230 TEXTBOOK OF BOTANY 



The percentage of water in leaves during dry periods has been found 

 to be 5 to 10 per cent less in early afternoon than in similar leaves at 

 night. Under moist conditions the daily fluctuations may be only 1 to 5 

 per cent. Obviously even if the stomates are open the rate of transpira- 

 tion is reduced, since the mesophyll walls are dryer by midday than they 

 were in the earlv morning. 



A dandelion plant rooted in a shaded ravine bottom may lose 5 to 10 

 times as much water as a similar plant living near the top of the south- 

 facing slope of the same ravine. The lower plant has a constant soil- 

 water supply; the upper plant has a very limited supply. A corn plant 

 in a moist region may lose more water in a season than a similar plant 

 in a dry region, simply because there is more water available. If the 

 corn plants in the dry region were growing in irrigated fields, they 

 would lose more water than the plants in moist regions. 



If the soil is dry and little water passes into the plant, transpiration 

 from the leaf and stem surfaces may lead to wilting and finally to the 

 death of the cells of the leaves, the stem, and the roots. This is death by 

 desiccation. The lower or older leaves usually dry out first and the 

 younger leaves last. 



Water-holding substances. Another eflFective factor that may retard 

 transpiration is the presence within the plant tissues of colloidal gels 

 such as pectic compounds, mucilages, and gums. When saturated, these 

 compounds, like saturated jelly, have little effect on the rate of evapora- 

 tion of water from them; but as their water content decreases, the force 

 by which the remaining water is held increases. Many succulents con- 

 tain these compounds. Cacti, which contain mucilages and are highly 

 cutinized, may retain sufficient water for life and renewal of growth for 

 a year or more. Even such thin leaves as those of tobacco, when re- 

 moved from the plant, may retain sufficient water to keep them pliable 

 for weeks in a room where most other leaves become desiccated and 

 brittle. 



Wind. Moderate air currents may accelerate transpiration by the re- 

 moval of more or less saturated layers of air from the immediate sur- 

 faces of the plant. When these layers have been removed further increase 

 in wind velocity has little or no effect on the rate of transpiration. 



The pull of transpiration. One of the consequences of transpiration 

 that extends throughout the plant is the movement of water to the leaves 

 from other organs. The net result of all the diffusions of water mole- 

 cules in leaves from vessels to mesophyll and epidermal cells is that 



