8 PLANT PHYSIOLOGY AND ECOLOGY 



the final response to a stimulus of either sort can only be deter- 

 mined by reference to the action of the other. An increase or 

 decrease in water loss produces a corresponding change in the 

 amount of water absorbed, and a change in water supply tends to 

 produce a consequent change in water loss. This is strictly true 

 only when the stimuli are normal. For example, a decrease in 

 humidity causes increased water loss, which is compensated, as a 

 rule, by increased activity at the root surface. Frequently the 

 water supply is insufficient to compensate for heavy or rapid water 

 loss, and the proper balance can be reached only by closing the 

 stomata. In the case of excessive supply or loss, neither compen- 

 sation suffices, and the plant dies. A change of structure, i.e., 

 adaptation to water stimuli, results when the compensation of 

 supply for loss or the reverse is more or less inadequate, but not 

 to a degree sufficient to cause death. In addition to this funda- 

 mental compensating action of water stimuli upon the plant as a 

 w^hole, water content affects the growth of roots in such a way 

 that the direction of growth is determined by the distribution of 

 the moisture of the soil. The rule is that roots turn and grow 

 toward the area of greatest moisture. This phenomenon is known 

 as hydrotropism : it will be discussed under absorption. 



10. Water content. The water content of a habitat is the 

 total amount of water found in the layer of soil occupied by the 

 roots. The water of lower strata may be raised, and ultimately 

 used by the plant, but it is not properly water content until it 

 reaches the roots. The water is found in the form of thin films 

 surrounding the soil particles. The amount depends upon the 

 thickness of these films. In soils that are saturated the films run 

 together, forming drops and masses of water. In air-dry soils 

 there is still a very thin film about the smallest particles. The 

 amount of water content varies most widely in different habitats. 

 Impervious rocks contain practically none, until cracks and rifts 

 are formed by weathering, ^larshes, ponds, streams, etc., repre- 

 sent habitats with maximum water. Dry habitats, such as prairies, 

 plains, gravel slides, sand-hills, etc., have a low water content, 

 varying as a rule from 3-15%. Wet habitats vary from saturation, 

 as in stream banks, wet meadows, bog hummocks, etc., where the 

 percentage ranges from 20-80% in accordance with the soil, to the 

 maximum found in bodies of water. IMoist habitats, meadows, 

 forests, and cultivated fields usualh^ contain from 15-30%. 



