50 
PLANT STUDIES 
to the danger. Perhaps the most completely adapted 
leaves of this kind are those of the "sensitive plants," 
whose leaves respond to various external influences by 
changing their positions. The common sensitive plant 
abounds in dry regions, and may be taken as a type of 
such plants (see Figs. 4, 41, 171). The leaves are divided 
into very numerous small leaflets, sometimes very small, 
which stretch in pairs along the leaf branches. When 
drought approaches, some of the pairs of leaflets fold to- 
gether, slightly reduc- 
ing the surface expo- 
sure. As the drought 
continues, more leaflets 
fold together, then still 
others, until finally all 
the leaflets may be 
folded together, and the 
leaves themselves may 
FIG. 43. Cotyledons of squash seedling, show- bend against the stem. 
It is like a sailing vessel 
gradually taking in sail 
as a storm approaches, until finally nothing is exposed, 
and the vessel weathers the storm by presenting only bare 
poles. Sensitive plants can thus regulate the exposed sur- 
face very exactly to the need. 
Such motile leaves not only behave in this manner at the 
coming of drought, but the positions of the leaflets are 
shifted throughout the day in reference to light, and at 
night a very characteristic position is assumed (see Figs. 2, 
3, 42), once called a " sleeping position." The danger from 
night exposure comes from the radiation of heat which 
occurs, which may chill the leaves to the danger point. 
The night position of the leaflets of Oxalis has been re- 
ferred to already (see 14). Similar changes in the direc- 
tion of the leaf planes at the coming of night may be 
observed in most of the Leguminosce, even the common 
ing positions in light (left figure) and in 
darkness (right figure). After ATKINSON. 
