CHAPTER II 

 THE WATER OF THE HABITAT 



8. Relation of the plant to water. The responses of the plant 

 to the water of its habitat are so numerous and so essential that 

 water must be regarded as the most important of all factors which 

 affect the plant. This is emphasized by the fact that practically 

 all indirect factors, i.e., soil, wind, etc., can influence the plant only 

 through their action upon water. Water is no more indispensable 

 to ordinary plants than is light or temperature, since a green plant 

 can not live and function if any of these is lacking. It is proper to 

 speak of it as more important, for the reason that water is the 

 immediate cause of a larger number of vital functions. Perhaps 

 the greatest value of water to the plant lies in its use as food. In 

 addition it is the vehicle by which solid foods, i.e., soluble salts, are 

 taken from the soil, and gases, carbon dioxide and oxygen, from the 

 air, and by which the foods made by the leaves are carried to all 

 parts of the plant. It is water that causes the stretching of the 

 cell wall by which growth is made possible, and it also gives the 

 rigidity so essential to stems of herbaceous plants. As a factor of 

 the habitat, though not as a stimulus, water is an important agency 

 in the reproduction of mosses and ferns and in the distribution of 

 the plant body, or seeds of water plants. In the form of humidity, 

 water regulates the loss of water from leaves. Finally, as is to be 

 expected from the above summary, water exerts a much greater 

 influence upon the form and structure of the plant than any other 

 factor. 



9. The nature of water stimuli. A terrestrial plant is con- 

 stantly subjected to the simultaneous action of water stimuli, the 

 water content of the soil acting upon the roots, and the humidity 

 of the air upon the leaves. Water content regulates the water 

 supply, humidity the water loss. The two are compensatory, and 



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