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52 THE CAUSES OF FLUCTUATIONS IN TUKGESCENCE 



individual pinna;. At sunset we find the primary rachis becoming extremely converged 

 to the axis, sometimes indeed so much so that the upper portion actually crosses the line 

 of the latter, the secondary rachises converging and sinking and the pinnules once more 

 rising so as to bring their upper surfaces into close contact, and their midribs to 

 assume a line directed upwards and somewhat forwards from the plane of the secondary 

 rachis to which they are attached. Here every part of the leaf from the primary 

 •achises to the pinnules moves, and in doing so, diurnally departs from, and nocturnally 

 reverts to, what was the permanent position at an earlier stage of evolution. 



In the case of Mimosa pudica,. as in that of Pithecolobnm saman, extensive periodic 

 movements take place in all parts of the leaves. Immediately before they begin to occur, 

 the permanent position is as follows: — The primary petiole is widely divergent from the 

 axis, the secondary petioles are closely converged to one another, and either follow the 

 line of the primary one, or ascend slightly from it, whilst the pinnules are, as they have 

 been from the outset, fully expanded, and have their midribs closely conveyed to the line 

 of the secondary rachises and directed upwards and forwards, so that the upper surfaces 

 of the lamina? of the opposite members of the series are in immediate contact. When 

 movements are established, they consist of diurnal convergence of the primary petiole 

 towards the line of the axis, diurnal divergence of the secondary rachises from one 

 another, and diurnal divergence, depression, and a certain amount of rotation of the 

 pinnules, so that their midribs come to be more or less in the same plane with, and at 

 right angles to, the secondary rachises, and their superior and inferior surfaces to look 

 directly upwards and downwards, alternating with nocturnal divergence of the primary 

 petiole from the axis, nocturnal convergence of the secondary rachises to one another, and 

 nocturnal reversion of the pinnules to their embryonic position. The movements in 

 Mimosa pudica and Pithecolohium saman are thus essentially similar in their nature, in that 

 in every part of the leaves they involve a diurnal departure from, and a nocturnal 

 aversion to, a previously permanent position. They differ, however, in detail, because in 

 the one case the latter implies divergence and in the other convergence of the primary 

 petiole in relation to the axis. This in no way affects the question of the nocturnal 

 position being one of reversion, but the difference is at the same time important, because 

 it implies a great difference in the stability of the diurnal position in the two cases. The 

 diurnal position is in any case an unstable one, depending as it does on a temporary- 

 victory of functional over passive structural resistance; and it is evident that in an 

 instance like Mimosa pudica, in winch the diurnal position of the primary pulvinus implies 

 that the mass of tissue making for it must overcome not merely the resistance offered 

 the opposing part of the pulvinus, but also that of the leverage of the distal parts of the 

 leaf, the instability must be greater than where leverage and special rise in turgidity, due 

 to functional causes, co-operate in determining the position. In Pithecolohium the mass of 

 pulvinar tissue in which functional activity preponderates lies above the flexible fibro- 

 vascular axis of the main pulvinus, whilst in Mimosa pudica it lies below it. The diurnal 

 excess in turgescence in the one case makes for divergence and in the other for 

 convergence of the petiole. But the weight of the distal parts of the leaves is constantly 

 making for divergence, and especially so during the day, owing to the increase in 



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gescence and weight which takes place under the influence of solar stimulation of 

 functional activity. In Pithecolobium, therefore, the diurnal relative increi.se in strength 

 of the functionally stronger pulvinar tissue, and the increased leverage of the distal parts 

 of the leaf coincide in making for divergence of the petiole, whilst in Mimosa pudica 































