290 PLANT PHYSIOLOGY 



_ p ercentage of hygroscopic water 

 ^ " 0^68 



_ perce ntage of water at full saturation — 21 

 ^ 2:9 



According to the data of mechanical analysis, the unavailable 

 reserve may be calculated as follows: 



q = percentage of sand X 0.001 + percentage of 



silt X 0.12 + percentage of clay X 0.57. 



Thus, different soils show a very different wilting coefficient. 

 For coarse sand, this coefficient is 1 per cent; for fine sand, 

 2 to 3; for loam, 5 to 10; for heavy clay soil, 14 to 16 per cent. 

 Regarding the adaptability of different plants, it has been found 

 that with sufficiently slow wilting, all of them leave in the soil a 

 perfectly equal amount of water, though the suction tension of 

 their root systems may be very different. This fact may be 

 explained as follows : When the capillary water, which is readily 

 available to all plants, is exhausted, the water-retaining forces 

 of the soil rapidly increase to several atmospheres. As the 

 suction tension of the plants usually does not exceed a few 

 atmospheres, differences in this regard between various plants 

 play almost no role. Moreover, the water loses its ability to 

 move through the soil; hence the soil particles that are not in 

 direct contact with the root hairs no longer give up their moisture 

 to them, however great the suction tension of root hairs may be. 



The. data of Briggs and Shantz are in perfect agreement with 

 those of Bogdanov, obtained at a much earlier date by studying 

 the relation of germinating seeds to soil water. Bogdanov found 

 that, independent of their nature and properties, the seeds of 

 various plants can germinate only when the water content of 

 the soil is no less than twice its hygroscopic capacity. His 

 method of calculating the unavailable water has been made use 

 of in agronomic practice. In calculating the amount of soil 

 water available to the plant, usually twice the amount of hygro- 

 scopic water is subtracted from the total amount present. 



67. Influence of Environmental Cond.ttions on the Absorbing 

 Activity of the Root System. Methods of Studying the Absorp- 

 tion of Water by Roots. Absorption of Water by Aerial Parts 

 of the Plant. — One of the environmental factors that strongly 



