110 SLEEP OF PLANTS. 
not quit their place of repose until after the heat of 
the solar rays is felt. The same idleness, as it were, 
is indulged by the trees with pinnate leaves, such as 
the mimosas and tamarinds, which close these organs 
half an hour before the sun goes down, and unfold 
them in the morning only after he has been some 
time visible. In our climates the leguminous plants 
open their leaves during the morning twilight. 
Humboldt seems to think that the humidity depo- 
sited upon the parenchyma by the refrigeration of 
the foliage, which is the effect of the nocturnal ra- 
diation, prevents the action of the first rays of the 
sun upon them. 
