NEARLY everybody will readily accept the statement that to grow a flowering plant we must have soil of some sort. We have talked about soil often enough, but we have, as yet, given it no sort of detailed consideration. Our use of the word implies a capability to support plant-life of some sort. We should not consider a quantity of powdered glass as soil worthy of the name; yet it would deserve it quite as much as a quantity of pure quartz sand with no admixture of any kind. We should need to add a great many other things before the word soil could be properly used to describe it. If we added a quantity of decaying animal and vegetable matter we should be on the way to having a soil. Until we do that our sand may be regarded as a very stable system, little liable to change of any kind : when we have done it we have, so to speak, inaugurated a dynamic system in which changes will be taking place constantly. By means of the destructive activities of the organisms living within the soil, the constructive activities of those with only theirin it are rendered possible.
The destructive effects of climate in reducing large masses of rock by the action of rain, wind, frost and so on, are exceedingly important, but need not detain us long. We must allow ourselves to begin at the point where the activities of living organisms are necessary to make a disintegrated rock-mass into a medium in which the roots of land-plants can thrive. Here it may be well to stress an important difference between plants living in the sea and those with their roots in the earth and the rest of their bodies in the atmosphere. The lower end of a seaweed is an attachment organ; its function is to hold on and not to gather supplies. For the land-plant the earth is practically the sole source of raw materials and so the roots must perform a double function : maintaining the plant inand collecting water with the necessary supplies of dissolved salts and gases. To make the point in a
slightly different way : the food-factories of the land-plant (the) are at a distance from the sources of supply and so need and, since they continue to survive, have a system of transportation both to and from those factories. In the sea, the food-factories are surrounded by their essential supplies and so the transportation is one way—away from the factories.
When there is any demand for supplies by the plant on the place of attachment, we may expect to find that the place will be altered by the activities of the plant. This is one of the ways to the formation of a real soil, capable of supporting a flora of. A landscape of rocks does not, at first sight, seem a very good situation for the growth of plants. The faces of the rocks themselves, when unsheltered from the sun, will be very hot and subject to great evaporation. In addition most rocks will hold very little moisture. The equipment necessary for survival in such a situation will include: water—low requirements; ability to conserve such water as is available; toleration of great amounts of light and heat; low habit of growth so that a strong -system is not necessary.
This equipment is not common among; so uncommon in fact that the colonists of bare rock-faces are nearly always alga? and lichens, which form no more than a skin or crust on the surface. The activities of these plants, coupled with those of the climate, provide material which accumulates in the crevices as soil. Such soil provides harbourage and supplies for the hardier sorts of flowering-plants having a stout root-system and a cushion habit of growth. These will lead the way to an accumulation of soil over the rocks themselves and thence to a plant-covering of grassy turf.