124 HOUSE, GARDEN, AND FIELD 



that the epidermis here becomes separated a little from 

 the tissues beneath, and that the white colour is due to 

 reflection of light from an extremely thin layer of air. 

 Buttercup leaves and many others show white patches 

 of the same kind. When the leaf is dipped into hot water, 

 the air is expelled, and the white patch disappears. 



Of what advantage is the trefoil leaf to the clover ? 

 Watch the leaves at sundown, and you will see that they 

 fold up when there is no more sunlight to be absorbed, and 

 when radiation of heat to the cold sky is to be feared. The 

 division of the leaf into distinct leaflets facilitates the 

 operation of folding ; two of the leaflets droop until they 

 become vertical, one edge being turned towards the 

 ground, and the other towards the sky ; then the third 

 leaflet folds over the other two, and forms a ridged roof 

 above them (Fig. 31). Instead of broad surfaces, only a 

 single edge, answering to the midrib of the central leaflet, 

 is turned towards the sky, and the radiation of heat is 

 checked in proportion as the radiating surface is diminished. 



The Darwins l ascertained by experiment the effect of 

 preventing leaves from going to sleep on a clear, cold 

 night. The leaves of a variety of plants were pinned 

 open on sheets of cork, or otherwise forced to remain in 

 the horizontal position. Many were hurt, and some 

 killed, while others, whose movements were not impeded, 

 either escaped, or at least suffered much less. Clover and 

 wood sorrel leaves, when pinned open, condensed large 

 drops of dew, a proof that they had become chilled, while 

 those which were unconstrained remained perfectly dry. 



How are the drooping and erection of the leaflets of 

 clover effected ? If we look carefully at the meeting- 

 place of the three leaflets, we shall see a kind of cushion, 

 and just beyond it three short cylindrical stalks (Fig. 32). 

 Part of each cylindrical stalk is different in texture from 

 the rest ; it is glossy, semi-transparent, and transversely 

 wrinkled. Here is the organ of movement for the leaflet. 



1 Movements of Plants, chap. vi. 



