THE PLANT AND ITS ENVIRONMENT 167 



becomes extraordinarily thick. Stomata are relatively few and are 

 usually either sunken in pits (Fig. 82), covered with a mass of 

 hairs, or otherwise protected (Fig. 83). In certain cases, notably 

 among arctic and alpine xerophj^es, the leaves and stems are 

 covered with a felting of hairs. In others, particularly the 

 xerophytes of saline soils, the vegetative organs become fleshy 

 and succulent (Fig. 84), Internally, most xerophytes have an 

 abundant development of woody tissue. 



Fig. 86. — Transverse section of the stem of a typical water-plant {Myrio- 

 phyllum). The fibro-vascular cylinder is poorly developed and the cortex is 

 provided with large air chambers. 



Hydrophytes. — At quite the opposite extreme from xerophytes 

 are those plants which are adapted to live nearly or quite sub- 

 mersed in water. These hydrophytes (Fig. 85) have root-systems 

 which are much reduced or may even be entirely absent. The 

 leaves, if submersed, are usually finely cut and dissected and are 

 very thin, Stomata are absent. The stems have become very 

 soft and weak and possess an exceedingly small amount of woody 

 tissue. In certain types, notably those in which the leaves 

 (or some of them) are exposed to the air, the tissues of the plant 

 are well provided with air passages (Fig, 86), 



Mesophytes. — Those plants with which we are most familiar 

 thrive best on a moderate supply of water and are known as 

 mesophytes. Living under conditions especiall}' favorable for 

 plant growth, they possess well developed root and leaf systems 

 and are generally large, thrifty, and fast-growing as compared 



