live either on the ground or on the bark of trees and shrubs.
- In general, the mosses have adopted another approach to the problem by becoming adapted to survival under conditions where they do actually become desiccated, but their cells do not die under these conditions. When a supply of moisture is resumed, they once again take up life where they left off weeks or even months previously. Clearly this mode of life is not one which will allow them to compete successfully with any other of the major plant groups which are deeply rooted in the soil and can keep going in periods of drought. However, one has only to look at moss-covered roofs, brick walls and paving stones to realize that there is a living to be had in habitats that appear to be despised by the other plants.
The remaining plant group which, except for mushrooms, does not find itself attractive to humans, is the fungi. Most of these produce a feeling of abhorrence although they carry out a most useful function, that of garbage disposal for the animal and plant kingdoms. If it were not for the fungi, the dead remains of all of the past animal and plant bodies would remain where they fell. Admittedly there are many animal scavengers but, in general, the main bulk of dead organic matter is disposed of by fungi and bacteria.
There is no doubt that the fungi are a very ancient group. Clearly recognizable fungal remains are seen fossilized among other plant material in rocks four hundred million years old. Even older fossil material looks as if it could be attributed to the fungi, but it is not yet certainly identified as such. It seems very likely, therefore, that fungi have been in existence as long as there has been material for them to dispose of. Most importantly they re-cycle plant nutrients and put back the necessary minerals into the soil for new generations of plants to grow.