THE GROWING PLANT 39 



cells ; growth is impossible in plants of which the foliage 

 is wilted. When the water supply is abundant, on the 

 other hand, and the absorptive power of the roots is 

 stimulated by a warm soil (101), the pressure within 

 the cells often becomes sufficient to force water from 

 the edges and tips of leaves. The drops of water that 

 so often sparkle on foliage in the sunlight of summer 

 mornings, commonly mistaken for dew, are frequently 

 excreted from the leaves. In young plants of the cala- 

 dium, water is sometimes ejected from the leaf-tips with 

 considerable force. 



The water of plants is almost wholly absorbed by the 

 root-hairs (100), the leaves having no power to take up 

 water, even in wet weather. The water of plants, with 

 its dissolved constituents, is commonly called sap, ex- 

 cept in fruits, when it is usually called juice. 



63. Distribution of food materials through the plant. 

 If we drop a bit of aniline blue into a glass of clear 

 water, it will not retain its form and size, but infinitely 

 small particles will become detached and move about 

 to all parts of the water in which it dissolves. This 

 movement will not stop until the bit has entirely disap- 

 peared, and until every part of the water contains exactly 

 as much of the aniline blue as every other part. This 

 equal distribution of the soluble material takes place 

 in response to the law of diffusion, that tends to cause 

 any soluble substance to become equally distributed 

 throughout the liquid in which it is placed. The liquid 

 in the meantime may remain stationary. The process 

 would be the same if we were to put in a very small 

 quantity of each of several soluble substances at the 

 same time. The movements of one of these substances 

 would not interfere much with those of the others. 



