WATER RELATIONS OF PLANTS 313 



In a wilted state, the plant transpires from one-fifth to one- 

 tenth of the water lost when in a turgid state. That is why 

 wilting must be regarded as one of the most effective means of 

 inhibiting transpiration in critical periods of the life of the 

 plant. 



Various plants show signs of wilting, or of loss of turgor, at 

 varying degrees of water loss. Those of open sunny habitats, 

 as for instance the sunflower or potatoes, may lose up to 25 or 

 even 30 per cent of their total water content without displaying 

 any signs of wilting. Such a water deficit may usually be 

 observed in these plants at noon on a hot day. Other plants, 

 chiefly shade plants, wilt with a loss of only 2 to 3 per cent of 

 their water reserve. This difference is determined by the fact 

 that in the first group of plants the cell wall is very distended, 

 like a rubber balloon, and retains a certain degree of elasticity 

 even with a considerable decrease in volume. In the second 

 group, the cell walls are under tension but not distended; conse- 

 quently, a small decrease in volume leads to collapse of the cell 

 walls. 



The capacity of losing a considerable amount of water without 

 wilting has the advantage of allowing the stomata to remain 

 open, the plant being thus able to continue photosynthesis. 



Besides decreasing the water loss, wilting leads to other deeper 

 internal changes, which finally result in an increase of the 

 capacity of the plants to withstand further harmful influences of 

 drought. These changes are known as ''drought hardening." 

 They will be further described during the discussion on the 

 drought resistance of plants. In drought hardening, the plant 

 increases its capacity for photosynthesis, transpiration, and 

 other physiological functions (Tumanov, 1926). In the culti- 

 vation of crop plants in arid regions, hardening of the plants in 

 their early growth stages is essential. In regions where moisture 

 is generally adequate, it is desirable to avoid the hardening of 

 plants such as tomatoes. If such plants are subjected to harden- 

 ing treatment, their growth is checked, and the stems become 

 hard and woody; and later, when their roots become established, 

 there is insufficient conduction through the wood to give a vigor- 

 ous growth. Hardened plants are stunted, whereas plants kept 

 always in rapid growth continue to grow rapidly and bear fruit 

 quicker than hardened plants. 



