WOOD SORREL 127 



should not wood sorrel adopt the method of closing of 

 clover, or clover that of wood sorrel ? I can only answer 

 these questions doubtfully and imperfectly. Clover, when 

 it goes to sleep, seems to be a little better screened from 

 the cold than the wood sorrel. Clover exposes little more 

 than one leaflet to the cold air ; wood sorrel all three 

 leaflets. Clover turns only one midrib towards the sky ; 

 wood sorrel the edges of all three leaflets, though in a 

 sloping position. Clover inhabits perfectly open situa- 

 tions, while wood sorrel is overshadowed by trees. It 

 would appear, therefore, that the leaf which, under 

 ordinary summer conditions, is most severely tried is 

 most completely protected. 



Clover, in taking the sleep-position, turns its proper 

 under-surface outwards ; wood sorrel turns the upper 

 surface outwards. This difference is perhaps connected 

 with a difference in the position of the stomates. In 

 clover the stomates are distributed over both surfaces of 

 the leaf, being more numerous, though smaller, on the 

 upper surface, while in wood sorrel they are restricted 

 to the under-side, which is concealed when the leaflets 

 droop. Hence wood sorrel, when it folds its leaves, screens 

 all its stomates, clover only about half of them. All this 

 accords with what we know in other ways, viz. that wood 

 sorrel is particularly sensitive to drought, while clover 

 endures drought very well. 



There is a general rule as to the sleep-position of leaves, 

 and clover observes this rule, while wood sorrel breaks it. 

 Any common leaf, which has a flattened shape, and takes 

 a more or less horizontal position by day, has its two faces 

 adapted to different functions. The upper face is crowded 

 with chlorophyll -corpuscles, and takes a darker shade of 

 green in consequence ; it is often protected from the 

 weather by a glossy cuticle. This surface is specially 

 adapted for the assimilation of carbonic acid ; we may 

 call it the assimilating surface. The under face of the 

 leaf is often hairy, or in other ways rendered unwettable ; 



