The Water Balance in Plants 



93 



The water balance and plant habitats. The place where a 

 plant grows naturally is called its habitat. The willow grows 

 beside a stream and the cactus 

 grows in the desert, each in its 

 natural habitat. If we put the 

 willow in the desert and the cactus 

 on a wet stream bank, both die. 

 This means that the conditions that 

 make up each habitat are favorable 

 to one kind of plant and not to an- 

 other. The conditions include not 

 only the kind of soil and the amount 

 of soil water, but also the evaporative 

 power of the air. In selecting plants 

 that may hve in a particular habitat, 

 the great importance of the dryness 

 or the moistness of the air is to be 

 kept in mind. Plants whose leaves 

 are soft and transpire water rapidly ^^^ ^^ ^ ^^^^, ^.,^ .,^ ^,^^ ^^, 



can succeed only in moist air, while in two and connected again with a 



those that have a low transpiration '"^e similar to that shown in Figure 



^ 49- If the roots absorb water more 



rate maintain a suitable water bal- rapidly than the leaves transpire it, 



ance even in a dry atmosphere, the mercury at D is pushed away from 



the plant, as m the illustration. If 



This is one of the reasons why on a the plant is set in bright sunshine, 



southern slope we find a set of plants '^^ transpiration win be increased 



and the mercury will then almost im- 

 that are different from those on the mediately be drawn toward the plant. 



northern slopes. 



Recent studies have shown that the leaves of plants growing 

 near the bottom of a ravine transpire water ten to fifty times as 

 fast as do those of plants growing higher up on an adjoining 

 southern slope. Doubtless, each year seeds of plants that grow 

 in the low ground germinate on the upper part of the slope ; but 

 each year the plants that spring from those seeds are eliminated 

 through their inability to get the water needed by their higher 



