IN THE MOToR ORGANS OP LEAVES 





secondary petiole, with the upper surfaces of tl of the opposite r«.w> in elost 

 contact with one anotlu r, and their surfaces fating directly outward-, the outer or 



posterior half of the laminae lying; vertically over the axillary half, ami the midrib 



pciuling din ctly forwards. When morements set in tiny had to progr. s>i\el\ 

 incrcu ing diurnal divergence, rotation and depression of the lnmimi which ultimately 



come to Ho in the same plane with the upper surface of the - udary petiole, and 



more or less at right nnglesto it, with their upper surfaces facing directly upward 

 alternating with a nocturnal resumption of the originally permanent p sition, or 



rather of that position slightly modified by the fact th;.t the midribs n< \v, as a rule. m 



*-■ 



longer point diroitly forwards, but eiightly downwards from the line «.l the soeondar\ 

 petiole. The movements of the pinnules thus ditl'er from those in the pimiuh s «.» 

 Pithccohthium taman, and are of the same natuie as those in the pinna- of Oiss,u nlata 

 in consisting not merely of alternating elevation and depression, but of alt< matin 

 elevation and depression combined with a considerable amount of rotation. 



The stiueunal features which determine the charu< tcr of the moTemtnts nl the seeoi I- 

 ary petioles are of the following nature. When an accumulation of pulrinar parenchyma 

 begins to be developed, it appears lirst ove tho up; r half of the base of the -oeondaij 

 petiole; and, together with the increasing loveiage ol the distal parts, n mis to give rise 

 to gradual deprei ion. At a somewhat later period accumulation b< rins over the lower 

 half, and this of course makes directly for elevation. In mature \,ut still freely ntotih 

 pulvini, there is little difference in the thickness of the c.-ll-v ills in the various portions 

 of the parenchyma; but the mass which lies to the axillary side of the axis is distin- 

 guished from the outer one by greater thickness (Date JH, Fig. 9), and by containing a 

 greater number of green chromatophores. It is this, together with the fact that tin 

 axillary masses on undergoing any increase in turgesoence press upon the r< rent rachif 

 in the case of all the pinna but the terminal ones, and against one another in tin 

 latter, that determine* the great diurnal divergence of the secondary petioles. Tin 

 external mass of pulvinar parenchyma is less bulky and poorer in chlorophyll, and 

 consequently undergoes less increase in turg« scenee diurnally than its opp onent ; and, 

 moreover, it is related basally to a mass of spon y tissue very poor in chlorophyll- 

 content (Plate III, Fig. 9), so that any rise in tnrgesce nc e which it do. undergo is in 

 reat measure expended in compressing this feeble tissue. With the absence of an\ 



S 



conspicuous structural differences between the -uperior and inferior parenchyma, and tm 

 presence of a relatively strong vascular axis (Plate HI, Fig* 8, 9), whirl, presents very 

 considerable resistance to both vertical and horizontal flexion, we find the amount of Ver- 

 tical movements becoming very inconsiderable. In Villmohhhim the secondary radii sea in 

 leaves at the stage in which movements are at a maximum are noctuiually directed back- 

 wards at acute angles to the line of the primary one; but in Leuoma they never 

 attain to a right angle to it, but are invariably directed forwards at % iy acute angle 



towards its apex. The really important struggle is that which is carried on between 



the strong vascular axis of the pulvinus and the mass of parenchyma lying to its 

 axillary side, and winch is diurnally able to divert it from its normal pa-ive direction 

 The axillary parenchyma, from its portion, its greater bulk, and its g 



chlorophyll, is able not only to do away with any effects of diurnal rise in fan 



sc< >n ( 



1U the external pad, but also to cause extreme divergence of 



In the ca«e of highly developed pulvini the vascular axis is reduced to the form of 

 soUd cord containing no medullary tissue. Where tins has a cylindrical form, as in ti 



Ann. Koy. Bot. Gard. Calcutta Vol VI 



