TEMPEEATUKE, LIGHT, AND MOISTURE 127 



characterized by weak stems, thin epidermis, few roots, little 

 conducting tissue, and large air spaces within stems and leaves, 

 all of which are necessary to fit them to their environment. 

 The leaves under water are usually narrow or much branched, 

 though those exposed to the air may be as large as, or larger 

 than, in other plants. Along with the true hydrophytes are 

 often found other plants that are xerophytic in structure, 

 though growing in water. These are sometimes known as 



Fio. 92. A group of hydrophytes, showing the zonation that often occurs 



In the water, spatter-dock (yuphar) and algse ; on the muddy shore, a helt of cat- 

 tails ; in the background, cottonwoods and willows 



xerophytic hydrophytes. They are, for the most part, plants 

 which absorb so slowly that they have been obliged to adopt 

 the structure of xerophytes in order to retain what mois- 

 ture they absorb. The scouring rush illustrates plants of this 

 type. The mesophytes occupy the middle ground between the 

 xerophytes and hydrophytes. They are the common plants 

 of our woods and fields, and though of far more economic 

 importance than either of the other classes, they are of much 

 less botanical interest. 



