FOLLOWING our principle of proceeding from the familiar to the less familiar, let us consider the state of things we may expect to find in an English woodland. Nobody would expect to go to what is called an oakwood and find nothing but oak trees. The oaks are merely the largest and most conspicuous occupants of the ground. One oakwood differs from another in greater or less degree, according as it is on the sandstone soils of the Midlands or the clay soils of Surrey. For purposes of illustration one place is pretty well as good as another, so let us see if we can get any enlightenment out of an oakwood on the Wealden clay in Surrey. We shall not expect to find anything like real oak-forest often; that is to say, a close canopy formed by the intermingling of the crowns of the trees over a large area. W^hat we shall probably find is a relatively small number of mature oak trees to the acre— say between ten and twenty. The rest of the population will
consist of various species of shrubs,, climbers and a host of humbler plants which very few people ever notice.
Much will depend, however, on the time of year at which our visit takes place and here we may stop to consider what are the changes in the general conditions for plants which account, in part, for the difference in aspect in May from that at Christmas-time. In May we shall see large numbers of plants active and about their business of maturingand storing food : in December we shall have to look hard to find any signs of life among the plant population. The reason for this is not far to seek. It is because of the lack of a sufficient energy supply, due to the low sun and the very short day. This is, perhaps, a rather cumbrous way of coming at the extremely familiar procession of the seasons, which nobody living in the British Isles, or in any similar high latitude, can have failed to notice. It is this very familiarity which allows us to pass over many fundamental facts without once thinking of their implications. If, instead of Spring and Summer, we think of the Period of Growth and, instead of Autumn and Winter, of the Period of Endurance, we may avoid some of the dangers of familiarity.
WHEN A PLANT MUST LIVE ON ITS SAVINGS IF a plant has not accumulated sufficient reserves of energy-producing materials during the Period of Growth its chance of tiding over the Period of Endurance until a renewal of supplies becomes possible is very small. In some plants (the) the reserves are concentrated in the : the rest manage by consuming themselves, much as a hibernating bear is fat when he goes to sleep and thin when he wakes up in the Spring. Since light is the ultimate source of energv for plants (and, indeed, for nearly all life on this earth) those plants in our oakwood which are nearest the sun will have least interference with their energy supply. On the other hand, the smaller plants will have to put up with very considerable interference with this supply, due to overshadowing by their tall neighbours. So it happens that many of the which manage to live in an oakwood are those which can take advantage of a short period of growth and flowering before their light supplies are seriously cut down by the -growth of the larger plants. For instance, the bluebell and the primrose, common inhabitants of our Wealden oakwoods, are plants which are able to make a quick start by using last
year’s stores to produceand before the bracken overtops them.
There is, of course, no suggestion that the plants knozv that the bracken is going to overtop them : it is reasonable to think though, that plants without the ability to store food-reserves or the capacity to exist on meagre supplies of light would not survive in such company. Furthermore, if the tall plants take the light they do something else at the same time; they screen the ground against evaporation and so give rise to a set of conditions suitable for plants which demand little light and much moisture. Such plants as the wood-sorrel, some ferns and many mosses come into this class.
We have so far tried to avoid professional technical terms, but there is no reason why we should do so if our understanding can be enlarged by a consideration of some of them. The branch of biology which deals with the dependence of species upon places, climates and other species, is called ecology. When we wish to discuss the intricate business relationships of man with man, town with country, people with people, we talk economics. Both words have a commonin the Greek word oikos which means house, but in both departments the inquiry goes far beyond mere domestic relations. Any population of living organisms may be thought of as strictly limited in size by the available resources of food and energy. These will include light, water and so on.
If there is any important change in these ruling factors the population must change too, either in numbers or in the species composing it. Consider, for instance, the city of Old Sarum which subsisted for centuries when liability to attack made its population relatively small and its strategic strength important. When the liability to attack became less the population grew beyond the capacity of the available water to support it and the restrictions of the military site became a nuisance to the civilians and the churchmen. As a consequence the city of New Sarum was founded in the well-watered meadows to the south.
PROBLEMS THAT HARASS BOTH MAN AND PLANTS OWING to the increased productivity brought about by the Industrial Revolution the human population of England and Wales about doubled itself in the fifty years between 1811 and 1861, whereas it took something like two hundred years to accomplish the doubling from the beginning of the
seventeenth century onwards. It is important to notice that the increase became possible not by the doubling of the native food-supply, but by the exchange of large quantities of manufactured goods for foodstuffs produced overseas. It is not suggested that anything exactly like this happens with a population of plant-species. There is no attempt to push plants into an anthropomorphic scheme, but rather to push man into a biological one—to show that his fundamental problems are not peculiar to him, but to all living things.
When, in our oakwood, for any reason, the population of tall trees over a given area becomes reduced, there is a consequent reduction of cover to the soil and this means increased light for the smaller plants and increased evaporation of water as well. The result will be that the moisture-loving, shade-enduring species will be penalised and their places taken by plants which can stand more light and are well-equipped to conserve their water-supplies.
The reduction of cover may bring other changes in its train. For instance, the more open character of the plant population will give free access to the larger animals so that the chances of survival of mechanically weak and unprotected plants will be reduced when the growth becomes very open. The tough, thorny plants will still survive and the ground-flora will consist of close-growing herbs. This change from one type of vegetation to another is commonly spoken of as degeneration, but again it is necessary to remind ourselves that no moral reproach is implied. It really means little more than that the change of conditions will cause some species to die out from that territory and allow others to get the upper hand.