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. 
