102 STOMATA. [CH. IV 



SECTION C. Stomata. Bloom. Lenticels. 



(117) Stomatal transpiration 1 . 



Cut a pair of similar well-grown leaves of Ficus elastica, 

 and when the bleeding of latex from the cut ends has 

 practically ceased slip about an inch of tightly fitting 

 rubber tubing over the leaf stalk, leaving J inch of tube 

 projecting ; then fold the free end down and wire it 

 tightly to the tube-covered stalk. In this way evaporation 

 from the cut end of the stalk is prevented : the wire ties 

 will also serve to hang up the leaves. Having weighed 

 them, hang them up close together in a dry room for 2 

 or 3 hours, when they must be again weighed. These 

 weighings give the ratio between the normal transpiration 

 of the two leaves. Now smear the lower surface of A and 

 the upper surface of B with vaseline, which should be 

 carefully rubbed on with a finger. Weigh the specimens 

 and leave them for 24 hours. It will be found that B loses 

 in weight something like 10 times as much as A. In one 

 experiment made in winter the leaf whose stomata were 

 closed remained green and fairly fresh for a fortnight, 

 while those with an ungreased lower surface were brown 



o 



and withered. 



(118) Stomatal transpiration observed with the horn- 



hygroscope. 



For demonstration purposes the well-known results 

 of Garreau 1 may be shown with the hygroscope de- 

 scribed in the 1st edition of this book, 1894, p. 94. 

 Instead of employing the awn of Stipa as an index of 

 transpiration, we now use the horn plates described in 



1 Garreau, Ann. Sc. Nat. 1850. 



