60 



PLANT STUDIES 



to tlio 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 Ije taken as a type of 

 such plants (see Figs. 4, 41, 171). The loaves 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 iinally 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 approaclies, until flnally nothing is exposed, 

 and the vessel weathers the storm by presenting only bare 

 poles. Sensitive plants can thus regulate the oxjiosed sur- 

 face very exactly to the need. 



Such motile leaves not only behave in this mannei' 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 h'igs. 2, 

 3, 43), once called a " sleeping position." 'I'lie danger from 

 niglit exposure comes from the radiation of heat which 

 occurs, wliii'li may chill the leaves to the danger })oint. 

 The night position of tlu! leaflets of O.niJis has l)een re- 

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

 tion of the leaf planes at the coming of night may be 

 observed in most of the Leguminosa;, even the common 



Fig. 43. CotylcdoHB of Bqliasli SL-L-dling, show- 

 ing positions in light (left figure) and in 

 darliness (riglit figure).— After Atkinson. 



