162 FOUNDATIONS OF BOTANY 



EXPERIMENT XXXI 



Through which Side of a Leaf of the India-Rubber Plant does Tran- 

 spiration occur? The student may already have found (Sect. 164) 

 that there are no stomata on the upper surface of the fig leaf which 

 he studied. That fact makes this leaf an excellent one by means of 

 which to study the relation of stomata to transpiration. 



Take two large, sound rubber-plant leaves, cut off pretty close to 

 the stem of the plant. Slip over the cut end of the petiole of each 

 leaf a piece of small rubber tubing, wire this on, leaving about half 

 of it free, then double the free end over and wire tightly, so as to 

 make the covering moisture-proof. Warm some vaseline or grafting 

 wax until it is almost liquid, and spread a thin layer of it smoothly 

 over the upper surface of one leaf and the lower surface of the other. 

 Hang both up in a sunny place in the laboratory and watch them for 

 a month or more. 



What difference in the appearance of the two leaves becomes 

 evident ? What does the experiment prove ? 



172. Endurance of Drought by Plants. Plants in a wild 

 state have to live under extremely different conditions as 

 regards water supply (see Chapter XXIV). Observation 

 of growing plants during a long drought will quickly 

 show how differently the various species of a region bear 

 the hardships due to a scanty supply of moisture. It is 

 still easier, however, to subject some plants to an artificial 

 drought and watch their condition. 



EXPERIMENT XXXII 



Resistance to Drought. Procure at least one plant from each of 

 these groups : 



Group I. Melon-cactus (Echinocactus or Mamillaria), prickly 

 pear cactus. 



Group II. Aloe, Cotyledon (often called Echeveria), houseleek. 



