WE are accustomed to speak of British plants and of American, Chinese and African plants. When we speak of British plants, we mean plants that are natives of the
British Isles, as opposed to cultivated plants. Of course, nobody supposes that plants are in the least concerned with political boundaries, and actually nearly all British plants are also found on the Continent. Nevertheless it is true that many species are found in America only, others in Africa only and so on. Any detailed treatment of the vast and interesting subject of plant geography is impossible within the limits of our space but we may make a few suggestive comparisons.
How is it that the British Isles are largely inhabited by human beings with relatively pale skins and not by kangaroos? One important reason is that the seas present an almost insuperable barrier to the kangaroo and not to men. We do not need to say that the kangaroo could not effect a successful colonisation, though that is probably true. All that concerns us at present is that the kangaroo lives in Australia and nowhere else because he cannot leave the continent without man’s assistance; he is neither a good swimmer nor a boat-builder. Other organisms are incapable of successful migration because they are poor climbers or are incapable of taking to the air.
Thisout of account the still more important inquiry whether a migrant can live in the country of its adoption. In human terms, having secured a passage to America, can a man find a place to live and the means of living in it?
In the plant world the migration of active adult individuals is by no means common, but movements of plants about the surface of the earth do occur in one way and another and some of these we must consider.
It is usual for plants to spend the whole of their lives in one place, but there are interesting exceptions to this. Among them are the rose of Jericho and the so-called ‘resurrection plant ‘of the florists. Both of these plants, though they are widely separated in any system of classification by relationship, have a very similar trick. Both of them are able, in a dry period, to curl up into a ball, in which condition they can be blown about by the wind with the chance of being carried to a more favourable locality. It is, however, far more common for plants to have organs of reproduction specialised for the production of units by which their offspring are enabled to occupy new territory. Chief among these units produced by plants living on the land, is the. The seed is a very complicated biological mechanism and may very properly be called the triumph of the land plant. It is one of the most
important means by which land areas are colonised and occupied by plant-life.
It should be observed thatshould not be confused with spores but, from the point of view which we wish to take at the moment, the distinction does not matter. The aspect of the matter which concerns us is, that a portion of a plant capable of being detached from the parent is also capable of facing the world on its own account. Since a grain of wheat is a kind of seed familiar to most of us and the adventures of the seed are fairly familiar in the parable of the Sower, it may well serve as an illustration for us now.