THE LOSS OF WATER BY THE PLANT 173 



wilting the loss of water is markedly reduced by the closing of 

 stomata as well as by the drying of the cell walls. Thus protecting 

 itself the plant loses only one-fifth to one-tenth as much water as 

 in a condition of turgidity. Wilting must be regarded, therefore, 

 as a most effective way of checking transpiration in periods most 

 dangerous for plants. 



Apparent wilting, or the visible loss of turgidity, is shown by 

 various plants at different degrees of water loss. Plants grown in 

 open places, as the potato or the sunflower, may lose 25 to 30 per 

 cent of their total water content without showing signs of wilting. 

 Such water loss has been actually observed in these plants during 

 the midday hours on all hot days. On the other hand, shade- 

 loving plants may wilt with a loss of 2 to 3 per cent of water. 

 These differences are due to the fact that in plants of the first group 

 the cell walls are much distended, similar to a blown rubber bal- 

 loon. Part of this tension may be preserved even with a consider- 

 able decrease in volume. In the second group, the walls may be 

 strained but not distended. Even a small decrease in volume 

 makes them collapse like a paper bag. 



The capacity to resist wilting, even when the water loss is con- 

 siderable, is to the advantage of a plant. The stomata will remain 

 open and the plant will be able to continue its work of assim- 

 ilation. 



56. The Conservation of Water of Desert Plants. Xerophytes, 

 Mesophytes, and Hydrophytes. — Water is very unequally dis- 

 tributed over the surface of the earth. There are places where 

 plants have always an ample supply of water at their disposal. 

 But there are also large tracts of desert with a very scanty vegeta- 

 tion, a few isolated shrubs scattered over a naked soil. The plants 

 of such dry habitats, where the soil contains but little water and the 

 air is hot and dry, have been called xerophytes. The opposite 

 type are the hydrophytes — plants characteristic of perpetually 

 moist habitats, as near a body of water or in a tropical rain forest. 

 The majority of plants of temperate climates and of moderately 

 moist habitats are of an intermediate type, the mesophytes. A 

 large majority of crop plants belong to this group. 



Since none of these ecological types are represented by a homo- 

 geneous group, they cannot be characterized by a few definite 

 physiological features. This is especially true of the xerophytes — 

 plants having adapted themselves to very severe habitats. Though 



