Basic structure and life cycles of plants

However apparently different in form or feature representatives of the higher plant divisions may be, in function they are remarkably similar. The universality of photosynthesis has already been emphasized; equally important are the processes of respiration and transpiration. These three basic areas are concerned with the exchange of gases from within the plant to the atmosphere (or vice versa) and, combined with other needs, determine plant form.

Basic necessities for plant growth are light, water, air, warmth and nutrients. While all are essential the differences in the availability of each combine to vary environment and, in turn, the plants that grow in any one place. Certain species therefore are more likely than others to inhabit a particular habitat but, in addition, unrelated plants of a distinctive (or restrictive) habitat are apt to exhibit similarities of form. This phenomenon of ‘parallel evolution’ helps to indicate how plants develop in response to the urge to be among ‘the fittest’. A remarkable example is the virtual identical appearance of certain New World cacti to species of African spurges which live in similar habitats. Only their flowers, which utterly betray their true relationships, can be seen to differ completely.

Whether adapted to live in a pond, 5,000m (16,400ft) up on Mount Kilimanjaro, in a desert or a tropical jungle, the basic parts of plants—their essential body —remains the same. The four vital components are: the roots, the stem, its leaves and the flowers. From the latter are the seeds developed and their successful production to ensure the continuation of the species is the entire raison d’etre of each and every plant. Plants do not grow to provide food for browsing animals and man, nor come out in glorious flowers for the benefit of bees or to satisfy man’s aesthetic sense. They do it for themselves utilizing anything or anybody to further those ends.

The basic parts and their myriad minor and major adaptions are miracles of effectiveness. They have to be: there is always another plant only too willing, and usually able, to take up an unused ecological niche.

The first thing to emerge from almost any germinating seed is a root. It has two jobs. First it anchors the growing plant in its place. (Seeds of plants such as mangroves develop their first root like a torpedo whilst still on the parent plant: a small round seed would splash into the water and drift away but a heavy hydrodynamic shape spears through the water and impales itself in the mud beneath ready to get on with the job of living successfully.) Second it is the search for and transport of water and the broad spectrum of nutrients obtained from the soil which are needed to supplement the carbohydrates that the green aerial parts manufacture. All go to building up the plant body, to maintaining it as a healthy organism, to facilitate its reproductive capacities.

The full extent of plants’ roots, as they divide, spread and divide again until the unicellular root hairs are reached, is seldom appreciated but it must be expected that the plant will possess as much subsoil growth as that seen in the air. Viewing a tree reflected in water is a good analogy: of course the exact shape will not be repeated—soil texture, rocky-substrate or a high watertable may prevent it but the top will only be as great as the root can support both structurally and physiologically.

After the development of the first root the germinating seed pushes up its first shoot. There may be a single leaf (this monocotyledonous beginning is a valuable diagnostic character of grasses, lilies, palms and a few other plant families) or a pair of seed-leaves (dicotyledonous plants) like two tiny ping-pong bats held on a thin stalk just above ground. At once, being green, they start to take in carbon dioxide, combine it with water that the young root supplies and begin the process of photosynthesis. Like antelopes that must run from the moment of birth, no time must be lost: the struggle, not perhaps to keep head above water, but certainly in the light, has begun.

The stem, which rapidly begins to thicken and grow (and all growth in girth or height is by division and multiplication of the cells, not by any elasticlike stretching) from between the cotyledons (in some plants the cotyledons remain in the seed below ground as inert food stores and the first photosynthetic leaf is a true one) carries unfolding leaves. In the angle of stem and leaf are buds which can grow into side branches to repeat the pattern. Gradually the typical shape of the plant is built up: the speed of growth varies from several metres in one season to the barely measurable and it will, as in all things, be related to habitat.

The stem spreads out its leaves so that they get as much light as necessary, to support structurally a considerable weight and to act as a complicated ‘plumbing’ system. Nutrients in solution have to be brought up from the roots and carbohydrates, produced in the green parts, have to be moved to where an energy source is required or stored until needed.

In herbaceous plants, whether they be annual or perennial the leafy parts are renewed either entirely from seed or from overwintering groundlevel buds each year. The stems of woody plants which persist, sometimes as vast trunks many hundreds of years old develop specialized methods of annual thickening to ensure preservation of their wood. But the extension growth of tiny twigs at the top of any such giant will be just the same as the first shoot it possessed as it left its seed.

The leaves, whether they belong to chick-weed at ground level or to a great forest tree perform the same functions. They are the power-horses, where the unique photosynthctic ability of plants primarily takes place. It is based upon the exchange of gases which pass in and out of the plant through the stomata— pores usually situated on the underside of plants’ leaves.

Monocotyledonous plants, typically, have vertical leaves with stomata on both sides; water-lilies have perforce to site them on the upper surface. Stomata also permit the escape of excess water as vapour, of oxygen as a waste product of photosynthesis in daytime and its ingress at night as a necessity of respiration. Stomata, too, are capable of movement, opening and closing in response to need and climatic conditions.

