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the power of independent motion, or the whole leaf may 
move. If there is danger from exposure to drouth, some 
of the leaflets will be observed to fold together ; in case 
FIG. 172. A heath plant (Erica), showing low, bushy growth and small leaves. 
the danger is prolonged, more leaflets will fold together ; 
and if the danger persists, the surface of exposure will be 
still further reduced, until the whole plant may have its 
leaves completely folded up. In this way the amount of 
