178 COLLEGE BOTANY 



common in many plants, especially the poplars. It reduces the 

 leaf surface and lowers the transpiration. The leaves on these 

 self-pruned twigs are usually green and apparently in a normal 

 condition. Twigs and leaves that fall in this manner do not 

 break off but grow off by the formation of a definite cleavage 

 plane in the same manner that leaves fall in the autumn. The 

 excessive transpiration is usually the result of hot, dry weather 

 periods known as drouths. The drouth periods frequently ex- 

 tend over large areas and form one of the most common causes 

 of crop failure in the Mississippi Valley States. We have al- 

 ready learned that excessive transpiration is controlled to some 

 extent by the rolling of the leaves, the reduction of the tran- 

 spiring surface, the modification of the stomata, the thickening 

 of the cuticle, the formation of a waxy covering, the formation 

 of trichomes, and the development of special devices for storage 

 (see page 175). 



Other Water Losses. Although transpiration may be 

 looked upon as the most important method by which the plant 

 loses water, it is not the only one. The plant also loses water 

 and compounds in solution by guttation, bleeding and secretion. 



Guttation. This term refers to the excretion of water from 

 the modified stomata. (frequently called water pores) along the 

 edges of the leaves of many plants, especially the cabbage, canna 

 and young corn. The water is given off in much greater abun- 

 dance than through the ordinary stomata and is usually more 

 prominent at night than during the day. The drops of water 

 on the margins of the leaves are sometimes very noticeable dur- 

 ing early morning hours and on plants grown in the greenhouse. 



Bleeding is the giving off of sap from wounds. It is most 

 common in the early spring. The maple-sugar makers make use 



