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66 . the CAUSES OP FLUCTUATIONS IN TUBGESCENCE 



of the rachis, the diurnal and nocturnal positions of all the individual pinnae 

 inT one and the same leaf are not uniform during the greater part of the period 



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w hich appreciable movements are present. This is clearly illustrated by the 

 following notes regarding the nocturnal positions of the pinnae in a single leaf. All of the 

 pinna? were closely convergent to the line of the petiole and depressed beneath it. In the 

 two terminal pairs depression was not so great as in the succeeding ones, and the halves 

 of the lamina were somewhat folded upwards. In the next four pairs depression and 



were extreme, and the lamina? were fully expanded, and had their upper 

 surfaces facing directly forwards to the tip of the leaf. In the next pair the upper 

 surfaces of the laminae were slightly inclined inwards, and in the succeeding three 

 pairs they faced almost directly inwards. In the two basal pairs depression and 

 rotation were already somewhat diminished, and the upper surfaces of the laminae were 

 consequently directed forwards, upwards, and inwards. In this leaf the distal pairs of 

 pinna? had not yet attained to the maximum of movement, whilst the basal ones had 

 already begun to decline from it. 



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The phenomena of movement in the leaves of Cassia Sumafrana indicate just as 

 as those of Cassia alala that they correspond with the interposition of a period 

 of instability between two periods of stable equilibrium in the tissues of the motor 



and that, so long as movements continue to occur, the assumption of the nocturnal 

 position is always owing to the action of a tendency to the resumption of what was 

 either the primary permanent position or the diurnal position at an earlier stage in 

 the evolution of the pinnae. In the initial periods of movement, the nocturnal 

 convergence, depression and elevation of the halves of the lamina give rise to complete 

 reversion to the antecedent permanent position. Somewhat later rotation comes in to 

 interfere with the completeness of the reversion and to replace folding of the lamina. 



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So long as folding is possible, the action of the portion of the mass of pulvinar 

 parenchyma making for the noctural position, which is prolonged into the under 

 surface of the lamina, assists in elevating the outer half of the lamina from 



the plane of the midrib; but when permanent expansion has been established, fold 



is no longer possible, and the same action tends to cause torsion of the midrib. 

 Rotation is, however, a comparatively transitory phenomenon here as compared with 

 the rotation of the pinnae of Cassia alata, because there is no excessive accumulation 

 of pulvinar parenchyma on the under surface of the outer half of the lamina, so 

 that the growth of the opposing tissue on the other side of the pulvinus com- 

 paratively rapidly provides sufficient resistance to prevent the occurrence of torsion 

 of the midrib and vascular axis of the pulvinus. Rotation is thus present only during 

 the period at which the mass of pulvinar tissue making for the noctural position 

 possesses its maximal relative strength. 



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In Cama Sumctrana, as in Canute alata, the movements of the leaves, although of 

 very considerable magnitude when at their maximum, are invariably slow and gradual. 

 In both cases the tissues of the motor organs do not provide any special filtmtive 

 facilities, either in the form of abundant pitting of the cell-walls or of an extensive 

 system of intercellular spaces (Plate VII, Fig. U), and hence any factors which tend 

 IT? I™ 6 , increments » extema l P^sure or to audden increase in 



any Tl2J W " J*^ ^ ° f ^ ™ ^P^ <* Pacing 



any immediately appeciable effects on turgescence. 



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