A LARGE number of factors, some more important than others, will determine whether or not our seed is going to grow up into an adult plant and produce seeds of its own account. We shall not try to consider all these factors, but we can quickly see how they may be divided into two groups. The first group will include those which are associated with the place—the kind of ground upon which the seed falls : the second, those associated with the climate of that place. Thus the ground may be granite rock or limestone, sand or clay; but the successful development of our seed will depend on the presence of sufficient sunshine and moisture. It is conceivable that any one of these kinds of ground may afford sufficient harbourage for the seed to germinate, that is, to begin its growth; but, as is noticed in the parable, the soil may not be enough for the plant to grow to maturity and produce seed of its own.

Again we may apply to the gardener; he knows that some kinds of plants will grow in any kind of soil, while others are very unaccommodating. The matter is complicated still further when we remember that the population of any tract of ground is rarely, if ever, of one species, but is usually of many. What is more there is, for every species, a combination of soil and climate factors in which it flourishes best and it is reasonable to expect that in any one place, at any one time, the combination of conditions will not be equally favourable for all the species which we may find occupying the territory.

Just as the occupations of particular sorts of men give a distinctive character to the territory they inhabit, so it is

with plants. We are all more or less aware of the highly competitive nature of our own largest groups—the great cities —and, in a general way, where men can get most, there they congregate thickest and there we shall find nearly every sort and kind of trade—except those concerned with the primary production of food. For our own communities we have names, as hamlet, village, town, and so on. The biologists have a highly developed system of names for the aggregations of species in the formation of which man has had no hand. Some of these are : socion, sociation, consociation, association, federation, formation. These terms are not likely to assist our present inquiry so they will be avoided; but they are exceedingly useful when the inquiry is sufficiently detailed. We need not push ours beyond the point at which we can see that a great deal still remains to be done.

It is necessary to make it quite clear that we do not even know what are all our units of vegetation, and it is, therefore, not surprising that we have, as yet, no precise system for naming them. After all, we have had reasonably precise records of every individual human being in Great Britain for over one hundred years—such a record as we can never hope to have for the plant population—and still the record is not as complete as we could wish. Add to this that the units of our human population can answer questions themselves and the problem assumes daunting dimensions.

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