WATER RELATIONS OF PLANTS 279 



plants, desiccation is lethal; their water expenditure must more 

 or less be balanced by an equal intake, and their water content 

 shows only small fluctuations. The ''water balance" is the 

 relation between the intake and expenditure of water in plants. 

 One of the most important conditions for the normal functioning 

 of higher land plants is the establishment of this water balance 

 without a permanent and significant deficit. This requires a 

 well-developed root system that will absorb water from the soil 

 with sufficient rapidity and a well-developed conductive system 

 that transports the water without impediment to the main 

 evaporating organs, the leaves. The plant must be provided 

 also with a system of protective tissues that on occasion can 

 inhibit the loss of water. The structure of all of these systems 

 of absorptive, conductive, and protective tissues is described in 

 detail in textbooks on plant anatomy, and it is assumed that the 

 student is acquainted with them. 



The amount of water that passes through the plant is immense. 

 Experiments have shown that during 1 hr. of a hot summer day 

 more water passes through a leaf of our common plants than the 

 leaf itself contains at any particular time. During the whole 

 vegetative period, a sunflower or corn plant evaporates as much 

 as 440 lb., or a barrel, of water (Fig. 81). Of the total amount 

 of this water, the plant assimilates but an insignificant fraction. 

 This becomes evident upon comparing the above figures with the 

 amount of water entering into chemical reactions during the 

 same period in the synthesis of carbohydrates. In the environ- 

 ment of Central Europe, for every kilogram of water absorbed 

 only 3 to 4 g. of dry matter are formed. If it is assumed that 

 the dry substance consists of carbohydrates and that half of this 

 weight is made up by carbon, the other half consisting of the 

 elements of water, then it will be found that of the 1,000 parts 

 of water that pass through the plant, only 1.5 to 2 parts are used 

 in the process of nutrition. The remaining 998 or 998.5 parts 

 pass through the plant in order to compensate for evaporation 

 and to maintain a sufficient degree of saturation in the tissues of 

 the plant. 



Such lavish expenditure naturally requires a no less rapid 

 absorption of water. This task is performed chiefly by the roots. 

 The real dimensions of the root system are much greater than 

 most people assume on the basis of such meager evidence as is 



