The Water Relations of Leaves 61 



best with a moderate water supply. In this group are in- 

 cluded the plants that yield most of our garden, field, and 

 meadow crops ; also most of the forms that are found in the 

 maple, beech, and elm forests of the Eastern states, and in 

 the fir and spruce forests of the canons and bottom lands of 

 the Western states. 



Xerophytes, hydrophytes, and mesophytes are readily dis- 

 tinguished as groups because of their great differences of 

 habitat and appearance. But it is not always easy to de- 

 cide whether a particular plant is a xerophyte, hydrophyte, 

 or mesophyte, because we find all gradations of form among 

 plants of the three classes. Nevertheless, these terms are 

 useful in describing the water relations of many plants. 



PROBLEMS 



1. How do plants that are wilted in the late afternoon of a hot summer day 

 recover their firmness during the night, even though there is no rain ? 



2. Where, near your home, do mesophytes, xerophytes, and hydrophytes occur? 



3. In what regions of the United States are mesophytes most common? 



4. In what parts of the United States are xerophytes abundant? 



5. In what parts of the United States are hydrophytes common ? What states 

 have very few hydrophytes? 



6. What xerophytes furnish useful products to man and animals? 



