The Green Plant 501 



Respiration occurs in plants whether green or colorless ; photo- 

 synthesis occurs only in the green parts of plants. 



Respiration occurs in the dark and in sunlight ; photosynthesis 

 occurs only under the influence of sunlight. 



In respiration, energy is released ; in photosynthesis energy is 

 stored. 



Transpiration. Transpiration is a term given to the evapora- 

 tion of water from living leaves. It is similar to perspiration from 

 the human skin and may be called a form of excretion. Transpira- 

 tion is a process of much importance to plants. 



Beneficial Effects of Transpiration. The process of tran- 

 spiration renders service to the plant in at least two ways. First, 

 it makes possible the continued ascent of water in the plant, which 

 is necessary to convey raw food material in solution to the cells 

 of the leaves and stems. Second, as evaporation has a cooling 

 effect, it may under certain weather conditions prevent the killing 

 of the leaves by too much heat. 



Harmful Effect of Transpiration. Excessive transpiration 

 destroys the life of plants. If water evaporates more rapidly than 

 it is replaced from the soil, the leaves soon wilt, and unless the 

 supply is restored the plant dies. This explains why it is necessary 

 to thoroughly water plants in hot, dry weather. Leaves are pro- 

 tected in several ways against this loss of water : by a waxy coat- 

 ing, by the growth of hairs on their surface, by the development 

 of a thick-walled epidermis and by other means. 



Economic Importance of Leaves. Leaves are of great 

 economic importance. Many of them, such as the leaves of the 

 cabbage and lettuce plants, supply man with food. Leaves, espe- 

 cially those of grasses, help feed all grazing animals. The leaves 

 of the tea plant serve to make a beverage. Leaves, such as sage 

 and wintergreen, supply flavorings. Others, such as peppermint, 

 boneset, wormwood and eucalyptus, serve as medicines. One of 

 the greatest services rendered by leaves, however, is that of keep- 

 ing the air pure by absorbing far more carbon dioxide than they 

 give off and giving out far more oxygen than they use. Thus 

 man and all other living things are dependent on green leaves for 

 the supply of oxygen needed to sustain life. 



