150 TEXTBOOK OF PLANT PHYSIOLOGY 



that their roots develop a normal suction and exudation pres- 

 sure. 



47. Absorption of Water by Aerial Parts of Plants. — The root 

 system is the special organ of water absorption. This does not 

 mean, however, that the other organs of the plant are unable to 

 absorb water. Every unsaturated cell imbibes water, if brought 

 into contact with it. Thus leaves, especially when wilting, absorb 

 water vigorously upon immersion, in spite of their cutinized epi- 

 dermis. Experiments have shown that a dry cuticle is almost 

 impermeable to water. When moistened, it swells and becomes 

 much more permeable. 



Leaves moistened by rain or dew are able, therefore, to imbibe 

 water. The dormant branches and buds of trees likewise absorb 

 water through their surface during thaws and spring rains. This 

 absorption, however, plays no important role in the general water 

 regime of the plant. The amount of water absorbed always 

 remains small in comparison with the total amount of water lost. 

 Moreover, the moistening of the cuticle increases the intensity of 

 tranpiration, thus accelerating the loss of water. This explains 

 the well-known rule that the watering of plants in direct sunlight 

 must be avoided, as it induces more rapid wilting. Cautious 

 watering of the soil without sprinkling the leaves, however, does 

 not produce harmful consequences, even when done during the 

 noon hours, if the water does not lower too much the temperature 

 of the soil. 



Only for some plants, whose existence is subject to the humidity 

 of the air, the absorption of water by the aerial organs is an impor- 

 tant means of water supply. Such are the so-called "epiphytes," 

 plants living on the surface of other plants or rocks, especially 

 on the trunks of trees. In the climate of Russia, only lower plants 

 are found in this group, chiefly lichens and some mosses. When it 

 rains, these plants absorb water with their whole surface and swell 

 considerably. During periods of drought, they lose all this water 

 and may become perfectly dry without losing their vitality. As 

 soon as they are moistened they again return to activity. Their 

 protoplasm, consequently, is capable of reversible desiccation. 



In the tropics, especially in moist regions with almost daily 

 rainfalls, epiphytism is rather common among higher plants, for 

 instance, the orchids and the Bromeliaceae in general. In these 

 plants a considerable amount of water is stored in the thickened 



