CHAPTER II 



THE INTAKE AND UTILIZATION OF WATER 



Under favorable conditions the entrance of water into the plant, 

 its translocation and its egress take care of themselves, without conscious 

 manipulation by the grower. At times, however, one or another of 

 these processes should be controlled to some degree and pathological 

 symptoms or conditions may arise which can be understood only through 

 a knowledge of these processes. 



WATER ABSORPTION 



Proper absorption, as the necessary prelude to the other processes, 

 is of obvious importance. Furthermore, it is the process with which the 

 grower is most frequently brought into contact. 



The Water Absorbing Organs. — The root is the absorbing system and 

 for practical purposes all the water which enters the plant is absorbed 

 through the root. There are indeed other sources from which moisture 

 can be obtained, such as, for example, the water resulting as an end prod- 

 uct of the oxidation of carbohydrates, which has been termed metabolic 

 water,^ but such sources are significant only in extreme circumstances. 

 The absorption of water by the root takes place chiefly through special 

 structures, the root hairs, which are extensions from the epidermal cells 

 of the root a short distance back of the growing point. The absorption 

 power of the root depends upon the extent of its surface and it is increased 

 to a marked degree by the presence of root hairs. The ratio of the 

 surface of the root supplied with hairs to one from which the hairs have 

 been removed has been calculated as 5.5 : 1 for maize and in the garden 

 pea 12.4 : l.^"^ These figures give some idea of the efficiency of the root 

 hairs for water absorption. Moisture can be absorbed by the root tip 

 and also through the surface of the root for some distance above the zone 

 of root hairs. In the older portions of the root the cortex and epidermis 

 die and peel off as a result of the formation of deep seated cork; hence, 

 this portion of the root is incapable of absorbing appreciable amounts 

 of water. 



The number of root hairs varies with different plants and, in the same 

 plant, with the conditions under which it grows. Thus, the development 

 of root hairs is reduced in wet soil, or in very dry soil and may be entirely 

 prevented where the root is in contact with water. ^^ This occurs in 

 certain plants, such as the cranberry, which normally grow in bogs where 

 the roots do not develop root hairs. 



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