Plants and Environment part 1

The land surface of our planet provides a remarkably wide range of climatic conditions, and all but the most inhospitable habitats have been colonized by plants. Where plants are established, insects and animals can follow and the greatest diversity of life of all kinds is to be found in the most ideal climates.

What makes a climate ideal for plant life? Basically the plant needs water, air, light, warmth and mineral nutrients — the first being the most vital — and where these are all easily available then life is at its most abundant. On our planet the closest to a paradise for plants is the tropical rain forest for here, with a high and regular rainfall and a permanently moist atmosphere, water is plentiful. Temperatures are equable and, as there is no time when growth cannot take place, there are always some plants in flower and seed at any time of the year. Light from the sun is good, though clouds cut down the direct rays of the sun, and where the natural forest cover has not been disturbed, most necessary minerals are present. Here plants have evolved and developed in equable conditions for millions of years.

Not all parts of the world, however, are so happily placed, nor have climates always remained the same. Plant life has therefore had to change to survive the different conditions. Through geological times the land masses of the world have changed their positions and even in the shorter period during which there have been flowering plants on the earth, the present day continents have split apart and taken on their present shapes and positions. During the last million years the earth has been subjected to a number of cold periods when ice sheets have invaded temperate parts of the world. All through these changes plants have adapted to the new conditions, or become extinct.

All plants are in a process of continuous change. A study of any population of wild plants will show that there are small but marked differences in size and shape of flowers, fruit and leaves, and in flowering times and general growth habit. These differences are controlled by the plant’s genes and are recognized as a part of the natural variability of the species. Sometimes too, a plant may be found in which one character is markedly different from normal. It may have double flowers, different coloured petals, a larger size or have extra fleshy leaves. These changes are due to an unexpected alteration in the pattern of one of the genes; this is known as a mutation. There is nothing unusual about mutations, they happen freoquently but in most cases they are of no particular advantage to the plant. Sometimes they may be quite disadvantageous, as in the case of double flowers, for example, where the stamens may be replaced by petals, thus making the male part of the plant sterile. Occasionally, however, the mutation confers a distinct advantage to the plant making it more successful than its fellows. This is most likely to happen during a period of climatic change. The single plant with the strange fleshy leaves that can store water may be the only survivor if droughts become frequent, while one able to flower and fruit in a shorter time will have an advantage if the climate becomes colder and the growing season shorter. In this way plants have evolved, those able to survive in a changed environment through some chance mutation making up most of the present flora of marginal areas.

Once beyond the area of high rainfall around the equator, the chief limitation to plant growth is the seasonal lack of water. This semi desert area lies within the tropics and temperatures are still high, in fact because there is no ameliorating cloud cover, they are often higher than on the equator itself. For several months of the year there is no rainfall, the dry period lengthening, especially away from the oceans, until the almost totally dry desert belts are reached. Plants living in this environment find excellent conditions for growth in the wet season, but only those with adaptations which enable them to survive the following dry period can live there permanently.

There are two usual ways in which plants survive long periods of drought, one is by modifications to their stems and leaves which help them to store water and resist the high temperatures above ground, the other is by spending the unfavourable season below ground or as seeds.

Succulents are the best known of the plants which keep active during a drought. They are able to do this because of their very restricted transpiration and their methods of water storage. Cacti are a very good example of a highly modified group of succulents. To cut down water loss their leaves are reduced to spines and their swollen stems, the surface cells of which contain chloroplasts, have taken over the role of leaves. These stems are short and thick, often near spherical in shape, and therefore have the smallest surface area possible for their volume. The epidermis itself has a thick layer of cetin so that transpiration can take place only through the stomata. Even the stomata are placed away from direct sunlight, often lying between the ribs or with a covering of hairs or spines, and to make doubly sure they do not allow too much water to escape they open only at night. One of the most obvious features of many cacti and other succulents is the presence of spines. Why they should have developed is a question which still puzzles many scientists, but they certainly confer an advantage in regions where grazing animals always search for moisture. An unprotected succulent would soon be eaten and so it is those with spines and those with acrid or poisonous sap, such as the spurges (Euphorbia), which have proved most successful.

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