50 



PLANT EELATIONS. 



to the danger. Perhaps the most comijletely 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, KiO). The leaves are divided 

 into very numerous small leaflets, sometimes very small, 

 Avliich stretch in pairs along the leaf branches. When 

 drought approaches, some of the pairs of leaflets fold to- 

 gether, slightly reduc- 

 't L| ing the surface expo- 

 sure. As the drought 

 continues, more leaflets 

 fold together, then still 

 otliers, 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 apj)roaches, until finally nothing is exposed, 

 and the vessel weathers the storm by presenting only bare 

 ])<ili_\s. Sensitive plants can thus regulate the exposed sur- 

 face very exactly to the need. 



Sueli motile leaves not only behave in this manner at the 

 eoming of drought, but the positions of the leafiets are 

 shifted throughout the day in reference to light, and at 

 night a very characteristic position is assumed (see h'igs. •,>, 

 3, 4:i), once called a " sleeping position. " Tlie danger from 

 night exposure comes from the radiation of lieat which 

 occurs, which may chill the leaves to the danger point. 

 The night position of tlie leaflets of Oxalis has been re- 

 ferred to already (see §14). Similar clianges in the direc- 

 tion of the leaf jflanes at the I'oniing of night may be 

 observed in most of the Lei/iimiiiosw., even the common 



Fig. 43. CoLyledoiis of squash 6ee(lliii<r, show- 

 ing positions in light (left ligure) and iu 

 darkness (right ligure). — After Atkinson. 



