172 TEXTBOOK OF PLANT PHYSIOLOGY 



55. Wilting and Its Physiological Significance. — When the 

 loss of water considerably exceeds the supply, a shortage of it 

 ensues in plant cells, and their volume and turgidity diminishes. 

 A considerable water deficit may lead to a complete loss in tur- 

 gidity. In such cases the plant wilts, i.e., its tissues lose their 

 normal rigidity and the leaves and young tips begin to droop. If 

 water is supplied in time to such a plant, its turgidity as well as 

 its normal life activities will be restored. 



Two types of wilting are distinguished: a temporary and a 

 permanent one. The first is observed when transpiration increases 

 to such an extent that the water supplied by the soil is no longer 

 able to replenish the loss. The leaves, as the organs spending 

 most of the water, lose their turgidity and wilt, while elsewhere in 

 the plant the moisture content still may be quite high. When 

 towards the evening transpiration decreases, the deficit is made 

 good and the plant usually recovers without additional watering 

 of the soil. 



Permanent wilting happens when the soil no longer contains 

 water available to the plant. No matter how slow the loss of 

 water may be under such conditions it always results in a gradual 

 loss of turgor of all organs of the plant, including the root hairs. 

 The latter, being especially sensitive to a shortage of moisture, are 

 destroyed easily, which breaks the close connection between the 

 root and the soil. After permanent wilting, the plant absorbs 

 water very slowly even from a very moist soil. Only after the 

 renewal of the root hairs is the former rate of water supply to the 

 plant restored. Moreover, with the desiccation of the chlorophyll- 

 bearing cells of the leaf, the green plastids are injured. They lose 

 for a long time their faculty of assimilating carbon dioxide. Per- 

 manent wilting proves detrimental to the nutrition of plants. 



Permanent wilting is also harmful to the other organs and tis- 

 sues of the plant, the growth of the newer parts commonly being 

 retarded. If wilting occurs in the early stages of fruiting, ripening 

 and filling of the grain is irregular, the yield is reduced, and the 

 grain itself is poorly developed. In some plants, like the cotton, 

 abscission of the flower buds may be observed. Such results, 

 though of less intensity, are produced also by temporary wilting. 



Although definite harm is done to the plant by wilting, espe- 

 cially when permanent, it should not be regarded as a pathological 

 phenomenon. Experiments have shown that in plants undergoing 



