CHAPTER VII 



ADAPTATION TO WATER 



"^ 163. The relation of structures to water. The functional 

 responses of the plant to water content and humidity may produce 

 modification of form, of structure, or of both. This may take 

 place in root, stem, or leaf, or, under intense conditions, may 

 occur in all of these. IModification is greatest in the leaf as the 

 organ of greatest activity, and of greatest exposure of surface to 

 the changing factors in the air. It operates upon the stem in 

 the degree that the latter carries on the functions of a leaf. The 

 root is changed least, owing to the greater uniformity of conditions 

 in the soil, as well as to its fewer activities. Under extreme con- 

 ditions, either organ may be lost. As would be expected, the 

 root is frequently lost when it is no longer needed for absorption, 

 and the leaf when the water supply is reduced to a minimum. 



164. Adaptation to a small water supply. A low water supply 

 threatens the functions of the plant, and consequently its growth 

 and existence, whenever the water loss is increased. The effect 

 of a deficiency in the supply must be met by changes in structure 

 which decrease the demand arising from w^ater loss, or by those 

 that increase th supply by adding to the absorption or storage 

 capacity of the root. These changes affect the form and size of 

 the various organs as well as their structure. Modification in the 

 form and size of leaf or stem lessens transpiration by reducing 

 the amount of surface exposed to the air. Changes of structure, 

 on the other hand, bring about the protection of epidermal cells 

 and stomata, as well as the internal tissues, from the factors that 

 promote transpiration. They also forestall the effects of excessive 

 water loss by storing water in specialized cells or tissues against 

 periods of low humidity. In a few extreme cases the epidermis 

 may be modified for absorbing water vapor from the air. 



144 



