LIGHT AND HEAT 



71 



a position which tends to check transpiration. The 

 stimulus causing the leaflets to move is the withdrawal 

 of light. The vertical position is also a protection against 

 excessive radiation and possible injury by frost. Radia- 

 tion is greater from a horizontal than a vertical surface ; 



FIG. 23. HORIZONTAL SHOOT OF MAPLE, WITH LEAVES ARRANGED IN 

 ONE PLANE. (REDUCED. AFTER KERNER.) 



there is therefore less risk of excessive cooling when the 

 leaves are in the night-position than if they remained 

 spread out as they are during the day. ^ 



Similar movements take place in flowers. The flower- 

 heads of many Compositse close at night, and only open 

 during certain hours of the day, when the weather is fine 



FIG. 24. LEAF OF WOOD -SORREL. 

 1, night -position ; 2, day-position of leaflets. 



and warm and the pollinating insects are about. In dull, 

 cold weather they remain closed. By this means pollen 

 and honey are protected from the rain, as well as from 

 marauding insects not engaged in pollination. 



The Effect of Light upon Growth. If potatoes are 

 allowed to sprout in darkness, not only is chlorophyll 



