i5o 



THE VEGETATIVE ORGANS 



middle leaflet now bends over them, folding the two sides 

 of the blade downwards and exposing the back of its midrib 

 to the sky. The amount of leaf-surface exposed to radia- 

 tion is thereby greatly reduced, and in keeping with this, 

 the lower and more exposed leaf-surfaces contain fewer 

 stomata than the upper and more protected ones. During 

 the day Clover leaves form an excellent mosaic, but at night, 

 when the leaflets are tucked in, the smallness of the space 

 they occupy is very striking. 



Compare with the Clover leaves those of Wood Sorrel 



Fig. 98. White Clover. 1, trifoliate leaf, day -position ; 2, 

 night -position ; in, motile organs ; 3, 4, and 5, inflorescences of 

 Clover : the flowers turn downwards after pollination. 



(Fig. 99) or a common garden Oxalis. In these plants the 

 three leaflets droop at night, hang vertically, apply their 

 midribs to each other, and so expose their upper and 

 protect their under surfaces, to which the stomata arc 

 restricted. They thus secure protection against cold, but 

 by a different method from that of the Clover. If these 

 plants are placed in the dark at midday they do not close 

 their leaves until the proper time ; their habit of going to 

 sleep at definite times has become fixed, and it takes some 

 days for them to become accustomed to changed hours. 

 Such movements are known as sleep-movements. 



The False Acacia (Robinia) furnishes another example 

 of motion in plants, and its leaf-movements should be 



