Physiological adaptations of plants

So far, only structural changes have been mentioned, and at the beginning of ecological studies this was all that was considered, early ecologists merely speculating on the significance of these morphological adaptations. More recently, however, more significance has been applied to physiological adaptations. For example, it was at first assumed that the anatomical features of desert plants would reduce transpiration (water loss), but it has since been proved that some desert plants have a very high transpiration rate. These recent experimental findings have more or less halted the study of relationships between form and environment, and have led to a great upsurge in research into ecological physiology. A morphological change cannot be considered an adaptation unless there is a corresponding physiological change which makes the plant more suited to its environment. Needless to say, there is a lot more to learn about the relationships existing between plant form, function and the environment. It is noticeable that the number of plant forms growing in any environment increases as the climate becomes warmer. This is thought to be because in a cold climate there is a very short growing season, and only species which can mature and reproduce in just a few weeks can survive. In a hot climate such as a desert, this selection for fast growth does not occur, and consequently there is a greater variety of morphological types to be found. These include succulents, mesophytic drought-evaders and highly drought-tolerant evergreens.

Quite different species in different environments will show the same adaptations. This is an example of convergent evolution, where two species have evolved the same adaptation independently. Evolution, after all, is merely plants becoming more highly adapted to their environment. It is believed that in many cases genetically fixed adaptations may outlive their usefulness, and this would explain such anomalies as succulent water plants. Once an adaptation has become a genetic character of a species, it cannot be discarded, although it may eventually disappear through mutation or natural selection if the plant reproduces sexually.

Tree euphorbias on the volcanic island of Tenerife display convergent evolution with cacti: both groups have evolved leafless water-holding stems to withstand dryness.

All plants are adapted to their natural environment, and man has made great use of this fact, although often unknowingly. The life cycles of crop plants are carefully geared to fit the prevailing climate by planting at the right times, irrigating and protecting from frost. As modern technology has improved, even greater controls have become possible. Plant flowering is determined by the number of hours of daylight, and by artificial lighting it is possible to get flowers out of season. This ability to use plant adaptations allows the great variety of foods which we now enjoy all year round.

Forest communities

By examining certain communities in more detail, one may develop a better understanding of vegetation and how it adapts to its environment. A typical community which shows many forms is forest. Forests are made up of at least three strata of vegetation. These are basically the tree layer, below this the shrub layer and at ground level the herb layer. Although in most cases there are more nonarboreal plants than there are trees, it is the trees which determine the environment beneath them. On a world scale, the increasing severity of the environment is accompanied by a decreasing diversity in forest flora. In a tropical rainforest there are over 40 tree species per hectare (100 per acre), and there may be as many as one hundred per hectare (250 per acre). Dominance by one or two species is rare unless man has disturbed the habitat. In direct comparison, within boreal forests, vast areas are found being completely dominated, often by just one species. These two types of forest are at the extremes of this community type, and are therefore probably the best examples to take to describe the diversity which may exist.

In equatorial regions the climate is continually hot and humid, and therefore provides the optimal conditions for plant growth. As a result not only are more species found, but also they tend to be larger. The tree canopy is usually composed of several substrata, each made up of trees of varying light requirements and height potential. Obviously those needing most light are the tallest and fastest growing. The shorter and slower growing species are positioned between their taller neighbours so as to obtain as much sunlight as possible. As growth conditions are so favourable, most tropical tree species are evergreen so that they can take advantage of the all-year growing season and not have a period of slow growth, as do the deciduous trees of more temperate regions. As the tree canopy is so dense (it can be up to 30m (98ft) deep), there is little light left to penetrate to the shrub or herb levels. This consequently limits the development of these layers to species which have become adapted to survive such poor conditions. It contains a large number of saprophytic and parasitic plants—these, after all, are the ideal adaptations to poor growing conditions. There are also large numbers of lianas and other epiphytic climbers which use the tall trees as a means of reaching the sunlight.

The temperate pine forests also have a very dense canopy, although here the lack of light penetration is also aided by the low angle of the sun in the sky. In natural pine woodland the trees are often found growing with birch, which has similar ecological requirements. In the single-species pine plantations there will be no shrub layer, and very few plants at ground level. Not only is there very little light here, but there is also a very poor soil as the pine needles have very thick cuticles, and these are slow to break down and add nutrients to the soil. The plants found here, again, will be saprophytes, parasites and epiphytes. Fungi are often the main species found, these do not need light as they are non-photosynthetic, and in any case they frequently have mycorrhizal relationships with coniferous trees in particular.

One of the best communities to show the relationships existing between the layers of a forest is the European temperate oak forest. Oak wood can exist under a variety of conditions, but the accompanying species will vary with soil depth and available nutrients, etc. In an area of high rainfall and with an acid soil, it would be common to find the ground beneath the trees completely covered by a dense growth of bilberry bushes (Vaccinium) accompanied by a thick growth of several moss species. Growing to a greater height, but shorter than the trees will be such species as mountain ash (Sorbus). Epiphytic lichens and ferns would grow on the oak trees, so that all available habitats would be exploited by suitably adapted plants.

Grassland communities

There are few types of vegetation in which grasses are not represented, and they have achieved an ecological dominance unrivalled by any other type of herbaceous plant. Grasses have an almost unique leaf shape, which coupled with their growth habit enables them to achieve the optimum photosynthesis for the light available. Their leaves are different from those of most other plants in that their growing point is at the base and not at the tip. This means that they can survive burning and still continue to grow. All grasses also have a large root system, far out of proportion to their shoot growth, which enables them to utilize to the fullest extent the available water and nutrients. They also produce large numbers of seeds on inflorescences which allow easy dispersal by the wind, and many also have forms of vegetative reproduction. All of these characters combined give the grasses a competitive advantage over most other herbaceous species with which they come into contact.

