18 STUDIES IN THE VEGETATION OP THE STATE 



to absorb a much larger amount of water in order to obtain 

 sufficient nutriment. On the other hand in times of drought 

 the soil solution may be too concentrated to be available for 

 the use of the plant. In alkaline and saline soils this takes 

 place with a much higher per cent of physical water present. 



3. Physiological Water in Soils. 



The term physiological water designates that portion of 

 the physical water available for the use of the plant. This 

 being the case it varies directly with the amount of physical 

 water on the one hand, and on the other it is limited only by 

 the plant's ability to draw water from the soil. This is 

 varied by the physical factors which affect the vigor of the 

 plant. The plant prefers an optimum of heat, light, etc. 

 This optimum varies with different species. If a plant is 

 subjected to a stimulus which is greater or less than the op- 

 timum it must either adapt itself to the new condition by a 

 slight change in its protoplasm and later in its structure, 

 or, failing to adapt its structure, it becomes less vigorous, 

 in the latter case it can, in times of drought, draw less 

 water from the soil before it dies. As a result the amount 

 of physiological water in the soil is less in this case than in 

 the former. The different causes of the variation of the 

 limit of physiological water will be discussed more fully in 

 the chapter on that subject. 



4. Non-Available Wate7- in Soils. 



The amount of non-available water in soils, as has been 

 indicated in ttie previous discussion, is varied widely by the 

 porosity and composition of the soil. Nearly all the water 

 in sand is available for the use of the plant, but the evapor- 

 ation and gravitation from soils of this nature is much more 

 rapid ; as a consequence the amount which the plant can use 

 is much less than it may obtain from a soil containing clay 

 mixed with sand or loam. The finer particles of the clay 

 hold the water so tenaciously that it is much longer in losing 

 its water content. In times of drought this gives the plant 

 in the clay soil a longer lease of life. 



