As with all life it seems likely that plants began as waterborne organisms. Changes in environ- ment caused the forms to spread to dry land and to colonize all but the most inhospitable terrain. And from land further forms have moved back into the water. The processes of adaptation, modification and opportunism have been and are continuous. Whenever man, since classical times, has started to study the world of nature he has had to try to classify, to list, to order, as much for his peace of mind as for the facilitation of the study. Any discussion of this truly daunting diversity is bound to do the same— although, of course, as time goes on and more work is done—the classification used becomes more sophisticated. Consensus is still not reached.
An accepted scheme is to divide the plant kingdom up into seventeen divisions, each subdivided into classes, subclasses, orders, tribes and families before it is possible to get down to actually naming a plant. Such a classification aims to do two main jobs in addition to making conversation possible about them. These are to suggest relationships and to indicate an evolutionary sequence: these roles, of course, are inter-related. Only the final two divisions consist of—the flowering process being the height of plant development.
In looking, then, at the world of plants and trying to understand their ordering, it is reasonable to start at the simplest and hence, in all probability, the most primitive in evolutionary terms. At the beginning come the bacteria, the first of the seventeen divisions. Bacteria, which are probably on the borderline between plants and animals, have no chloro- phyll, no clear nucleus in their unicellular bodies and reproduction is by simple division.
The succeeding seven divisions comprise a world in themselves, generally unknown and unregarded but fulfilling their own diverse roles species by species and playing their part in the great multidimensional web of organic life. These are the algae. They vary in size from the microscopic to the massive. They include the extraordinary diversity of the seaweeds, the biggest of which can grow up to 90m (300ft) in length in tropical seas. As non-, algae frequently reproduce by simple fragmentation of their vegetative bodies but a sexual stage is also usual (as, for example, the ‘conjugation’ of spirogyra).
Their success in adapting to their environment should need no emphasis though the potential difficulties might be considered. There is no necessity for aquatic species to be able to abstract carbon dioxide and oxygen from the water; there may be greatly reduced light and in addition marine plants have to deal with the problem of living in a highly concentrated solution of salts where ‘normal’ plants could never survive.
The division called the Mycophyta is a difficult one to place. These are the fungi which do not photosynthesize and, possessing no chloroplasts, are not green (the brown and red seaweeds have pigments which mask their chlorophyll). Fungi have taken a cuckoolike path to success and exist as two types: as saprophytes which live on the dead remains of organic material as do mushrooms and moulds on old bread. Other moulds and mildews are crippling diseases of higher plants and hence are parasites. Some parasites are said to be facultative: having attacked and killed their host, they then continue to live on its decaying body—a gory sequence in anthropomorphic terms. Not needing to photosynthesizc, fungi do not need light for growth and hence mushrooms can be cultivated in cellars.
Lichens, whose growth on walls and roofs helps man’s artifacts to blend in with their surroundings, are odd organisms in that each species is a combination of an alga and a fungus which pair symbiotically to exist in an utterly unlikely marriage. In areas of highlarge lichens of bizarre form festoon trees and cover rocks. (Habitat and environment are apt to encourage similarity of form in widely differing plants. The famous Spanish moss of the Florida Everglades which so resembles a big grey lichen, is neither this, nor a moss, but a flowering plant of the pineapple (Bromeliaceae) family. To assume relationships from general appearances is hazardous.)
The next division in the plant kingdom is the Bryophyta, which are separated into the mosses and liverworts. The use of the plural should be noted. So often we talk of having ‘moss in the lawn’ or seeing liverwort by the riverbank without appreciating that these are just two of a huge number of species in a diversity of forms. Most are lowly and flourish in damp spots, often in positions of poor light intensity. They are thus adapted to succeed as the lowest layer in, for example, a woodland stratification.
Mosses are usually leafy in form (especially when examined under a lens) whilst most liverworts consist of a flat plate of tissue, only a few cells thick, which bears the separate male and female reproductive organs. Much reproduction, however, is by simple fragmentation of the plant itself. Almost any piece broken off will continue to put out rhizoids (primitive) if in a suitable spot.
It is not until the Ptcridophyta are reached (cvolutionarily speaking) that we generally start to notice, and hence accept, clear differences between one plant and another. True, we are wise to distinguish, in the fungi, between mushroom and deathcap; but mostly we lump everything fungal together as ‘toadstools’, just as all marine algae become ‘seaweed’.
The Pteridophyta includes the ferns whose range of morphological form is not only great but of such beauty to the human eye as to be deemed worthy of cultivation. At this point the gardener enters and we really start to take notice of what the plant kingdom offers.
Ferns, of Course, vary in size from tiny plants of rock crevices to ‘trees’ in tropical jungles. Tree-ferns, although having apparent trunks up to 10m (33ft) high, have not really grown wood as we know it; this is the prerogative of higher. The pteridophytes also include various fernallies of which the club-mosses, the selagincllas and the horsetails (Equisetum species) are most distinctive.
The true ferns have a peculiar lifecycle, or, one might say, life-tandem which clearly indicates their early origins. Everyone will have noticed the dark dots or bars on the underside of fern fronds producing dustlike spores. These spores will germinate, not into a parent-resembling fern plant, but into a small green thallus like a liverwort. And like a liverwort this will bear sexual organs, the fusion of whose gametes (or sexual cells) will cause the development of a new recognizable fern plant. This unique sequence is known as the alternation of generations—sporophyte to gametophyte back to sporophyte.
All the great divisions so far mentioned depend very greatly upon water; not just as an ingredient of the photosynthetic process or as that which the ‘roots’ take up from the soil. High humidity, if not a full watery medium, is essential to life. Otherwise dessication and death quickly sets in. This reverts again, therefore, to the habitat of most of these early groups, the non-flowering plants and to their, sequentially, primitive status.
Any informed knowledge of evolution (the process by which it is maintained that the myriads of plant and animal forms were not individual creations but a continuum of development in response to habitat and environmental change) is likely to be based, or at least helped, by fossil records. Unfortunately, yet inevitably, the most primitive organisms, because of their very nature, do not fossilize. Yet it is well established that the flowering and seeding process which we consider to be so typical of plant life is a late, and, it would seem, cumulative development. Of the number of species which must have existed and are now extinct and even of the current sum of the world’s flora the-bearing plants must be seen numerically, to have a small place.
However, their success in variation in form, in size and in adaptation makes them paramount. These seed-bearing plants comprise just two divisions in the seventeen of the plant kingdom. The first, and more primitive, are the Conifcrophyta (often called Gymnosperms: literally ‘naked’). These include a few small palmlike tropical trees, the cycads, but mainly the conifers, which, as their name suggests, carry their seeds on woody cones familiar on pine, cedar, spruce, hemlock, larch and so on. Conifers are all trees, or at least dense woody shrubs, with a worldwide distribution.
Their evolutionary age has been conveniently shown by fossil records of species still existing. Scientific excitement, comparable with that which zoologists enjoyed when a coelacanth was fished up alive off Madagascar, was intense when seeds of Mctasequoia glypto-stroboides reached Europe from Hupeh in 1948. The tree had hitherto only been known from Carboniferous fossils over 200 000 000 years old.
The higher flowering plants comprise the last division, the Magnoliophyta (or Angio-sperms). They begin, evolutionarily speaking, with the beautiful magnolias (fossil records of magnolia relatives are frequent) and progress through all forms, in all habitats to virtually cover the earth. The life-style of each reflects its own particular success-story (recall Darwin’s maxim of the survival of the fittest) and hence they may be woody or herbaceous, aquatics or desert plants,or living for millenia and every conceivable intermediate stage.