LNTERNAL REDISTRIBUTIONS OF WATER IN PLANTS 297 



water at the expense of older leaves, fruits, and roots. The root hairs are 

 often the first cells to lose water in this way. Maintenance of many plants 

 in a state of permanent wilting for more than a few days usually results in 

 death of the root hairs due to loss of water from them. This is one reason 

 why recovery of many plants from permanent wilting takes place very slowly 

 even after water again becomes available in the soil. 



Indications that water often moves from fruits into leaves when an in- 

 ternal water deficit exists in a plant have often been observed. Most such 

 observations have been made upon detached fruit-bearing branches. If two 

 approximately equal-sized cut branches of a lemon tree, for example, one 

 bearing fruits, the other not, be hung up and allowed to dry out the leaves on 

 the fruit-bearing branch will remain fresh and green long after those on the 

 other branch have wilted and withered. Obviously the leaves on the fruit- 

 bearing branch are obtaining water from the fruits. Loss of water from the 

 leaves, although slow, is much more rapid than from the fruits. Due to this 



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Fig. 77. Daily variations in the diameter of lemon fruits. Data of Bartholomew (1926). 



slow transpirational loss the leaf cells soon develop a greater diffusion pres- 

 sure deficit than the cells of the fruits on the same branch and transference 

 of water from the fruits to the leaves occurs. Concurrently with this loss 

 of water from the fruits a gradual shrinkage in their diameters can be detected. 

 Essentially the same results can be obtained in similar experiments with the 

 fruit-bearing branches of other species. 



Similar internal movements of water from fruit to leaves occur in intact 

 plants. One of the most striking investigations of this phenomenon has been 

 made by Bartholomew (1926) who studied the diurnal expansion and con- 

 traction of lemon fruits while still attached to the tree. These measurements 

 were made by means of an auxograph, an instrument which automatically 

 records variations in the diameter of fruits and other plant organs. 



The auxographic record obtained in one of these experiments is shown in 

 Fig. 77. As shown in this figure the lemon fruit began to contract in volume 

 each day at about 6:00 A.l\i. and continued to shrink until about 4:00 p.m. 



