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 ; 
Fie. 23.—HorizontaL SHoot or MAPLE, witH LEAVES ARRANGED IN 
OnE PLANE. (REDUCED. ArrER 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. 5 
Similar movements take place in flowers. The flower- 
heads of many Compositz close at night, and only open 
during certain hours of the day, when the weather is fine 
Fic. 24.—Lzear or Woop-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 
