These comprise the effects of plant on plant and the relation between animals and plants. Of these bio logical factors, however, the most important is the influence of man. It is largely due to man that the present tree limit in the alps is considerably below its former altitude, whilst even in com paratively unsophisticated regions of the tropics considerable changes have resulted from the practice of migratory cultivation and the artificial extension of grassland at the expense of forest, both for domestic animals and the encouragement of wild game. The profound effect of rabbits on natural vegetation has been well exemplified in New Zealand and Australia, whilst the attacks of field mice, mollusca and various parasitic fungi, inter alia, con siderably affect the efficiency of reproduction in many wild species. On the other hand, the beneficial effects of earthworms, moles, ants, etc., is analogous in nature to the action of the plough in maintaining adequate aeration of the soil. Many flowers depend upon insects for their pollination, and hence the area of a species may be restricted by that of its pollinating agent.
In addition to the effects of climatic, edaphic (soil), and biotic factors, there are the historical factors to which allusion has al ready been made, and in this connection it must be recognized that the vegetation of yesterday has in large part been responsible for the conditions which determine the vegetation of to-day. The habitat is not static, but dynamic, and the present is but the link which joins the conditions of the past to those of the future.
The success or failure of the individuals of the species in one area will depend not merely on its ability to flour ish in the environment there present but also on its capacity to modify that environment and itself to become modified. The study of this relation of the individual to its surroundings is termed autecology. A few examples will serve to illustrate the importance of the life history and structure of the organism in determining its fitness for a particular station or habitat. Thus annuals are an especial feature of deserts, where the short rela tively wet season is occupied in rapid growth and reproduction whilst the dry hot season is passed in the seed state. The same quality has enabled many plants to become weeds of arable land and others to survive the rigours of a cold season. By contrast the vegetation of arctic regions is characteristically rich in peren nials, for the growing season is so short and the temperature so low that growth is a slow process, and the annual species which occupies a considerable period in attaining the reproductive state is at a disadvantage in competition with perennial types in which the flower rudiments are often formed already in one season pre paratory to their expansion in the next. Many of the herbaceous plants of woodlands exhibit exceptionally early leaf expansion, and hence their leaves are receiving light and manufacturing food material for weeks, and sometimes months, before the canopy of foliage of the shrubs and trees is formed above. This quality
of precocious leaf development has enabled its possessors to occupy woodlands in which the light intensity during the summer montns may be less than 1% of that in the open.
The possession of a relatively small leaf surface, a tufted habit, low growth, or hairy leaves are a few amongst many external features which tend to reduce the rate of water loss from the leaf surface (transpiration) and thus enable their possessors to occupy drier areas than would be possible for them in their absence. Such features are often accompanied by a microscopical internal struc ture also tending to check the rate of water loss, and plants having an aggregate of such feature are often termed xerophytes, from their capacity to grow in arid situations. Many plants, on the other hand, are entirely devoid of such transpiration checks and even exhibit features which tend to promote water loss, and of ten possess means of secreting liquid water. Such plants are quite unsuited to dry situations, but are admirably adapted for the humid situations in which they flourish.
Some species owe their success to a remarkable capacity for vegetative spread, as, for example, Mercurialis perennis and Epilo bium angustifolium, others to a copious seed output, as, for in stance, the foxglove (Digitalis purpurea), of which one plant may produce a million seeds in a single season. Contrast this with the seed output of another woodland species, Allium ursinum, which often does not exceed 4o seeds per annum. The rosebay willow herb (E. angustifolium) will often spread at the rate of 6ocm. a year by means of its rhizomes and at the same time produce some 50,000 seeds, whereas its congener, E. montanum, only pro duces from 4,000 to 53,00o seeds in a season and spreads vegeta tively about I•I cm. per annum. The equipment of species in the struggle for existence is thus very unequal.
Many seeds and fruits are dependent on wind action for their distribution, and some, such as the para chute-like fruits of the goatsbeard (Tragopogon), the plumed seeds of the willows and willowherbs, or the winged seeds of the tropical Bignoniaceae, are wonderfully fitted for this mode of dis persal. Others, such as the burr-fruits of the goose-grass (Galiurn aparine), enchanter's nightshade _(Circaea) and herb bennett (Geunr urbanum), become attached to the coats of animals by means of their hooked appendages, whilst the seeds of many berries are swallowed by animals and deposited in their droppings, often at great distances from their source. Here, however, it should be noted that the crops of birds on migration are usually empty and their plumage remarkably clean, so that bird carriage is not so important an agent as might be supposed. Still other seeds or fruits, like those of many waterside plants, have a buoy ant structure and are carried to new habitats by means of streams and ocean currents. The efficacy of this means of dispersal is shown by the carriage of seeds of Entada scandens from South America by ocean currents, which frequently deposit them along the west European seaboard in a viable condition.