

72 THE CAUSES OP FLUCTUATIONS IN TUEGESCENCE 



amount of rotation, seeing that at an early stage of development the upper surface 

 of the primary rachis faces directly inwards to the axis, just as it does in the case 



of Pithecolobi 



o 



Where the growing shoots are either erect or more or less ascendin 

 „ they are in Pithecolobium saman, simple divergence, or divergence accompanied by a limited 

 amount of rotation of the primary petiole, will suffice to cause the upper surfaces of 



the primary rachises to face directly upwards ; but where the axis is more or less 



horizontal, it is clear that simple divergence could only produce such an effect in 



the case of petioles which originally faced directly downwards, and then only when 

 earned out' in very excessive degree, and that in all other cases rotation (and 

 frequently excessive rotation) must necessarily come into play. The ultimate 

 position of the leaves is not, however, entirely due to rotation of the petioles, 

 but also to the occurrence of a certain amount of spiral movement in the 

 growing axis, which in the course of development undergoes a sort of uncoiling, 

 so that eich successive leaf at a certain period comes to be more or less laterally 

 situatBd. A mass of pulvinar parenchyma is gradually developed over the outer face of 

 the tase of the petiole. The accumulation of tissue is not, however, uniformly distri- 

 buted, but predominates on, that side which is for the time being the inferior one; 

 and, with this, rotation of the petiole gradually sets in aud advances through a 

 quadrant, so that the originally inferior aspect comes to be axillary, and its structurally 

 axillary one comes to look directly upwards. But, owing to this rotation, the predomi- 

 nant mass of pulvinar parenchyma which was originally inferior necessarily assumes an 



axillary position, and consequently makes for divergence in place of lateral elevation 

 of the petiole. In the mature leaves a great mass of axillary pulvinar parenchyma 

 is very conspicuous; this, however, does not belong to the structurally axillary nice of 

 the petiole, but to that lateral aspect of it winch originally was directed downwards. 

 The extreme inequa I ity ■ in the development of pulvinar tissue in respect to the two 

 sides of the vascular axis of the pulvinus, together with the weakness of the latter, 



is illustrated in Plate III, Figs. 5, 6 





Whilst periodic movements are relatively inconspicuous in the primary petioles, they 

 are highly developed and very persistent in the case of the secondary rachises and 

 pinnules. As in other instances, they are primarily absent ; but from the time at 

 which they appear, they normally continue to be carried out with unimpaired vigour 

 during the entire period in which the leaves retain a healthy character. In this 

 respect the leaves resemble those of Mimosa pudica much more closely than do any of 

 the leaves which have hitherto been considered. Before the period at which periodic 

 movements set in, the secondary petioles are primarily directed upwards and forwards 

 from the line of the primary one, and are so closely convergent as to have their inner 



faces in contact with one another (Plate III, Fig. 7). Somewhat later they become 



somewhat divergent whilst still retaining their ascending direction (Plate III, Fi<z;. 7). Both 



divergence and elevation next undergo progressive diminution ; and, immediately prior to the 

 onset of periodic movements, the secondary petioles are once more closely convergent 

 to the Hue of the primary one, and lie in the same plane with it or slope slightly 

 downwards from it. When periodic movements do make their appearance, they give 

 rise to alternating nocturnal convergence and depression and diurnal divergence and 



elevation to the plane of the petiole. The pinnules, like those of Ht/iecoiobhtm 

 saman and Mimosa pudica, are from the outset fully expanded and, until the onset 

 of periodic movements, are directed forwards in a line parallel with that of the 



•* 



