STRUCTURE AND WORK OF PLANTS 19 



19. The work of the leaf : temporary responses. On exces- 

 sively dry days plants such as wheat and corn sometimes 

 wilt, since they are transpiring more water than they are 

 securing. If the soil becomes very hard, the water passes 

 into the air quite readily ; but if the soil is kept well pulver- 

 ized upon the surface, more soil water is held and a larger 

 supply is available. Observations made upon a garden that 

 is constantly cultivated during hot, dry weather, and upon 

 one that is not so cultivated, show a great difference in ability 

 of the plants to withstand drought. In a cornfield on a dry, 

 hot day the leaves of the corn often roll into rather tight 

 tubes. This form of leaf exposes less surface to evaporation 

 and consequently loses less water than would the fully ex- 

 panded leaves. This habit is doubtless of advantage in main- 

 taining a balance in water supply. 



In setting out young orchard or shade trees, nurserymen 

 recommend that the branches be well pruned ; otherwise the 

 leaves may soon grow in such numbers that they will tran- 

 spire more water than comes into the newly transplanted 

 trees, which do not have their ordinary amount of absorbing 

 root surface. Obviously newly transplanted trees and garden 

 plants should be kept especially well watered until their root 

 systems are well formed. 



20. Respiration. The work of respiration in both plants 

 and animals is commonly associated with the interchange of 

 gases between the exterior and interior of the living body. In 

 plants the interchange of gases may take place through the 

 leaf or through other parts of the plant. This interchange,' 

 however, is no longer regarded as the fundamental thing in 

 respiration, since respiration takes place in active, living pro- 

 toplasm in all parts of the plant. It consists in decomposition 

 of protoplasm or of some of its parts, or, as is supposed by 

 some physiologists, it may consist in decomposition of food 

 materials that have not yet become protoplasm. Through 

 respiration complex plant substances are broken down, and 

 the energy released by this decomposition is the energy by 



