IX THE MOTOR ORGANS OP LEAVES. 



.7 



tissues as favouring or obstructing the access or escape of fluid. Taking ■ any highly 

 turgid cell, in which a great fall in the osmotic capacity of the Cell-nap * ' in, a 



propo 



loss in turgescence must ultimately ensue owing t> the filtrative dischar 



^ 



»~ 



caused by the elastic ivcoil of the cell-walls; but the rate at whicli the litharge 1 

 conducted must be regulated by the facilities which the latter provide! for Ultra' on, 

 and by the degree to which ready amtmmodtlion for the exapmg fluid i> ji cut 



Tin's being so, tho rate at which movements occur ought to b» ir a recognisable relation 

 to the degree of jutting of the cell-wall- and to the opt nn» - of the pulvium parenchyma 

 and direct evidence that this IS actually tho case is not wanting. The movement* which 



are executed by the pinna: of Cassia alata are <• ainly, in so far as magnitude i* 

 concerned, fairly comparable with those occurring in any part of iho leaves of Mimosa 

 l>ndica ; but the rate at which they are conducted in the two cases is very different 

 the movements in the fornn r being v« ry slowly carrii I out :is Ciunpared with thow 

 in the latter. Coinciding with this difference in rate of movement, we fin 1 very 

 conspicuous differences in the structural details of tho pulvini; the pulvinar tissue in 

 Casbia alata being throughout dense and < >mpo d of cells which are provided with 



a limited number of small pits, whilst in Minima put] be* tho de. per strata of the 

 tissue are not only very open in t. \ture, but are composed of dementi chars terised 

 by an abundance of huge pits. (Plate VI, Figs. 3, 4, l&J 



The facilities for filtration mast evidently bear a direct relation to tho extent to which 



the cell-walls are pitted; and where extensive pitting is combined with v« ry cousid* rable 



thickness and strength of the cell-walls elsewhere, as it is in the primary pulvini of 



Mimosa pudica, filtratiee facilities naturally attain a maximum (Plate VI, Figs. 3, 

 12: Plate VII, Fiff. .">). In the case of the movements o. curriug in the leaves of 



)ITC 



Leuccena glauca and Cassia sumatrana, we also find very clear i'videu<<- of a c 

 pondence between rate of movement and textural facilities for filtration. The move- 



ments in Lett" na glauca are relatively rapid, those in Cassia sumatrana comparatively 



slow and coinciding with this we find the pulvinar parenchyma much more conspi- 

 cuously pitted in the former than in the latter case (Plate VII, Figs. 2, 4). 



It is clear that turgescence must be maintained with greater difficulty when 

 filtrative facilities are great than where they are small, and consequently that the 

 stability of any position maintained as the result of the active turgescence of a i. iss 

 of ti-sue must vary inversely with the filtrative facilities which the latter provide-. This 

 being so, we have reasons to conclude that, where filtrative facilities are great, we ought 

 to find conspicuous movements occurring under the influence of conditions which ar 

 incapable of inducing such results where filtrative facilities are relatively small. It 

 need not, therefore, surprise us to find conspicuous movements occurring in the leav< - 

 of Leucama glauca, and even more in those of Mimosa pudica under conditions in which 

 no appreciable movements occur in those of Cassia alata and other similar leave*. A 

 single rou"-h breath of wind will, under favourable circumstances, cause a certain amount 





of alteration in the relative position of the various parts in the leaves of Lrucwna 

 glauca and very extensive alterations in those of Mimosa pudica; but the agitation 

 necessary to secure perceptible displacement in the leaves of Cassia alata or Cassia 

 sumatrana must be frequently repeated. The disturbance to which the leaves are 

 primarily subject by the wind may be the same, and nny in all cases give rise to like 

 increase in the external pressure to which the turgid pulvinar tissues are exposed ; but 

 the amount of fluid auually discharged from the cell cavities under its influence must 



Anx. Roy. Box. Gard. Calcutta Vol. VI. 



