82 THE CAUSES OF FLUCTUATIONS IN TURGESCENCE 



transpiratory loss in weight and leverage of the pinna?, and this is of course equivalent 

 to increased relative strength in the portions of the primary pulvinus which make for 

 elevation. 



The excessive elevation of the primary petioles during the later hours of the night 

 is owing to a similar disturbance in the relations existing between pulvinar strength and 

 distal leverage. The diminution in distal leverage in this case is not, however, owing 

 to exce»ive transpiratory loss in the distal parts of the leaves, but to continued absence 

 of assimilation and coincident continued respiratory loss. The position of the primary 

 petioles at sunset is no doubt partially determined by the fact that the inferior mass of 

 parenchyma contains an excess of tissue liable to rapid filtrative loss of fluid on the 

 cessation of the active assimilatory formation of unstable osmotic products consequent on 

 removal of solar stimulation, whilst transpiratory loss continues to proceed, and a certain 

 amount of departure from the maximal nocturnal position might naturally be expected to 

 occur under the influence of cessation of transpiratory loss and continued root-supply. The 

 departure would not, however, proceed so far as it does and lead to maximal elevation just 

 before sunrise, were it not for the steady decrease in weight of the distal portions of the 

 loaf dependent on continued respiratory loss; for the hours of darkness correspond to a 

 period of fasting during which continuous expenditure takes place without corresponding 

 addition of new materials derived from extraneous sources, and consequently loss in weight 

 must occur. On renewed solar stimulation the pulvinar tissue increases in turgescence and . 





absolute strength, and the inferior mass of parenchyma undergoes relative increase in 

 strength in respect to the superior one, but not in proportion to the increased weight of 

 the distal parts of the leaf which takes place in consequence not merely of their increased 

 turgescence, but to the gradual accumulation of fixed products of assimilation which takes 

 place within them under the influence of sunlight. Maximal elevation can thus no 

 longer be maintained, but progressive descent sets in and attains a maximum when the 

 removal of solar stimulation occasions general loss in turgescence at a period when the 

 accumulation of the fixed products of active assimilation has attained its maximum. The 

 accumulation of assimilatory products in the pulvinar tissues does not render them more 

 powerful to resist leverage save in so far as it affects the osmotic properties of the 

 cell-sap; but the accumulation within the distal parts of the leaf renders them heavier, 



and therefore more resistant to the elevating action of the inferior mass of pulvinar 

 parenchyma. 



No such anomalously excessive departure from the primary nocturnal position occurs 

 in the secondary rachises and pinnules. The reversion towards the nocturnal position 

 exhibited by them during the later parts of the night is limited in extent and is of 

 precisely the same nature and origin as that occurring in nyctitropic structures generally. 

 It is owing simply to the fact that, on the removal of solar stimulation, greater filtrath 



loss of fluid takes place from those tissues which make for the diurnal position than 

 irom their structurally stronger opponents during the period in which 



pirati 



continues, and that during the succeeding period of continued absence of transpiration 

 and uninterrupted root-supply all the tissues alike have the opportunity of fully satisfying 

 the osmotic capacity which they possesa apart from solar stimulation, which implies a 

 certain amount of increase in relative strength in the tissues which make for the diurnal 

 position and which had previously suffered most from filtrative loss. But, in the case 

 ot the primary pulvini, this occurs during a period in which the tissue which gains in 



