70 



FLUCTUATIONS IN TURGESCENCE 



of the inferior one are. There is thus ample provision for a relative^ excess of 

 diurnal tumescence in it, and this, in co-operation with the diurnal rise in leverage 



in the distal parts of the leaf accompanying solar stimulation, is quite sufficient to account 

 for the occurrence of diurnal divergence. The inferior mass shows no distinct evidence 

 of any considerable excess in relative structural strength; but it is aided in its tendency 

 to give rise to convergence by the strength of the axial mass of wood and medullary 

 tissue, and consequently nocturnal convergence is for a time extreme. The wood ap- 

 parently plays a much more important part here in determining the assumption 

 of the nocturnal position than it does in the case of the secondary and tertiary 

 pulvini. The struggle in the primary pulvini is, to a great extent, not so much between 

 the opposed masses of pulvinar parenchyma, as between the superior mass which tends to 

 cause divergence and the wood which makes for convergence in the effort to regain its 

 original direction in relation to the axis. For a time the nocturnal loss in turgescence 

 in the superior mass of pulvinar parenchyma, and the coincident diminution in leverage 

 of the distal portions of the leaf, allow the strong woody tissue to resume its original direction 

 to the axis, and even to pass beyond it owing to the adjuvant action of the inferior 

 parenchyma; but the movement rapidly diminishes and ultimately disappears with the 

 continued increase in bulk and structural strength of the superior parenchyma, leaving 

 the primary petiole in a permanent position of extreme divergence or even more or 

 less inclined backwards from the line of the axis. So long as the movements continue 

 to occur, they lead very conspicuously to that assumption of the maximal nocturnal 

 position during the earlier part of the night, followed by gradual departure from it 

 in spite of continued absence of solar stimulation, which, as has already been pointed 

 out, is so characteristic of nyctitropic movements generally. 



The pulvinar tissues in Pithecolobium are relatively dense (the system of intercellular 

 spaces being comparatively limited), but pitting of the cell-walls is very much more conspi- 

 cuous and abundant than it is in the pulvini of either of the species of Cassia (Plate VII. 



Figs. 3, 8), and with this we find a capacity for much more rapid movements in the 

 leaves. At a period in the evening at which leaves of the species of Cassia are only 

 beginning to show indications of departure from the diurnal position, those of Piiht* 

 colobium saman are already in the fully developed nocturnal one. This may be partially 

 due to the greater facilities for active transpiration which they present in connection with 

 their great excess in stomata; but the fact that similar phenomena present themselves in 

 connection with continued agitation by showers of violent rain indicates that it is mainly 

 determined by textural facilities for redistribution of liquid. An amount of agitation by 

 wind or tropical showers winch hardly produces any appreciable effects on Cassia alata, 

 suffices to produce maximal movement in the secondaiy petioles and pinnules of Pithe- 

 colobium. The total amount of displacement involved in the transition from the diurnal 

 to the nocturnal position in both cases alike is very great ; but the rate at which it can 

 be effected is very different owing to the presence of structural peculiarities, which in 

 the case of Pithecolobium are of a nature to permit of redistribution of fluid by 

 nltrative discharge from the cell-cavities of the pulvinar parenchyma taking place much 

 more rapidly than is possible in the case of either of the species of Cassia, 



In so far as strictly nyctitropic movements-movements dependent on fluctuations 



m turgescence, determined by variations in degree of solar stimulation-are concerned, 

 another factor, however, comes into play besides the mere structural facilities for the 

 escape of fluid from the cell cavities of the motor organs, mid for transpiratory loss 









