118 PHYSIOLOGICAL BOTANY. 



have withered and lost much weight, and that the salt will 

 have become moistened and much increased in weight. It 

 is clear that what the salt has gained is the water that has 

 been lost by the leaves in the form of vapour. This action 

 is known as Transpiration. It is always going on from the 

 surface of all parts of plants which are exposed to the air ; 

 above all, leaves are the organs where the greatest loss of 

 water takes place. In adult leaves the watery vapour is 

 principally carried out from the stomata, which regulate 

 transpiration to a great extent, by their opening and clos- 

 ing according to different circumstances. 



As the cell-walls of the epidermis of submersed plants are 

 not cuticularized, these plants wither rapidly when exposed 

 to the air owing to the evaporation of water from their 

 whole surfaces. 



2. RESPIRATION. 



If some living water plants as Spirogyra be put in an air- 

 tight flask filled with water made free from oxygen by boil- 

 ing, they will perish after a few days. 



If a flower-pot with a living plant be put in a closed 

 vessel filled with carbonic acid gas, the plant will die after a- 

 few days. 



From the preceding experiments it will be understood 

 that oxygen is indispensable to the life of the plant. 



If a funnel filled with moist seeds of the Daidzu be in- 

 serted into the mouth of a bottle containing a solution of 

 caustic potash, the seeds will gradually commence to germi- 

 nate, the clear solution will slowly turn whitish, and a rise 

 of temperature may be observed in the germinating seeds. 



In the above experiment the potash solution is turned 

 whitish by absorbing the carbonic acid gas which is evolved 



