] 5 cunmnguam ON FLUCU' ATIOXS is tlkgkscence. 



cf the leaves of seedling plants are for some time very feebly developed, there being only from one 

 to two piumn present, and these bearing only a very few pain of pinnules, so that distal leverage is 

 probably much leu influential in them than it is in fully developed leaves with their four pinnae and 



multitudinous pinnules. 



The phenomena whioh are presented by the leaves of Mimosa pudica, when grown under normal 

 tropical conditions, and especially when grown in the open ground so as to permit of indefinite 

 extension of root-surfaoe, certainly render it very hard to imagine how a belief in the occurrence 

 of deep depression of the primary petiole as a normal nyctitropic phenomenon should have arisen. 

 Had it been present, it would have been quite anomalous and a striking example of an exception to 

 the rule that the position of the various parts of nyotitropio leaves during the earlier part of the night 

 s one which was either the permanent position in very young leaves or the diurnal position in some- 

 what older but still immature ones. But, as a matter of fact, deep petiolar depression does not occur. 

 And the slightly ascending, horizontal, or slightly descending position which really does occur affords 

 ■ striking example, not of deflation from, but of conformity to, that rule. The position, 



however, even in plants with unlimited oxtension of root-surface, is normally an extremely unstable 

 ono owing to the coincident depression of osmotic capacity within the pulvinar tissues at a time 

 when they halt still to contend with very considerable distal leverage ; and hence very slight 

 disturbances, which are quite incapable of inducing deep depression in unloaded leaves exposed 

 to solar stimulation, readily suffice to give rise to it now. As has been pointed out in Appendix 



0i thl diftarbtnee in tht liquid-equilibrium in an axis which attends the amputation of its distal 



extremity gives rise to much more constant and extensively propagated movements of petiolar 

 depression when it occurs after the leaves have assumed the initial nocturnal position than when it 

 oomes into play whilst they are still subject to solar stimulation and in the maximal diurnal one. 

 Fn is, perhaps, affords an explanation of the origin of the belief ; for it is conceivable that deep petiolar 

 depression may spoi aneously occur in the case of pot-plauts with limited root-surfaces and exposed to 

 abnormally dry nocturnal atmospheres, the limitation of supply of water and continued active trans- 

 piratory loss co-operating to play the part of slight mechanical disturbances or other extrinsic 

 agencies disturbing the liquid-equilibrium of the tissues. Even if this be the case, however, it in no way 

 affects the faot that, under normal ciroumstauoes, deep depression of the petiole is not a normal nycti- 

 tropic phenomenon. 



II.— That when the primary petiole is deeply depressed during the day in consequence of 



"stimulation," its leaf hangs hose and pendulous; whereas when in the same position 



during the early part of the night it is rgid 

 It is hardly necessary to discuss this question, seeing that, in plants grown under normal 



circuni- 



stance*, the position of deep depression of the petiole, whether it arises diurnally or nocturnally, makes 

 its appearance under precisely similar circumstances and is induced by the same factors. It 



however, be poiuted out 



the leave! 



regard 



may 



loose and pendulous when petiolar depression has arisen diurnally in consequence of 



the action of any so-called ** stimulant " agencies 



III.-That sudden exposure to absolve darkness, apart from any other disturbing conditions, 



is attended by the occurrence of sudden movements in the leaves. 



The following notes show the nature of the results of experiments in which plants whose leaves 



were m maximal diurnal position, were suddenly introduce. J«J *l v i * 7? P ' 



a u i-it.1,1 > ow auuueujy introaucea into the absolute darkness of a photographic 



dark-room in which black velvet curtains covered avpti tl, ft om „n : a <• , puowgrapiuc 



i t i- x.l ,. us covered even the small windows of cobured class affording the 



only source of light under ordinary circumstances:— g g 



" f^T f five min » te8 ' du«tion »o appreciable change in the 



irrea. Alter an exposure of fifteen minutes a certain 



VIII—After 



position of the various 



Sachs Vi>rlesun K eu ; Vorlesung, XXXVII. s. Utk 









