WATER OF SOIL IN ITS RELATION TO PLANTS 193 



perhaps tlie most reliable. While these investigators found 

 in general that plants on heavy soils exhibited a low transpira- 

 tion ratio, hasty conclusions must not be drawn. Since the 

 fine-textured soils contain more nutrient materials, it is prob- 

 able that this is also a factor. 



101. Amounts of water necessary to mature a crop. — 

 Although it may be seen from the transpiration ratios cited 

 that the amount of water necessary to mature the average 

 crop is very large, a concrete example under humid condi- 

 tions may be cited to advantage. A fair estimate of the dry 

 matter produced in the above-ground parts of a forty-bushel 

 crop of wheat would be about two tons. Assuming the tran- 

 spiration ratio to be 300, the amount of water actually used 

 by the plant would amount to 600 tons to the acre, or about 

 5.2 inches of rainfall. This does not include the evaporation 

 that is continually going on from the soil surface, which might 

 very easily amount to as much more. The demand in total, 

 to say nothing of run-off and drainage, is at least equal to 

 10 inches of rainfall. 



102. Role of capillarity in supplying the plant with 

 water. — A query arises at this point regarding the mode 

 by which this immense quantity of water is supplied to the 

 plant. The rootlets, especially their absorbing surfaces, are 

 few in number as compared with the interstitial angles that 

 contain most of the water retained in the soil. How, then, 

 does the plant avail itself of water not in immediate contact 

 with its rootlets? This question has been anticipated in the 

 discussion concerning the capillary equilibrium which tends 

 to occur in all soils. As soon as the rootlet begins to absorb 

 at one point the film in that interstitial angle is thinned. 

 A considerable convexity of the water surface occurs at that 

 point, resulting in an inward pull, which causes the water to 

 move in all directions toward that point. Thus a feeding 

 rootlet by absorbing some of the moisture with which it is in 

 contact, creates a condition of instability which results in 



