TRANSPIRATION 89 



Gentle winds of three to ten miles per hour increase the 

 rate of transpiration markedly, by carrying the moisture 

 from the surface of the leaf as fast as it diffuses through the 

 stomata, but further increases of wind velocity have little 

 effect. 



The transpiration rates that are highest and most likely 

 to cause serious damage are brought about by a combination 

 of conditions such as occurs in the summer; high tempera- 

 ture, bright sunshine, and winds. If the soil has plenty of 

 moisture the plants that are well established and are adapted 

 to our conditions can withstand even our most drying con- 

 ditions. Occasionally in the Middle West where the tem- 

 perature may be higher, the humidity lower, and strong winds 

 blowing, the plants have been killed even with adequate soil 

 moisture. 



More than 90 per cent of the moisture is lost from plants 

 through the stomatal openings of the leaves and young 

 stems. As explained in Chapter 11 they close and the plant 

 wilts when the turgor pressure is reduced to zero or nearly 

 zero. Their closure is gradual and diffusion is of such a 

 nature that it is not retarded until they are more than half 

 closed. This slow method of regulation may save the life of 

 the plant by preventing its desiccation, but wilting also re- 

 tards the growth by reducing the food supply, hence it is 

 better practice to regulate the soil moisture instead of expect- 

 ing stomatal regulation of the loss of moisture. 



The water requirement of plants is the relation of the 

 water absorbed by the plant during the growing period to 

 the dry weight of the plant. It is a unit of water economy. 

 Plant species differ quite widely in this respect under uniform 

 conditions, and the same species differ widely as the con- 

 ditions vary. The water requirement for a number of plants 

 was determined by Briggs and Shantz. A few of the approxi- 

 mate values follow: sorghum 280, corn 350, wheat 480, oats 

 600, alfalfa 820, ragweed 950. These large figures mean, for 



