CH. IV] BLOCKING OF STOMATA. 109 



green, but assumes the very dark tint of an injected leaf. 

 In the first case thawing produces injection with air, in 

 the second with water. The explanation given by Moll 1 

 is that the cells of the mesophyll, in freezing, give up 

 water to the intercellular spaces, and that when they are 

 thawed the cells absorb the melted ice from the intercel- 

 lular spaces, which then fill up with air or water as the 

 case may be. 



(122) Blocking of stomata by water 2 . 



One arm of a bent glass tube is gently pushed into the 

 cavity of an onion (Allium cepa) leaf and is there firmly 

 secured by a ligature of soft cotton or worsted. The other 

 end of the tube is held in the mouth, and the leaf is 

 immersed in water : by blowing gently, bubbles are forced 

 out of the stomata on the external surface. Now clean 

 the bloom from a zone of the leaf, which may be done by 

 gently rubbing it with a plug of cotton-wool dipped in 

 warm water. 



On again immersing the leaf and blowing, it will be 

 seen that the air does not come out of the cleaned zone, 

 which is now thoroughly wetted owing to the removal of 

 the bloom. 



When onions are not available the flower stalk of 

 Narcissus answers well. A rubber tube can be slipped 

 over the cut end, and the stalk plunged upside down, 

 flower and all, into a jar of water. The stalk should not 

 be cut too near the ground but where it begins to be 

 hollow. 



1 Archives Neerlandaises, Vol. xv. 



2 Sachs' Physiologic, French Trans., p. 280. 