The grassland community structure is much simpler than that of the forest. It is dominated by the field or herbaceous layer. There are two main naturally occurring grassland communities, these are savanna and prairie. The tropical savanna consists of often quite tall grasses, although they seldom exceed 5m (16¦ 5ft), growing with the occasional shrub. These communities are subject to heavy grazing and fires, which give the grasses the upper hand in the environment. The temperate prairie grasslands are controlled by the same factors, but the grasses found growing here are usually smaller. In both the prairies of America and the steppes of Russia, man has taken advantage of the grasses’ natural dominance of the area, and has turned much of the land over to cereal production.

Adaptations to extreme conditions

Some vege’tation has become adapted to survive the more severe conditions existing on certain parts of the earth’s surface. Plants growing on the coast, for example, must survive high salt concentrations in their substratum, and often periodic battering by the waves. Salt-tolerant plants (halophytcs) are also in the unique position of being surrounded by water which they are unable to use, because salt-water is of no use to plants. They therefore have to be adapted to conserve freshwater and to survive in waterlogged soil which reduces the amount of oxygen available to their roots. Halophytes of the temperate regions are usually small herbaceous plants. By being small they offer little resistance to the wind and waves which batter their exposed habitat, and therefore are less likely to be damaged. They often have fleshy leaves and stems made up of large cells for storing water. It is also common that they have a reduced rate of transpiration as a further aid to freshwater conservation. In the tropics, coastal vegetation is quite different; mangrove swamps are found here. The plants are large and woody with long aerial roots to support the bulk of the plant above the surface of the water. Having aerial roots also solves the problem of obtaining oxygen when growing in an oxygen-deficient waterlogged soil, as these plants can just as easily absorb this vital element from the air as from the soil.

Plants growing in an alpine environment have to survive some of the most severe conditions. They are often on exposed cliffs, can be subjected to extremes of temperature and usually have to grow on a poor shallow soil. They have overcome most of these problems by adopting the rosette or cushion growth habit and as a result they are little affected by high winds. They may often have shallow rooting systems which spread widely under the surface so as to obtain as much nutrient as possible from what soil there is. Most alpines have a short flowering period so that they may produce flowers even during the very short ‘summers’ in these areas. They are also perennials, so the species may survive when con- ditions do not allow annual reproduction.

Desert plants also tend to be perennials, as annual reproduction is not always possible, because of insufficient water. This same factor has led to the evolution of several forms of xerophytic plants. To conserve water, plants have developed numerous adaptations. Many have become succulents, composed of large cells with the ability to store water, for example, the cacti. Some have stems which curve inwards to trap water, and others have hairy surfaces for the same purpose. Other adaptations serve to reduce the amount of water lost from the plants, including the development of smaller and fewer stomata (the microscopic pores on the leaf through which water is lost and gases are exchanged), and slower rates of transpiration. As water loss occurs through the leaves it is usual for these to be small and in many cases they are reduced to mere spines, as in cacti.

Hydrophytes are adapted to a completely different environment, as these are plants which live in water. They must in some cases even be capable of obtaining their oxygen and carbon dioxide from the water in which they grow. In still or slow-flowing water, the problems are slightly less as here there will be some build-up of silt in which the plants can root and from which they can obtain nutrients. For example, the water lily (Nymphaea) is rooted in the silt and has its leaves floating on the surface of the water. In a fast-flowing stream there will be no soil, only rocks, and any plants found growing here are often algae which attach to these rocks and are filamentous so as to move with the water and not become damaged or dislodged. Other plants such as Lemna, the duckweeds, merely float on the surface with small roots hanging down into the water.

Man’s influence on plant communities

Few communities seen today are completely unaffected by man, who has both influenced established communities and created new ones. The most interesting for the ecologist are those which he has created, as here they have a chance to see the primary colonization of land and the subsequent development of vegetation. Man has created large barren afeas with his slag heaps, and only by applying his knowledge of natural communities has he been able to vegetate them. The railways have added much to the richness of plant communities. Trains are the perfect agents of seed dispersal, and when the line is disused the sheltered areas remaining have a very rich flora. Where the face of the earth has been scarred by gravel workings, the usual result is areas of barren lakes. These can eventually be colonized, supporting a very varied plant and animal population. Unfortunately, there are also many results of industry which will remain as scars on the landscape for some time. They include the waste from chemical industries and slag heaps containing heavy metals. Here the adaptations to enable plant growth are so great that it will take many years for even a few plants to become established.

Plant communities exhibit a surprising complexity. Trees do not just grow on a certain hillside because there have always been trees there, but because’ at that time and with the prevailing conditions, trees are the best-suited vegetation. This may not always be the case as the environment is constantly changing. The study of plant communities has changed dramatically in recent years. Originally plant ecologists merely observed and described what they saw. Now it is realized that the dynamics of the community are more important. Before a description can be drawn up, one must ask how the plants involved co-exist. The modern trends in this field now have a strong mathematical bias, as by using figures one can standardize the method of description more easily.

Plants are the basis of each food chain, thus plant life is important to us all. Perhaps if more were known about how plant communities work we may be able to solve some of our food shortage problems. The plant community is the basis of all plant life, its understanding is the culmination of all botanical studies.

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