32 



HORTICULTURE FOR SCHOOLS 



lessened 

 smaller 



by the guard-cells shrinking together and leaving 

 opening. The quantity of water transpired by 



plants is very great, 

 in many species be- 

 ing equivalent in a 

 season to several 

 hundred times the 

 weight of dry mat- 

 ter in the plant. 



The second func- 

 tion of leaves is res- 

 piration, in which 

 the leaves take in and 

 give off the same kind 

 of gases as do human 

 beings when they 

 breathe. Leaves re- 

 spire more or less all 

 the time, both day 

 and night. In respi- 

 ration, oxygen is 

 taken in from the air 

 and carbon dioxide is 

 given off. The pur- 

 pose of respiration is 

 to supply energy. 



The most impor- 

 tant function of the 

 leaves is photosyn- 

 thesis, or the manu- 

 facturing of food 

 material by the aid of 

 light. As light is 

 necessary for photo- 

 synthesis, the plants 



