130 HOUSE, GARDEN, AND FIELD 



frequently. It is easy to alter the incidence of the light by 

 cards, and to show that the leaf -stalk bends chiefly at the 

 ring. Sometimes the ring is of different depth on different 

 sides, showing that it has become permanently adapted to 

 a particular attitude. Each leaflet has its own special 

 organ of movement, which answers to the wrinkled stalk 

 at the base of each clover leaflet (Fig. 34 a, a'). Just below 

 the ring at the base of the leaf-stalk, is the place where the 



FIG. 33. Stem of wood sorrel, with leaf- buds, flower-bud, bases 

 of old leaves and roots ; also a detached leaf in the sleep-position, 

 and a fruit with its stalk. Slightly enlarged. 



leaf breaks off at the approach of winter ; the leaf-base 

 will then be found to be packed with food-materials, which 

 were formed in the leaflets, passed down the leaf-stalk, 

 and stored up as a supply for the winter. 



The leaf-stalks and flower-stalks of wood sorrel are 

 moderately hairy. The edges of each leaflet are set with 

 a regular row of hairs, and so are the short stalks of the 

 leaflets. It looks as if these hairs would hinder the passage 

 of air through the narrow space between the closed leaflets, 

 and still further diminish the loss of water by evaporation. 



