* 



r 



63 



THE CAUSES OF FLUCTUATIONS IN TUEGESCENCE 



of the leaf. In the case of five large leaves in which the actual 



the distal portions 



number of secondary rachises and pinnules was 



twelve to sixteen, and 



of the latter f 



ted, that of the former varied from 

 ity-eight to one hundred and four? 



y 



ely indicates that the amount of leverag 



telling on the 



primary 



which ve 



pulvinus in favour of divergence must be very 



that, in 



parenchyma (Plate I, Fig. 14), it not unfrequently leads to the leaf ultimately becoming 



permanently directed more or less backwards towards the base of the axis. In the 



operation with the great prog 



considerable: and so consideiable is it 

 development of the mass of axillary 



case of the primary petiole and rachis . th 



is 



al reversion to a previously 



permanent position, only slightly modified by a certain amount of increase m con- 

 vergence ; but in that of the secondary ones considerable modification is present owing to the 

 fact that for some time the amount of nocturnal depression goes on steadily and 

 progressively increasing. The nocturnal position of the pinnules, on the other hand, is 

 one of pure reversion, and the alternate movements of diurnal depression 



and 



al elevation which they undergo are the exact parallels to the alternate unfolding 



and folding of the laminae of 

 appreciable rotation similar to that 



young pinnae of the spec 



Cass 



T her 



is n o 



occurring 



in 



th 



case of these pinnae, becaus 



the 



opposing masses 



of 



pulvinar tissue making respectively for the diurnal 



an 



el 



nocturnal positions lie directly super 



d inferior, and are related to the upper and 



der surfaces of both halves of the lamina alike. The inferiorly situated mass of 



parenchyma : 

 action of the 



aided 



m giving rise 



to nocturnal elevation of the 



pmnu 



axis of the pulvinus, which constantly tends to resume 



by the 

 original 



direction to the secondary petiol 



magnitude that the 



It is thus only after it has attained considerable 



superior mass of pulvinar parenchyma is able to overcome the 



resistance of the inferior one and of the vascular bundle; and, even when it has 



5 de- 



attained its maximum development 



P 



of the 



increase in bulk 



pmnuh 

 of th 



to the 



it is, as a rule, incapable of 

 e of the secondary petiole, 



i 



nferior one, which takes 

 16 and 17 of Plate I. 



superior mass of pulv 



securing complete 

 The great relative 

 parenchyma as compared with the 



during maturation of the leaf 



is 



d 



in Fig 



The 



■ 



pposed masses of parenchyma 



tl 



posed altogether superiorly and inferiorly; but yet 



secondary pulvini 



lly (1 



are not movements of 



and of ele v 



vergence, 

 which the 



pie depressioi 

 i and diverg 



movements which actually occur 



and elevation, but of dep 



and 



con- 



This is owing to the peculiar relation 



pu 



nected (PI 



bear to the bevelled petiolar surfaces with which 



IV. . Fi 



& 



The 



they 



are 



superior mass of pulvinar parenchyma, as the figure 



shows, is so situated as to lie so much above the level of the bevelled surface of the 



primary r 



achis th 



act appreciably 



any alterations in its relative strength and 



giving rise to movem 



of elevation 



P 



can 



nly 



ferior pad, which is 



d depression : but 



e 



ally in close contact with the bevelled surface 



diverg 



turgescence must tend not merely to gi\ 



rise to elevation, but 



i 



any 



4 



► to 



of the secondary petiole. In the normal passive position, as determined 



by purely structural feature 



the 



almost parallel with th 



al vascular bundle of the . secondary pulvinus lies 



line of the primary petiole 



and 



any rise in turg 



the axillary portion of the inferior mass of pulvinar parenchyma must tend 



ce m 

 diver- 



gence, 



o win g 



to the relation which the 



of 



another. The movements 



are not in 



th 



case 



the rachis and pulvinus bear to one 

 complicated by the presence of any 





I 



t 



