204 EXPERIMENTS WITH PLANTS 



ber of stomata occur (as shown by the air-pump or by 

 microscopical examination) ? 



Does the water escape through the openings only? 

 Select a plant in which there are very few stomata, or 

 none, on the upper surface of the leaf (many species 

 of Begonia, Holly, Willow, Poplar, Lilac, Tulip Tree, 

 Mahonia, Oleander, Scarlet Eunner, etc.). After mak- 

 ing sure, by testing with the air-pump or by examina- 

 tion with the microscope, that the stomata are absent 

 from the upper surface, remove several leaves from the 

 plant and treat one lot by covering both the stalk and 

 the lower surface of the leaf with vaseline or grafting- 

 wax ; treat another lot by covering the stalk and the 

 upper surface of the leaf ; treat a third lot by cover- 

 ing the stalk and both surfaces of the leaf. Attach to 

 each leaf a stout piece of thread (of about the same 

 weight in each case), and weigh (taking care not to 

 rub off any of the vaseline in the process) . Suspend 

 the leaves by the attached pieces of thread, and re- 

 weigh from time to time. Does it appear that much 

 more water has escaped from the under surface than 

 from the upper! Has there been any loss from the 

 leaves which were entirely covered? If so, it indicates 

 the amount of experimental error due to the incomplete 

 sealing with vaseline, and this should be diminished as 

 much as possible by any suitable means. (It must be 

 remembered that unless care is taken some of the 

 vaseline may rub off, or in a sunny place some may 



