NYCTITROPIC MOVEMENTS 683 



to be depressed, even on the cessation of the stimulating day- 

 light, may, in some cases, be short-lived, and in others may 

 continue for a considerable period. The return movement 

 will in the latter case be somewhat delayed. Again, the 

 internal energy which hastens recovery may, in certain cases, 

 bring about the utmost erection of which the leaf is capable 

 at some time earlier than the ensuing morning. Thus, for 

 example, in the response of lliophytum, the highest position 

 (fig. 273) was attained at about 7 A.M., while in the records of 

 these particular Mimosa leaves the corresponding point was 

 reached very much earlier, that is to say at 1 P.M. 



Periodic impulses contributed by the plant as a 

 whole. — In addition, however, to those periodic impulses 

 whose seat is in the responding leaf, there is another concor- 

 dant impulse contributed by the plant as a whole which 

 intensifies the diurnal movement. The plant is subjected 

 during the day to the stimulus of light, which causes con- 

 traction of the exterior tissues, and thereby tends to drive 

 the water inwards. The turgidity of the cortex is thus 

 progressively diminished during daylight. But at night the 

 water which has thus been driven into the central reservoir of 

 the plant will flow outwards. In this also the absorbed 

 energy will play an important part. The rhythmic activity 

 of the cortical tissues being thereby increased, they will suck 

 water outwards from the central reservoir, just as the rootlets 

 suck it from the ground. The result of this alternation of 

 external stimulus during the day, and internal stimulus during 

 the night, will be a periodic inflow and outflow — a diminu- 

 tion and increase of tension — the first half of the cycle being 

 completed in the daytime, and the second half in the night. 



I have shown (p. 46) that the leaf of Mimosa is 

 erected whenever the internal hydrostatic tension is arti- 

 ficially increased, and depressed when it is diminished ; and 

 we have seen how such variations are induced by the alter- 

 nation of day and night, not only in individual petioles, but 

 throughout the plant as a whole. The alternate ebb and 

 flow of the water, from the central reservoir, will thus be 



