Plants and Environment part 2

Trees and shrubs within semi desert areas have their own defenses against drought. These usually take the form of a deciduous habit, the plants losing their leaves as the hottest season commences, together with the ability to store water within their roots and occasionally their trunks. The famous Baobab (Adansonia digitata) of Africa has an almost bottle-like stem. Other plants spend the difficult season completely dormant. The most obvious examples are the bulbous plants which remain underground during summer. Species of Gladiolus follow this pattern in the wild as does the Chincherinchee (Ornithogalum thyrsoides) and many tulips (Tulipa). Annuals sidestep the problem by germinating with the first rains, growing, flowering and seeding in as little as four or five weeks, ready to face the summer problems as seeds. In near total desert conditions where rainfall does not occur regularly, the seeds can remain dormant for many years, awaiting the right conditions. In total deserts, nothing can grow.

In cool, temperate regions where rainfall is more regular, it is still often excess water loss which presents most problems for the plants. In these areas the warmth of summer is never sufficient to cause more than short-term difficulties while droughts are rare. In winter, however, growth stops and the plant’s metabolism slows down very considerably. Water loss must therefore be reduced if there is to be no deficit. The deciduous habit is again one of the commonest and most obvious adaptations, the loss of leaves effectively stopping almost all transpiration. Herbaceous plants carry the loss of leaves one stage further by dying down to ground level or below in autumn. This is one of the commonest adaptations to be found in extreme continental conditions such as those found in the central parts of North America and Asia. Many of Europe’s garden herbaceous perennials, like michaelmas daisies, (for example Aster novibelgii), sneezeweeds (Helenium), lupins (Lupinus) and coneflowers (Rudbeckia) come from the North American prairies.

Where the growing season is short, many trees occur which are evergreen but have small, hard needle- or scale-like leaves. These are the conifers. Transpiration takes place through the stomata which have found success through a development which has enabled them to live without any need for light. Photosynthesis cannot take place without light, but the small group of plants known as saprophytes flourish without manufacturing their own food, living on the decaying vegetation around them. They are not parasites, as the leaves from which they draw their supplies are no longer attached to a living plant. Yellow Bird’s-nest or Pinesap (Monotropa hypopitys) and Ghost Orchid (Epipogon aphyllum) both grow in this way.

Even in moist places there are areas where plants are liable to suffer from drought. Sand dunes are a familiar example. Here the rain water drains rapidly away and the nearby sea water being inimical to the growth of most plants. Once again some plants remain active whilst others take avoiding action. Very deep roots are characteristic of sand dune plants, as are the thick layers of cuticle in the epidermis. The leaves of Sea Holly (Eryngium maritimum) are an excellent example of this. Marram Grass (Ammophila arenaria) shows a number of adaptations which have made it one of the most successful of dune plants. It has a deep and ramifying root system and if the plant is partly buried by blown sand, it can send out adventitious roots from its stems and then grow upwards from this point to free itself from the rising dune. This habit makes it an excellent sand binder and on an eroded sandy coast its roots can often be traced for as much as 10 metres down into the sand. The leaves of Marram, like those of other coastal grasses, have their stomata sunken in the epidermis on the underside of the leaves. In hot sun the whole leaf rolls in upon itself from the sides, thus creating a tube in which the humidity can be maintained at a higher level than outside. In cool, moist weather the cells expand and the leaf flattens out. As in deserts, the sand dune plants which avoid the drought season do so as either seeds or bulbs.

A few flowering plants have succeeded in colonizing those places in the shore which are periodically submerged in salt water and one ;genus of higher plants, the eel grasses (Zostera) actually lives in salt eater. These plants are known as halophytes and almost all are ucculent in character. Many are able to absorb water from rain and dew as well as from the soil without taking in too much salt. Examples are Saltwort (Salsola kali) and the glassworts (Salicornia).

Too little water can be fatal for many plants, but so also can too much. Few plants can withstand being submerged in water for long periods and many are not even able to grow in waterlogged soil. The problem in each case is the intake of oxygen and carbon dioxide. A small number of plants which live permanently in water are able to extract from it the necessary gases; milfoils (Myriophyllum) and hornworts (Ceratophyllum), which live totally under water, are both able to do this. Plants with floating leaves have stomata on the upper surface only so that there is no danger of waterlogging, and many have long, relatively flexible petioles (leaf stalks) so that the leaves can adjust to changing water levels. Another adaptation found in many plants with submerged roots is the presence of large intercellular air spaces in which air can be stored. This is typical of many pondweeds and water lilies.

As well as water, light and air, most plants require small amounts of certain minerals and these they absorb through their roots. There are, however, some habitats where such minerals are totally lacking and these have favoured the plants which have the most unusual adaptations of all — the parasites and the insectivorous plants. Only a small number of species are total parasites, but many rely on a host plant to obtain for them essential minerals even though they themselves draw water from the soil and make their own food by photosynthesis. Yellow rattles (Rhinanthus), Lousewort (Pedicularis sylvatica) and eyebrights (Euphrasia) are all semiparasites which have feeding roots or haustoria which fix themselves upon grasses. Mistletoe (Viscum album), though independent of the ground, has green leaves and is able to manufacture its own food. Such plants as broomrapes (Orobanche), dodders (Cuscuta) and Toothwort (Lathraea squamaria) are total parasites, having no chlorophyll and consequently relying on their hosts for al their food.

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