makes increased support necessary. In this combination of conducting and strengthening tissues, with the distribution of the two functions among different cells, the highest efficiency with the greatest economy of material is possible. There is no limit to which the plant can increase in size, provided only it preserve, from year to year, a layer of reproductive cells (the cambium) from which new cells developing into new conducting and strengthening elements may be formed.
In comparing the conditions under which water and land plants live this must be added. In the water, conditions change slowly and in regularly recurring periods. On land they change not only in regularly recurring periods but also frequently and suddenly. Submersed aquatics fall into a smaller number of species than do the plants living between the tide-marks. These again are numbered in fewer species than are land plants. The vertical distribution of aquatics is limited by the light to a few feet; the vertical distribution of land plants is limited by the temperature to a few thousand feet. Within this greater vertical space there is far greater diversity of conditions than in the shallow layer of water in which plants can live. This greater diversity of environment has been the cause of the greater diversity among land plants. But land and water plants, were they not sensitive to all the influences which combined make their environments, and had they not reacted to these influences, would never have attained the diversity which they now possess. The dependence of all living things upon water, and their power of reacting to all the influences of their environment to which they are sensitive, are the most striking phenomena displayed by animals and plants.
