Transpiration and Water Movement 



95 



the part of such plants to respond to these conditions by 

 the production of a protected surface or of growth-forms 

 tending further to reduce the water-loss. 



When transpiration is excessive, leaves commonly droop. 

 This is an indication of wilting. It is usually regarded as 

 a further protection against water-loss. At all events, such 

 leaves obey an obvious physical law. The leaves of corn 



FIG. 28. Stomatal apparatus in leaf of carnation. 



and some other plants " roll " under the same conditions. 

 Here the tensions are different. It is often stated that in 

 corn this rolling results because the stomata occur on the 

 upper surface, but the data previously cited indicate that 

 corn may exhibit more stomata on the lower than on the 

 upper surface. 



Plants which are commonly able to maintain themselves 

 to advantage in arid situations may be modified in one or 

 more of a variety of ways, some of which are as follows : 