Yet this is not enough. The need to have some control of water loss, which in severe drought may lead to wilting, dessication or even death, gives the reason for much of the incredible variation in leaf form. Xerophytes (plants naturally adapted to dry habitats) often have silky or white woolliness over their leaves. The pores are thus protected and the pale colour helps to reflect heat. Sometimes stomata are situated in deep depressions. Sometimes, for example in marram grass (the classic sand-dune binder), leaves are rolled into a spill with the pores only on the inside. The variations seem almost infinite, the ultimate being that leaves are dispensed with altogether, as in brooms and gorse in temperate climates and the majority of cacti. Gaseous exchange and photosynthesis therefore has to be possible through stomata on the stems.

Much of the work that plants do. especially those that are likely to live for some years, is concerned with food storage and a whole range of modifications of root, stem or leaf have evolved. Roots may develop swollen tubers as in dahlia, shoots may dive back into the ground and swell into stem-tubers. Potatoes are these (the fact that they have ‘eyes’, or buds, which grow into further shoots prove that these are in fact stems). Corms of crocus or gladiolus are compressed stems which husband their food-store below ground for a resting season. Bulbs of daffodil and many other plants act in the same way, but here the resting food-store comprises layers of swollen leaf-bases. These storage organs double as methods of vegetative reproduction. The disadvantage to the plants themselves of their leafy tops or underground food stores is their palatability to herbivorous animals. Man has eaten potatoes (tubers), onions (bulbs), carrots (the overwintering swollen tap-root of a typical biennial plant), taro yams (a horizontal swollen stem or rhizome) since the earliest times. So it behoves plants to defend themselves if they can.

This they do physically with a vast armoury of spines developed from shoots, prickly leaves (if anyone doubts the reason for the barbarity of holly leaves let him note how they become less and less frequent up the tree as danger from browsing animals decreases). Stinging hairs of nettles and other members of the Urticaceae and Loasaceae families are highly sophisticated as are the acrid juices of spurges as repellants to prospective hungry animals. More economical of effort but no less sophisticated are forms which sufficiently resemble the truly unpalatable, such as deadnettles, which are utterly unrelated to true nettles, as to be left alone. All this is necessary if the culmination of the plants’ growth pattern is to be reached.

That culmination, of course, is successful sexual reproduction through the flowering and fruiting process. Such success, so vital to the continuation of the species is attended by so many difficulties and dangers that, just as it is the culmination of the plants’ efforts so is it the apex of their evolutionary ingenuity. The basic needs are apparently simple; to produce male and female gametes (sexual cells) and to ensure their union (fertilization) so that a developing embryo may grow in a seed. The seed has then to be satisfactorily dispersed so that it has a good chance of growing into a new plant. A less immediately noticed need, but no less vital for the ultimate success of the species, is that gametes should come from different plants of the same species to offer the chance of genetic development. A fear of in-breeding has been the cause of a number of clever mechanisms in plants and behavioural patterns or social taboos in higher animals.

The flower is the apex of plants’ aspiration. Its separate parts can be shown to have evolved from leaves and, although not all flowers possess all parts, they can be seen to have specialized jobs to perform. The outermost ‘leaves’ are sepals which protect the rest of the flower while in developing bud. Sometimes they support the comparatively big open petals whose role it is to make the whole organ conspicuous. This is often helped by the presence of scent and nectar. The scent may or may not be attractive to man — for man is not the object of the exercise but small creatures which will transfer male cells (pollen) to female receptive areas (stigma).

Within the petals are the pollen-producing stamens and finally, like a queen in the centre of her court, the stigma surmounting the ovary. Size, numbers, shape, arrangement, colour and scent of all these parts vary almost with the number of flowering species that exist. Floral parts, indeed, are one of the main criteria of plant classification. The variations, however, are not vagaries of extravagant evolution but essays in complcmentariness and even symbiosis. Each minor alteration is more likely to encourage a suitable pollinating creature (insect, bird or mollusc) to programme its way of life in an associated manner. When Angraecum sesquipedale was discovered in 1862, Darwin predicted that a moth would have to exist with a proboscis long enough to reach its nectar (at least 25cm). The moth was indeed discovered — forty years later!

Some plants depend on the less predictable pollinators of wind or water and while no effort has to be expended upon petals or nectar, vastly greater quantities of pollen must be produced to offset the unpredictability of the medium.

Eventually, then, pollination is effected and fertilization takes place. The seeds have merely to develop and ripen, bearing within them the genetic complement of their species. The parent has yet one more role (in its whole life if an annual; for that season if perennial) to fulfil — the dispersal of the seeds. And here the final and superb scries of modifications and adaptations appear. Ovaries develop explosive powers and seeds are shot out with considerable force, seeds grow parasols and are carried off by the wind, others float long distances in water. Those bearing hooks are caught in the coats of passing animals. Palatability of fruits is developed into a virtue coupled with seedcoats hard enough to survive digestive tracts of birds; a hedge soon develops on the line of a wire fence.

Although it might seem that each species is an individual intent upon nothing but its own success, it must be seen that in the continuously competing world of plants there is also a delicate balance in which each species, uniquely occupying its niche, is yet making this possible for others. Few plants live in isolation and together they build for themselves and, in passing, for the world of animals and man, the environment in which all organisms can live.

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