LAND AND WATER PLANTS. 245 



makes increased support necessary. In this combination of conduct- 

 ing and strengthening tissues, with the distribution of the two func- 

 tions among different cells, the highest eflicicncy 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 j^eriods 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 environ- 

 ments, and had they not reacted to these influences, would never 

 have attained the diversity which they now possess. The depend- 

 ence of all living things upon water, and their power of reacting 

 to all the influences of their environment to which they are sensi- 

 tive, are the most striking phenomena displayed by animals and 

 plants. 



