2i6 ROUND THE YEAR 



to prevent the drying-up of the leaf during summer 

 heat or parching wind. The stomates, by which 

 water-vapour is transpired, open, not upon a free 

 surface, but into a closed chamber, in which the air is 

 still, being further secured against frequent change by 

 numerous hairs, which project into it and guard the 

 opening. The exposed surface of the leaf bears no 

 stomates, and is overspread by a layer of cork, which 

 renders it impervious to moisture. That Crowberry 

 is really effectively protected against drought there 

 can be no doubt. I gathered a plant in May, and 

 hung it up in my study. After eight days it was still 

 green, and capable of forming new buds when placed 

 in damp earth. I have never known Crowberry or 

 Nardus to perish by drought. 



Kerner, in his Natural History of Plants, has dis- 

 cussed- the possible uses of the rolled leaves of the 

 Crowberry and other plants. He thinks that they 

 keep the evaporating surface dry, and allow air to be 

 exhaled from the leaf even when it is drenched with 

 rain. One explanation does not absolutely exclude 

 the other ; it is conceivable that the rolled leaf may 

 be serviceable under the opposite conditions of too 

 wet and too dry. But I am inclined to think that 

 the leaf would not be rolled up so completely if 

 exclusion of water were the main object. A far less 

 elaborate contrivance suffices in other cases to keep 

 the under-surface of a leaf dry. 



Another consideration inclines me to look upon the 

 rolled leaf as specially a protection against drought 

 and excessive transpiration. The in-rolled, stomate- 

 bearing surface is often notably reduced. Striking 



