STRUCTURE AND WORK OF PLANTS 10 



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

 si\-ely dry da3s plants such as ^^'lleat and corn sometimes 

 wilt, since they are transpiring more water than the}' are 

 secnring-. If the soil becomes Aery hard, the •\\ater 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 a^'ailable. 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 (lav tlie leaves of the corn often roll into rather tieht 

 tubes. This form of leaf exposes less surface to evaporatiim 

 and consequently loses less water tliaii would tlie fully ex- 

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

 taining a l)alauce in water supply. 



In setting out young orcliard or shade trees, nurserymen 

 recommend tliat the branches be well pruned ; other^^•ise the 

 leaves may scjon grow in such numbers that they A^-ill tran-. 

 spire more \\-ater than comes into the ne^\'ly transplanted 

 trees, which do not ha"\'e their ordinary amount of absorbing 

 root surface. Obviously newly transplanted trees and garden 

 plants sJiould l)e kept especially well \vatered until their root 

 systems are \\'e]l formed. 



20. Respiration. The work of respiration in both plants 

 and animals is commonly associated A^ith the interchange of 

 gases between the exterior and interior of tlie li\ing bod}-. In 

 plants the interchange of gases may take place, through the 

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

 howe^-er, 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 



