HUTCHINSON'S POPULAR BOTANY 



FIG. 353. WOOD-SORREL (Oxalis). 

 During the day the leaflets spread out from the leaf-stalk ; 



Fia. 354. WOOD-SORREL. 



at night, and duiing rain, they fold down close to the 

 leaf-stalk. 



dry climates ; the reverse is the 

 case, the transpiring surface of such 

 leaves being much smaller than if 

 they were flattened out into thin 

 and spreading forms. Yet even 

 the smaller surface needs to be 

 protected, and this it is which 

 gives its value to the flinty armour. 



The vertical position which 

 many leaves assume is likewise a 

 means of checking excessive trans- 

 piration. One has met with the 

 expression, " the shadoidess forests 

 of Australia," and the phrase is not 

 inappropriate, for the leaves of 

 many Eucalyptus-trees and Acacias 

 (the chief timber trees in Aus- 

 tralian woods) do not assume a 

 horizontal position like the leaves 

 of most European forest-trees, but 

 are placed vertically on edge ; and 

 thus the shadows which they cast 

 at midday are reduced to a mere 

 line. This, as we need scarcely 

 add, is due to the fact that the 

 rays of light fall upon the up- 

 turned edges of the leaves and not 

 upon the broad surfaces of the 

 blades.* The latter, indeed, escape 

 altogether the meridian sun, though 

 they get the full benefit of his less 

 scorching rays at the beginning 

 and close of the day. 



The interesting Compass-plant 

 (Silphium laeiniatum) should not 

 be forgotten when speaking of the 

 position assumed by leaves in refer- 

 ence to transpiration. Longfellow's 

 Evangdine contains a graceful de- 

 scription of the plant (though he 

 confuses flower and leaf), which 



* This is also a protection from the in- 

 jurious effect on the chlorophyll corpuscles 

 of the intense sunlight. 



