Like all living organisms, plants must in order to survive be able to adapt to the environmental conditions under which they live. These conditions include extremes of temperature, availability of water and nutrients, and the supply of light. Not only must plants ensure their individual survival but also that of their seeds and spores if the species is to be perpetuated. Successful adaptation is essential for the evolution of new species, because it is the “fittest” or best-adapted species that survive to pass on their genes to subsequent generations. Similarly, those plants that are best adapted to their environments become dominant.
Not all plants, however, are equally well adapted to their environments. There is a great difference in efficiency and effectiveness of adaptation among species. The grasses of the prairies and steppes, for example, have adapted to continuous attacks by grazing animals for whom they provide a staple diet. The growing point of each grass is close to the ground, with the result that the plant survives even when the foliage higher up is cropped. In contrast, the Calabrian primrose (Dendrobium biggibum) is an unsuccessful species on the verge of extinction. It is unable to reproduce fast enough to survive being eaten by animals and being picked by people.
Types of adaptation
Plants adapt to their environment in many different ways. In deserts, where high temperatures and lack of water are problems, cacti and other succulents survive by storing fluids in their swollen stems. A waxy cuticle protects the plant against high rates of transpiration. Plants that resist such dry conditions are called xerophytes. Some annual herbs in arid areas limit their entire life cycle from seed germination to seed production to the few weeks of the rainy season. These annuals include many members of the daisy family (Compositae).
Life in saline environments also requires special adaptations. Plants that live in these conditions are called halophytes. Salt-marsh plants, such as the glasswort (Salicornia fruti-cosal are regularly covered by seawater; they have adapted in various ways to withstand the build-up of salts within their cells that results from the influx of water caused by osmotic pressure. In certain freshwater plants, survival is possible only if the leaves are not completely submerged. The giant Amazon water-lily (Victoria amazonica) has air-filled honeycomb structures on the underside of its leaves that cause them to float.
Other plant adaptations are specific to food and light requirements. Epiphytes are plants that use other plants as a means of support. In forests they grow on trees in order to be nearer the light, drawing nutrients in solution through their roots from sites such as hollows in tree bark. The bird’s nest fern (Asplenium nidus) is an example of an epiphytic plant that relies on its host rather than the soil for its nutritional requirements.
Carnivorous plants, such as sundews (Dros-era sp.), have adapted to environments with poor nutrient status by catching and “digesting” insects rather than relying on the soil to provide nitrogen and other minerals. Parasitic plants have evolved in such a way that they take what they need directly from plants that can provide it. In doing so, however, they often sacrifice their independence. Most have no chlorophyll and, thus, cannot survive on their own by means of ordinary photosynthesis. The pink-stemmed common dodder (Cuscuta epithymum), for example, lives as a parasite on hops and nettles, twining itself around its host’s stems and penetrating them with small projections that extract the host’s nutrients.
Evolution and adaptation
The Darwinian notion of “the survival of the fittest” operates at different levels in a group of plants. Within a plant community, for example, different species and individuals of the same species have to compete for the available resources. All may succeed by means of different adaptations, or some succeed and others fail. The more severe the “selection” pressure on individuals, the more restricted is the range of possible survivors. Thus, when the selection pressure is reduced, as in a garden, many variations occur.
Selection, and therefore survival, takes three main forms. Directional selection is the process by which plants are directed or “pushed” toward an optimum set of characteristics. Once these have been achieved, by natural selection, that situation as it currently exists is maintained and is known as stabilizing selection—the second form. Any plant not reaching the optimum has a decreased chance of survival unless environmental conditions change, in which case selection again becomes directional.
The third type is known as disruptive selection in which there are two or more sets of optimal characteristics for a particular environment. This occurs where temperature or moisture levels, for example, vary in the environment. Each optimal set stabilizes as before, but the divergence in characteristics may become so great that new subspecies or even new species evolve.