IN THE MOTOR ORGANS OF LEAVES 



M 



The periodic movements of the haves of Mimosa pudic* are not only typically nveti- 

 tropic in respect of the nocturnal position being one of reversion to a previously permanent 

 one, but also in respect of the phenomenon of the attainment of the maximal nocturnal 

 position during the first part of the night and gradual departure from it subsequently 



quite apart from any photic influence. The amount of reversion towards tho diurnal 



position exhibited by the pinnae and pinnules is comparative^ slight, but elevation of 

 the primary petiole during the later hours of tho night advances so fa that in plants 

 exposed to normal conditions the maximal elevation for the twenty-four hours is attained 

 immediately before sunrise. This at first sight appears to bo an nnomalous phenomenon, 

 but in reality it may be readily accounted for. The maximal elevation which occurs before 

 sunrise is not to be regarded as indicating that the inferior parenchymal ti-ue of the pri- 

 mary pulvinus then attains it maximal turgesconce. and, consequently, its maximal strength. 

 It merely indicates tho presence of relative maximal strength in relation to distal 

 leverage. The inferior mass of pulvinar parenchyma has constantly not merely to eon- 

 tend with the depressant action of the superior one, but also against tho leverage of tin 



distal portions of the leaf. Both of these are subject to variations, and the action of the 

 leverage is so important as an agent in determining the exact position at any given linn 

 that any considerable reduction of distal w< ight may not merely equilibrate a temporary 

 increase in relative strength of the superior ma^s of pulvinar parenchyma, but may well 

 go farther and secure that the entire resistance which the inferior m; > lias to ovk mane 

 shall actually attain a minimum under the circumstaia s. 



The importance ,of distal leverage as a factor in affecting tho posiiiori of the 

 primary petiole quite apart from any alterations in the restive strength of the opposing 

 masses of pulvinar parenchyma, is indicated very clearly by the phenomena which follow 

 amputation of the pinnse of fully expanded haves (vide Appendix A). The first effect oi 

 this normally is to give rise to rapid and excessive depression of the jotiole; but 

 elevation soon sets in and advances so far that the petiole not merely rapidly regains its 

 original position, but becomes much more elevated or convergent to the hue <»f the axis, 

 than it was whilst the leaf was intact. The effects of alterations in distal leverage on 

 the position of the primary petiole are also very conspicuous in cases in which the pinmc, 

 in place of being removed, are merely exposed to condition- determining considerabk 

 diminution in their weight. When a plant, the haves of which are fully expanded, is 

 suddenly transferred to an atmosphere very much drier and considerably warmer than 

 that to which it had previously been exposed, elevation of the pinnules begins almost 

 immediately and advances rapidly until they have completely assumed the nocturnal 

 position. This phenomenon is clearly owing to Loss in active turgeseence connected with 

 sudden and excessive transpiratory loss accompanied by obstructive expansion of the 

 gaseous contents of the water- vascular system. But the primary petioles show no tendency 

 to assume the position proper to the early part of the night. They show no tendency 

 towards depression, but, on the contrary, gradually become very highly elevated. It 

 cannot be imagined that this is the result of any increase in the absolute strength of tin 

 mass of inferior pulvinar parenchyma which makes for elevation. Tin? tissues in the 

 pulviui, whether petiolar or pinnular, must alike be subject to diminution in turgeaeem » 

 and consequent loss of absolute strength under the circumstances to which th< 

 plant is exposed; but the loss in strength in those pulvinar tissues in the primary 

 pulvinus wliich make for elevation is more than counterbalanced by tho exccssiv< 



A.nn. Eoy. Eot. Gaed. Calcutta Vol. VI. 





