THE RELATION OF PLANTS TO WATER 153 



of the character of mesophytes. There are, therefore, as in the 

 case of xerophytes, all gradations between true submerged 

 hydrophytes and mesophytes. This is readily observed along 

 the borders of streams and lakes, where the vegetation is often 

 divided into more or less clearly defined zones (Fig. 74). 

 In the water will be found both floating and attached hydro- 

 phytes of a typical character, while along the shore line a zone 

 of amphibious plants may graduate into the typical grasses, 

 sedges, and cat-tails of a marshy area. Farther back from the 

 stream or lake typical lowland mesophytes often pass insensibly 

 into grass and tree associations, inhabiting the drier hills or 

 uplands bordering the water and marsh areas. The limitations 

 of the text will not admit of a more extended treatment of the 

 wonderful and interesting adaptations in the form and structure 

 of plants to the water conditions of the soil and the atmosphere. 

 Enough has been said, however, to indicate the profound effect 

 of water as a factor in producing the types of vegetation which 

 inhabit the various climatic regions of the earth's surface. The 

 problem of the way in which water as a causal agent has been 

 able to mold plant life is one for the students of variation, adapta- 

 tion, and evolution to solve. We know simply that the fittest 

 plants for each particular water environment in any given habitat 

 have been selected for survival in the struggle for existence. 



