WORK OF THE LEAVES: TRANSPIRATION 



93 



light retards transpiration. In darkness or weak light the stomates 

 tend to close. For modifications of the leaf which retard the loss 

 of water see paragraphs 148, 149. 



Fig. 83. 



Rhododendron maximum in freezing weather, the leaves rolled into tubes and 

 hanging directly downward. 



157. Whenever possible the student should make obser- 

 vations on the loss of water by plants and the effect on the plant. 

 This can be done sometimes during excursions. Compare the 

 conditions of plants in very dry periods in dry soil with the same 

 plants during a moist period. Plants grown in pots * or small boxes 

 can be set in a dry room and left without watering; some can be 

 placed where there are dry currents of air. These can be compared 

 with others where the air is quiet and the soil is kept moist. Plants 

 with thick, leathery leaves, like the rubber plant or the purslane, 

 can be set in a dry room along with plants with thin delicate leaves 



* The pot and soil should be sealed hermetically with rubber cloth to 

 prevent evaporation of water. 



