HI THE MOTOR OKGANS OP LEAVES. 



07 





CHAPTER IX. 





^Tltc mobcmrnt 



the Icabc 



s 



pthccolobium Salman anb 



^cucacna glauca 



Tho movements of tlio leaves of Pithtcololium taman are distinguish, d from tin 

 occurring in the leaves of cither of tho species of Cassia which have been treat' <1 of in 

 the previous pages by much greater persistence and by the relative rapidity with winch 

 they are executed. The movements of tho primary petioles, although originally very 

 conspicuous, do undergo rapid diminution at a eomparatively early period in tho life 

 of the leaves, and are practically absent latterly; but those f the secondary pitiohs 

 and pinnules are very much more persistent, and it is only in very old 1< uvea that 

 they become inconspicuous. When the movement* are at 



nocturnal positions are of the 



following nature : 



a maximum, the diurnal and 



diurnally, tho primary petiole and 



rachis are very widely divergent from the axis, lying almost at right angles to it; the 



secondary petioles and rachises are also v ry widely divergent from, and )i. almost in the 

 same horizontal piano as the primary ones, and the pinnales are in t lie name plane and 



have their midribs diverging from the lino of the strondajry rachises ;it angl s of 

 rather more than 45 degrees. Nocturnally, the primary petiole is so highly convergent 

 that it not unfrequently actually crosses the line of the axis; tho secondary petioles 

 are converged and so deeply depressed that th y come to point more or 1< hack ward 

 towards the base of the leaf (Plate VII, Fig. 7); the pinnules are folded almost directly 

 upwards, so that the upper surfaces of those of tho opposite sides of the secondary 

 rachises are in close contact, their midribs being directed upwards and forward- at 

 vertical angles with the line of the Secondary rachises of almost the same degree of 

 inclination as that of the diurnal horizontal one; in other words, they have under- 

 gone almost pure elevation apart from any appreciable rotation. 



When dealing with the subject of nyctitropic movements generally, it 

 been pointed out that the nocturnal position in tins case is socially clc 

 reversion to what was the permanent one at a period immediately preceding the 

 time at which periodic movements first begin to make their appearanc , and that tin 

 diurnal divergence of the primary petiole is essentially connected with the gradual 



h 



s 



already 



on 



f 



development of a great mass of axillary pul 



diurnal rise in turg 



the primary pu 



parenchyma. 

 i must, how 



The action of th< 

 er, in this case be 



greatly 



aided in effecting divergence by the coincident increase in turgescence and 

 the position of the 



p udica 

 pulvin 



the distal portions of the leaf, winch imply corresponding increase 

 leverage here aids in establishing the diurnal position, and renders 



f Mimosa 

 piimarx 



it extent 



in distal leverage. The 



it more stable than it would otherwise be, or than 



le 



aves 



ke th 



which distal leverage opposes, in place of aidi 



which makes for the diurnal position ; and to it, no doubt, to a 



the 

 gre; 



is 



d 



the phenomenon of rapid diminut 



d 



sarly disappear 



f movement in 



the primary petiole 



Larg 



leaves are provided with from six to eight secondary 



rachises which bear from three to eight or nine pairs of pi 

 the latter being smallest in tho basal rachises ai 



the 



ber of 



d 



increasing progressively .towards 



Ann Roy. Bot. Gard. Calcutta Vol. \ 



