312 PLANT PHYSIOLOGY 



turgidity is reestablished, and the plant resumes its normal 

 vital activity. 



There may be distinguished two types of wilting, temporary 

 and permanent. The first may be observed in cases when exces- 

 sive heat and dryness of the atmosphere cause such an increase 

 in transpiration that water is absorbed from the soil too slowly 

 to compensate for expenditure. In this case, loss of turgor and 

 wilting are displayed by the organs that lose water most rapidly, 

 such as leaves, while the other organs of the plant remain turgid. 

 As soon as transpiration is decreased, for instance, toward the 

 evening, the water deficit decreases, and the plant recovers with- 

 out any additional moistening of the soil. Temporary wilting 

 is not so very injurious to plants; however, it may decrease the 

 yield, for during wilting, photosynthesis and growth are retarded. 

 This may often be observed with sunflowers and sugar beets, 

 pumpkins, and other agricultural plants. 



Permanent wilting takes place when the soil does not contain 

 any more water available to the plant. Under these conditions, 

 transpiration, however weak it may be, leads to a gradual loss 

 of turgor in all the organs of the plant down to the root hairs. 

 The root hairs apparently are very sensitive to a lack of water 

 and die quickly. This leads to a disturbance in the contact that 

 existed between the root and the soil. This is the reasor why 

 plants, when abundantly watered after permanent wilting, 

 absorb water very slowly ; only when new root hairs have formed, 

 is the usual rate of water absorption reestablished. Further- 

 more, the withdrawal of water from the chlorophyll-bearing 

 cells injures the chloroplasts, which for a long time decrease or 

 even completely lose their photosynthetic ability. For this 

 reason, wilting is very harmful to the nutrition of plants. 



Permanent wilting is no less injurious to other organs and 

 tissues. It retards the growth of the young organs. If wilting 

 occurs in the beginning of seed formation, the filling and ripening 

 of grain are irregular, the yield decreases, and the grain is of low 

 quality. This phenomenon is sometimes observed, also, after 

 temporary wilting, though to a lesser degree. 



Wilting, especially permanent wilting, is definitely harmful to 

 plants. Experiments show that it is accompanied by a very 

 sharp decrease in the water loss, as a consequence of the closing 

 of stomata as well as of the incipient drying of the cell walls. 



