50 



PLANT RELATIONS. 



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 

 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." One danger from 

 night exposure may come from the radiation of heat which 

 might chill the leaves too much ; but the night position 

 may have no such meaning. The leaflets of Oxalis have 

 been referred to (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 



Fig. 43. Cotyledons of squash seedling, show- 

 ing positions in light (left figure) and in 

 darkness (right figure).— After Atkinson. 



