288 THE INTERNAL WATER RELATIONS OF PLANTS 



In general diminution of water content is greatest in the leaf cells. The 

 condition commonly called wilting is induced whenever the shrinkage in the 

 volume of water in the leaf cells is sufficient to cause them to lose all or 

 most of their turgor. 



Wilting as a visible phenomenon is confined chiefly to species in which 

 the leaf tissues are composed largely of thin-walled, parenchymatous mesophyll 

 cells, and in which the leaves are maintained in their usual firm, expanded 

 condition principally by the turgidity of such cells. External manifestations 

 of wilting can also be observed frequently in young succulent stem tips, floral 

 parts, and even fruits. Root hairs also wilt very commonly, although such 

 wilting usually cannot be observed except under experimental conditions. 



In many species of plants the leaves are supported largely by lignified 

 tissues. Examples of species bearing such leaves include many of the evergreens 

 such as pines, holly, mountain laurel, etc. and numerous sclerophyllous species 

 common in the semi-arid regions of many parts of the world. Such leaves 

 wilt just as do parenchymatous ones in the sense that a marked loss of turgor 

 may occur in the leaf cells. The wilting of such leaves is not usually char- 

 acterized by the drooping, folding, or rolling which are the visible symptoms 

 of wilting leaves composed principally of parenchymatous tissues. 



Several stages of wilting are commonly distinguished. Even on days 

 upon which visible wilting does not take place incipient luilting is of frequent 

 occurrence. Incipient wilting corresponds to only a partial loss of turgor by 

 the leaf cells. Wilting which becomes visibly apparent in parenchymatous 

 leaves may be of either of the temporary or permanent types. Temporary 

 wilting, like incipient wilting, is due to a transient excess of the rate of 

 transpiration over the rate of absorption of water. In temporary wilting loss 

 of turgor and resulting volume shrinkage of the leaf cells is so marked as to 

 become visibly discernible in the behavior of the leaves. In the leaves of most 

 species transient wilting corresponds to a complete or nearly complete loss of 

 turgor. Both incipient and transient wilting are to be distinguished from 

 permanent wilting which refers to wilting which results, not from a temporary 

 excess of transpiration over absorption, but from an actual deficiency of water 

 in the soil. Plants do not recover from permanent wilting unless the water 

 content of the soil in which they are rooted is increased. 



As a general rule the leaves wilt first, because they are the organs from 

 which the great bulk of all water loss occurs, but the decrease in turgor gradu- 

 ally spreads throughout the plant as the internal deficiency of water becomes 

 more severe. Loss of turgor is thus general, although not necessarily equal, 

 throughout all of the tissues of a plant whenever wilting of any considerable 

 duration occurs. Any living cell in a plant may wilt, if this term is used to 



