Ill 

 an 



convergence 



to any considerable extent. There is 



90 THE CAUSES OF FLUCTUATIONS IN TUMESCENCE 



this case also is one which is principally maintained between the fibro-vascular tissue 

 d the axillary parenchyma, the external parenchyma from its unstable foundation being 



incapable of making actively for 

 therefore no difficulty in seeing why rise in tumescence in the pulvinar parenchyma, 

 although it be fairly uniformly distributed, should be followed by movements of diver- 

 gence of very considerable magnitude; and there are structural reasons to account for 

 the fact that, as a rule, movements of any kind should be conducted only gradually. 

 Both in their structural features, and in the characters of the movements which they give 

 rise to, the secondary pulvini of Mimosa pudica much more closely resemble the pulvini 

 of common nyctitropic leaves than either the primary or tertiary pulvini do. The 

 amount of tissue favouring exceptionally rapid redistribution of fluid which they contain 

 is trifling in comparison to that present in the primary pulvini, and it is not normally 

 exposed to nearly so much resistance as it is in these. In the primary pulvini there is an 

 abundance of tissue liable to exceptionally rapid filtrative loss in turgescence, and strug- 

 gling with excessive resistance dependent on intrinsic pulvinar conditions and distal lever- 

 In the secondary pulvini both the amount of such tissue and the resistance to 



indeed were the 



age. 



strange 



i 



which it is exposed are very much less, so that it would be 

 capacity for rapid movement, alike in the two cases. So long as the primary petiole is 

 not deeply depressed, leverage does not come into play in making for convergence of the 

 secondary rachises, and with this we frequently find sudden movements of them almost 

 entirely absent, and, as a rule, at the utmost very limited; but when deep depression of 



to act, and a correspondingly 



of the 



distal leverage begins 



convergence 



the primary petiole does occur, 



increased tendency to the occurrence of rapid and considerable 



secondary rachises manifests itself under circumstances leading to decreased turgescence 



in the secondary pulvini. 



The tertiary pulvinus differs as strikingly in structural features from the secondary 

 one as the latter do from the primary. The entire 

 vertically so as to present an 



organ 



is greatly compressed 

 and its fibro-vascular bundle 

 has the form of a broad flattened strip traversing the centre of the parenchyma 



elongated elliptical figure, 



(Plate VI, 



Fig. 



10 



The 



following are 



the measurements of the various parts of 



the section from which the figure was obtained: 



Transverse diameter of entire pulvinus 



• ■ • 



Thickness of parenchyma to the left of the fibro-vascular bundle 



• • • 



Vertical diameter of entire pulvinus 



• • • 



jj 



)> 



" 



>> 



» 



jy 



of superior parenchyma 

 of fibro-vascular bundle 

 . of inferior parenchyma 



# • • 



• • • 



Thickness of cell- walls of inferior parenchyma 



• • • 



?? 



55 



JJ 



of superior 



5> 



• « • 



• • • 



• • ♦ 



• • * 



• • t 



• • 



0735 m.m. 



0-12 



0-48 



Breadth of fibro-vascular bundle 



• • • • • • 



Thickness of parenchyma to the right of the fihro- vascular bundle. . . 0*135 



0-39 

 0-18 

 006 

 0-15 



0-0054 



0-0009 



>> 



55 



55 



>> 



>» 



55 



55 



55 



55 



It is evident that the extreme 



tical 



compression of both the entire pulvinus 

 and of the fibro-vascular bundle provide even greater facilities for vertical flexion 

 than are present in the primary pulvinus. 



The 



with 

 may 



organ 



li 



any excessive 



distal leverage 



not, however, to 



g 



gle 



regard 



its position as determined simply by the 



as the primary pulvinus has, and practically we 



conflicting forces of the infer 



