Plants and Environment part 3

Dodder is perhaps one of the most interesting of the total parasites. It starts its existence normally, the seed germinating and producing a club-shaped taproot which fixes the plant in the soil. The shoot then begins growing, not straight up as is usual in seedlings, but with a circular movement. As soon as it touches a support it encircles it. If, however, the stem fails to find a suitable plant nearby, it is not rigid enough to grow upright and falls back to the ground. At the same time, the roots and the lower part of the stem die off, the shoot re-roots and begins to move upwards again; this pattern can be repeated until the shoot has moved up to 20 cm from its original position. Once the leading shoot is curled around its host it grows small, sucker-like so-called feeder roots called haustoria. These push against and penetrate the tissues of the supporting plant and the dodder then draws all its nourishment from it. If the shoot fails to find a host plant, it dies.

Of all the plants which are unable to manufacture sufficient of their own food the most unusual are the insectivorous or carnivorous plants. These replace unavailable nutrients by trapping small insects, dissolving their body juices and extracting the necessary minerals from them. Sundews (Drosera) have hair-like tentacles on the edges and upper surfaces of their leaves. These hairs end in sticky glands and when an insect lands on the leaf it is caught in the stickiness and held firmly by the slowly closing tentacles. Butter- worts (Pinguicula) have sticky glandular leaves which roll slowly inwards from the margins when insects are trapped on them, and the even more dramatic Venus’s Fly Trap (Dionaea muscipula) from North America has leaves which seem to be hinged along the mid-rib and can snap shut rapidly over their prey.

Wherever plants are living they are rarely alone, but have to compete vigorously with others. Most plants produce great numbers of seeds every year and of those which germinate, only a tiny percentage can ever reach maturity. Whenever a bare patch of ground is created, plants compete with each other to cover it. This is only too familiar to the farmer and gardener who has prepared ground for a new crop. At first, crowding can be beneficial for the young plants, providing a protected environment for those at the centre, but soon competition will begin to affect their development and those unable to maintain sufficient root growth will weaken. Finally one or two plants will outgrow the others, eventually shading and crowding out their rivals. In the wild, plants live in mixed communities, many different species making use of the various niches which may exist in one habitat. In forests, tall trees make up the top storey while beneath them grow smaller trees, shrubs, herbs and mosses, all with differing demands upon available light and food. Even natural grass- and has tall and short grasses as well as a soil level community of mosses and liverworts. There are, however, some plants which are lot closely surrounded by others. Pliny the Elder noted that no plants really thrive under a walnut (Juglans) tree. We know now hat its leaves contain a substance known as juglon; this is transferred to the ground when the leaves fall and effectively inhibits the growth of other plants. Plants secrete a wide variety of compounds, some poisonous, others fragrant, while some have medicinal properoties. What, if any, value these substances have for the plants which produce them is often not at all obvious.

After several million years of development, most plant communioties live in a delicately balanced state, always in a process of change — a change that takes place so slowly that it is barely perceptible. Animals too have their place in this pattern and it is only within the last 10,000 years, a brief moment compared to the existence of plant life on this planet, that Man has begun to dominate all other forms of life. The very word civilization means the culture of towns, and it is town-dwelling Man who is responsible for the great destruction of the natural world which goes on today. Rural Man lived in harmony with his surroundings for many thousands of years, conscious of his dependence upon plants for food and shelter. In a few parts of the world this is still true today, but urban civilized Man now dominates and his power is immense.

To feed his ever increasing numbers, Man must grow more and more food. In doing this, natural plant communities are destroyed and replaced by single-species crops (monocultures). Monocultures have a number of dangers: first, their cultivation means that at some time during their life cycle the soil must lie bare; this brings a great danger of erosion, especially in tropical regions where soils are thin and rainfall often torrential. The second danger is that of disease. A pest which hitherto may have been barely noticeable in its effect upon one among hundreds of species of plants, suddenly finds itself surrounded by hectares of its food pdant. To combat this, Man uses chemicals which kill many other creatures as well as the intended pest. In the same way weeds are destroyed and along with them plants which may be of great value. Here lies a great danger. Insects and plants which play a vital part in the maintenance of life on this planet are destroyed because Man is not aware of their importance, nor of the long term effect of the chemicals he is using. Who, 40 years ago could foresee the dangers of DDT or of the other hydroocarbon poisons? The irony is that these are dangers, not just to other forms of wild life but in the long term, through the gradual build-up of residual traces, to Man himself.

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