MOTOR 



63 



occurring 



in excess m 



continues to undergo diurnal and nocturnal fluctuations so long as the leaf retains its 

 vitality unimpaired, these are no longer accompanied by displacement! of tho pinnae, 

 because they now affect the entire pulvinus alike in place of 

 particular portions of it. 



The movements of the pinna; in Cassia alata, although so extensive and complicated 

 when at their maximum, are always carried out comparatively slowly. Owing to this we 

 find that the leaves have hardly begun to show appreciable displacement of their pinme 

 at a time in the evening when those of Mimosa pudica, Lcuccena gfauca and I* it hccolobium 



sanian have already completely assumed the nocturnal position. Correspondin 



rr 



they 



hardly show any appreciable movement under disturbances by wind or rain which suffice 



to cause complete movements in the leaves of these plants; and, on 

 from the axis, in place of immediately showing 



being 



detached 

 the effects of arrested water-supply as 

 the leaves of Mimosa pudica and Leucama glauca do, they only slowly and gradually 

 assume the nocturnal position. This retardation of movement is the consequence of the 

 structural features of the pulvinar tissues. The masses of pulvinar parenchyma are 

 dense, the intercellular spaces are very small, and the cell-walls are provided only with 

 few and small pits, and consequently any filtrative escape of fluid from the turgid cells, 

 whether it be of a normal character or the result of loss of osmotic capacity in the 



or of rise in external pressure, can take place only gradually and slowly. In 

 order, therefore, to give rise to any appreciable movements, tho causes of increased 

 filtration or of obstructed water-supply must continue to act for a much 

 than it is necessary that they should act in cases where filtrative facilities are present 



cell 



sap 



longer 



time 



in 



high degree. 



The pinnae of Cassia sumatrana also exhibit nyctitropic movements 



which, although 



neither so complicated nor so persistent as those of Cassia alata, are very conspicuous 

 during a certain period in the life of the leaves. Fully developed leaves have from 

 nine to twelve pairs of pinnae, which differ from those of Cassia alata in certain 

 important respects. In place of being practically sessile, they are provided with distinct 

 secondary petioles measuring 0*12" X 004", and they never possess conspicuous laminar 



expansions of pulvinar tissue (Plate III, 



Fig. 



4). 



Nyctitropic movements are verv 



extensive in young leaves after they have attained a certain degree of development; 



but they very soon 



diminish in amount, altering 



in character as they" do so, and are 



practically absent during the greater part of the life of the leaves. 



Owing 



to the very 



transitory character of the movements, the nocturnal positions of the successive pairs of 

 pinnae in one and the same active leaf are by no means so uniform as they are 



exhibiting 



the 



in the case of Cassia alata, the distal, latest-developed pinnae often 



maximum of movement at a time when the movement in the basal ones has already 



undergone great diminution. As usual, nyctitropic movements are entirely absent up to 



a certain period; after this they begin 



to manifest themselves with rapidly 



increasing 



intensity, very soon attain a maximum, and then rapidly decrease and disappear. 



In pinnae in the earlier 



stages 



of development the halves of the laminae 



are 



permanently folded up and in close contact with one another, and the midribs are 

 slightly divergent and somewhat elevated from the plane of the common petiole, their 

 keels at the same time facing directly outwards, so that the edges of the folded laminae 

 of opposite pinnae are in close relation to one another (Plate III, Figure 1). At this period 

 there is no evidence of any accumulation of pulvinar tissue in the secondary petioles. 



i 



