LEAVES 



71 



from the tissues generally. There can be no question that the pulvinar tissues in 

 Mimosa pudica provide much greater facilities for filtrative redistribution of fluid than 

 those of Pithecolobium suman do, and that the lamina- contain an excess of stomal* j and 

 with this we find a corresponding increase in capacity for rapid movement in connection 

 with the incidence of mechanical agencies favouring increased local filtration owing 

 to increased pressure. But yet the assumption of the nocturnal ]M>sition occurs later 

 under normal conditions in Mimosa than in Pithecolobium. The only explanation fat 

 this apparent anomaly must lie either in differences j„ the stability of the osmotic pro- 

 ducts which aro formed under the influence of solar stimulation of the tissue*, or in 

 differences in the degree of protoplasmic stimulation and activity att 

 degrees of illumination in the two cases; and as we find that tho leaves oiMwiosa a t 

 only assume the nocturnal position later, but resume tho diurnal one earlier, than 

 those of Pithecolobium do, and that when once they have begun to show movements 

 in either direction the latter arc executed with relatively great rapidity, there can be 



ending p:i 



little doubt that the second explanation is the correct ono. The activity of tho pro 

 toplasts in Mimosa must apparently be fully called into play and maintained by 

 degrees of illumination which are incapable of producing an equally Stimulant action 

 on those of Pithecolobium. 



In Lcucoena glauca the movements in the secondary rachises and pinnules present 

 a general resemblance to those occurring in the correspoixling parts of tho leaves of 

 Pithecolobium saman ; but from tho outset the primary petioles, although provided with a 

 most conspicuous pulvinus, fail to exhibit any appreciable periodic movements, and 

 in place of these show steadily progressive divergence and rotation. The absence of 



periodic movements is no doubt partially, at all events, connected > ith tho fact that 



the primary petiole is considerably weaker in relation to the other parts of the leaf 

 than it is in the case of Pithecolobium. Jn the latter the primary petiole contribute- 

 about 20 per cent, to the total weight of the leaf, whilst in Leucccna it contribute s 



half as much; and, as ttis difference coincides with corresponding differences in 

 the strength of the vascular axis of the primary pulvinus, it is evident that in Leucama 

 the action of the latter as a factor making for convergence in consequence of its 

 tendency to resume its original convergent position in relation to tho axis will be 

 proportionately feeble. The leaves are ordinarily provided with from four to seven 

 secondary rachises f each of which bears from nine to fifteen pairs of narrow pinnules 

 which are set on at comparatively wide intervals ; so that, when the leaf is in a con- 

 dition of full expansion, each of them is completely isolated from its neighbours, and 

 there is an absence of any overlapping of successive pairs like that which is ordinarily 

 present in the leaves of Mimosa pudica under similar circumstances. 



Owing to the very lax habit of the plant, the growing portions of the shoots 

 are almost invariably more or less horizontally disposed, and it is in connection with 

 this that a necessity for excessive rotation of the primary petioles has arisen. 

 Rotation does present itself in the case of certain leaves of Pithecolobium saman, but 

 the phenomenon is not of constant occurrence as it is in Leucama glauca, and its 

 predominance in the latter is owing to the horizontal disposition of the axis on 

 which the growing leaves are situated. The mature leaflets are practically arranged in 

 two rows, one lying on either side of the axis, and with their surfaces facing directly 

 upwards in the same direction as the upper surface of the axis. This position, however, 

 necessarily implies the antecedent occurrence of a varying, and in most cases excessive 





