Water Requirements 131 



the camel, and it thrives under conditions which would 

 perhaps eliminate the great majority of the grass species 

 of the semiarid United States. 



Mesophytes. - The mesophytes occupy an intermediate 

 position with respect to water requirements. They con- 

 stitute therefore the chief elements of the terrestrial flora, 

 and in fact the main crops and herbaceous vegetation of 

 the earth. Likewise the species constituting the typical 

 forests of northern Europe and America, as well as most of 

 those of tropical regions, would be classed in this category. 

 In other words, we have in this group a great majority of 

 those plants which constitute crops in the usual sense of 

 this term. The relative abundance of plants requiring 

 an intermediate amount of water results in a tendency 

 to consider them as the normal plants, whereas others may 

 be regarded as abnormal, or as specially adjusted to per- 

 sistence under intensified conditions. 



Hydrophytes. - - All plants growing wholly or partially 

 submerged are denoted hydrophytes. Typical members 

 of this group, such as the water lilies, or water millfoil, 

 exhibit modifications of structure which are of much 

 interest. It is important to refer again to the fact that 

 soil water is not pure, and must of necessity contain sub- 

 stances in solution. The water of most streams, ponds, 

 and inland lakes contains relatively small quantities of 

 organic matter, and invariably small amounts of many 

 mineral compounds; such fresh waters support one type 

 of vegetation. Ponds which are common in the typical 

 bog regions of the northeastern United States and else- 

 where may contain organic materials produced under 

 peculiar conditions, which apparently serve to make the 



