TRANSPIRATION 



231 



[Chap. XXIV 



water, because of the cohesion of its molecules when confined in small 

 tubes, is pulled forward in the veins of the leaf and the vessels of the 

 stem. It is this pull upon the continuous water columns in the xylem 

 vessels of stems that accelerates the movement of water to the leaves at 

 the tops of trees. 



If during rapid transpiration the upward movement of water is 

 stopped by compressing the stem or by severing the stem from the roots, 

 the water in the veins of the leaf and in the vessels of the stem is soon 

 depleted. No more water passes to the mesophyll cells and the epidermis. 

 Transpiration continues and leads to the loss of turgor of the mesophyll 

 and epidei-mal cells. The stomates are closed by the loss of turgor of the 

 guard cells, and the rigidity of the leaf declines — the leaf is "wilting." 



On a clear warm day in midsummer the same sort of thing happens in 

 an intact plant, but it does not always proceed so far. If the rate of 

 transpiration is more rapid than the rate at which water enters from 

 the soil and is drawn up the stem into the leaf, turgor decreases and 

 stomates gradually close. Cuticular transpiration alone continues, and 

 the transpiration rate is greatly reduced. Water continues to move up 

 from the roots through the stem and into the leaf cells; and after a time 

 the water content of all cells is restored, and the cells become turgid. 

 Under the conditions assumed in this paragraph the lea\ es may not 

 regain their customary rigidity before evening. The stomates, how- 

 ever, may remain closed until after sunrise the following morning. 



During dry periods in summer, water may move to the leaves from 

 nearby fleshy fruits on a tree, and as a result the fruits become shriveled. 

 The water is kept from the fruits by the pull of transpiration from the 

 leaves (Fig. 80). 



Fig. 80. Diagram of daily increase and decrease in diameter of lemon fruits 

 attached to the tree. Transpiration from these fruits is negligible. The daily shrink- 

 age was due to movement of water from the fruit to adjoining stems and leaves. 

 During the night the water content of the fruit was restored. After E. T. Bar- 

 tholomew, 1926. 



