

86 



CAUSES 



circumstances as the strength of the mass of pulvinar parenchyma making fo 

 elevation increases, and hence, under normal circumstances, maximal elevation of the prim 



under the same 



r 



y petiole does 



de with maximal strength 



f th 



parenchyma making for 



vation. The strength of the latter 



far as it has to contend with the opposin 



of pulvinar parenchyma, rises under the influence 

 apparently more than discounted by the 



of solar stimulation; 

 ident rise in distal 



but this increase is 



everage 



so that 



* 



gh its strength 



© 



attains an absolute maximum under such circumstances, the 



which it has to overcome 



it is 



relatively weaker in respect to the total resistance 



under the influence of continued absence of photic stimulation. 



The structural peculiarities of the leaves of Mimosa pudica do not, however, merely 

 provide facilities for the occurrence of extensive movements of the primary petiole in 

 connection with variations in the degree of solar stimulation of assimilatory function in the 

 tissues, but also for the ready occurrence of such movements under the influence of various 

 other conditions. The primary petiole is constantly in a position of extremely unstable 

 equilibrium winch is determined by the action of the superior pulvinar parenchyma and the 

 leverage of the distal parts of the leaf on one hand, and of that of the inferior pulvinar 



parenchyma on the other. 



of the opposing masses of parenchyma must inevitably lead to movements of the petiole 



Any alterations in distal leverage or in 



the relative strengths 



in one direction or another. Any increase in distal leverage must tend to induce 

 depression or divergence of the petiole, not merely in connection with coincident com- 

 pression of the tissue of the inferior pulvinar parenchyma, but because of the increased 

 filtrative loss of turgescence which this implies; and any 

 sponding elevation up to the point at which the cells of the inferior parenchyma have 



decreased leverage to corre- 



attained the greatest degree of turgescent increase in capacity of which they are capable. 

 But variations in distal leverage do not take place solely in connection with variations 

 in the conditions of assimilatory activity of the tissues. * Any addition of weight to the 

 distal portions of the leaf whether of intrinsic or extrinsic origin, or any additional 

 pressure on the upper surface of the petiole, imply corresponding increments of leverage, 

 and hence we find that, especially when these are developed suddenly, they are liable to 

 be followed by depression of the leaf. On the other hand, we have already seen how 

 rapid loss of weight in the distal parts of the leaves, as the result of excessive 

 evaporation there, may lead to extensive movements of elevation. 



The tis-ues of the primary pulvinus not only provide facilities for the occurrence 

 of extensive movements in the primary petioles, but they are further especially adapted to 

 permit of these movements being executed with great rapidity under certain circumstances. 

 The fibro-vascular bundle is permanently weak, soft and flexible, and the mass of pulvinar 

 parenchyma which underlies it presents excessive structural facilities for rapid filtrative 

 redistribution of large masses of fluid within its deeper stratum, with its thick-walled 

 and elaborately pitted cells and great system of intercellular spaces. We have already 

 seen that rate of movement is directly related to the degree in which the pulvinar tissues 

 present structural peculiarities favouring rapid alterations 



m turgescence, 



and that the 



tissues in the primary pulvini (and specially in those portions of parenchyma underlying 

 the fibro-vascular axis of the primary pulvini of Mimosa pudica) are endowed with such 

 peculiarities in exceptionally high degree. The superior mass of pulvinar parenchyma, 

 aided by the leverage of the distal parts of the leaf, is constantly making strongly for 



depression of the primary petiole. 



Elevation is only maintained by means of active 



