THl!J WORK OF LEA VES 207 



ble has passed into the bottle, admit another by raising 

 the end of the tube above the water for a short time. 

 When the rate of water absorption and evaporation 

 (usually called transpiration) becomes fairly constant, 

 transfer the apparatus to the sunlight and take observa- 

 tions (make the experiment also with a leafy branch). 



If an increase in the rate of transpiration occurs 

 in the sunlight, it may be attributed either to the 

 opening of the stomata or to the increase in trans- 

 piration (through the already open, or partly open, 

 stomata) due to rise in temperature. In most cases 

 both causes cooperate to increase the amount of tran- 

 spiration. We can see why it is of advantage to the 

 plant to have the stomata open in sunlight, since it 

 is then that starch-making goes on, and if the openings 

 of the leaf were closed the absorption of carbon dioxide 

 would be impossible. We can also see why the more 

 rapid evaporation of water which occurs when the 

 stomata are open may be an advantage, since it leaves 

 behind in the leaf mineral substances (just as' boil- 

 ing water in a tea-kettle leaves behind a crust of lime 

 and other mineral substances which are dissolved in 

 the water) which, as we have already learned, are of 

 value to the plant. 



The stomata usually close when the supply of 

 water begins to run low, and we can readily see why 

 this is advantageous, since otherwise the leaf would 

 soon wilt, and may even do so in spite of the closing 



