ioo GROWTH AND WORK OF PLANTS 



osmotic substances are more active in the guard cells, they be- 

 come turgid, curve backward and keep the stomates open. 

 Humid air, darkness, or weak light retards transpiration. In 

 darkness or weak light the stomates tend to close. For modifi- 

 cations of the leaf which retard the loss of water see paragraphs 

 15, IS 1 - 



Fig. 86. 



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

 directly downward. 



161. Practical observations on the effects of transpira- 

 tion under different conditions. Whenever possible the 

 student should make observations on the loss of water by plants 

 and the effect on the plant. 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 



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

 prevent evaporation of water. 



