* 



62 



THE CAUSES OF FLUCTUATIONS IN TUMESCENCE 



• 



latter become absolutely weaker in 



the absence of insolation, they at the same time 



become relatively stronger than they were before, and the previously existing equilibrium 

 being proportionately disturbed, movements necessarily occur until a new position of 

 equilibrium has been established. 



As we have just seen, the nyctitropic movements of the pinnae of Cassia alata are 

 extensive and highly complicated at a certain period in the life of the leaves. 



eaves, and gradually diminish and 



of loss of 



very 



They are, however, entirely absent in very young 

 disappear 



in mature ones long before these 



begin to show any 



other signs 



vigour 



On their onset they make their appearance first in the basal pinnae 



and gra- 



dually extend thence outwards along the course of the petiole. In leaves at the period 

 when movements are just beginning to appear, all the pinnae save the basal ones have 

 the two halves of their laminae permanently folded up, with their upper surfaces in close 

 contact with one another, and the midribs directed upwards and forwards from the plane 

 of the petiole. The distal pairs of pinnae are in close contact with one another and with 

 the sides of the petiole, whilst the proximal ones are more or less divergent, the diver- 

 gence increasing in passing downwards towards the base of the rachis. In the youngest 

 pinnae there is as yet no sign of any accumulation of pulvinar tissue. The midrib lies 

 in a groove on the upper surface of the lamina, and the basal ribs of the outer side of 

 the latter emerge clean and clear from it. In the basal pairs of pinnae the laminae are 

 partially unfolded during the day, the surfaces of the outer half being still almost verti- 

 cal, but those of the inner one lying nearly horizontal. Their midribs are still directed 

 forwards and upwards from the plane of the petiole, but much less so than those of the 

 distal pinnae. There is already a conspicuous mass of ochreous pulvinar tissue on the 

 outer side of the keel of the midrib, and extending thence over the bases of the lower 

 ribs on the under surface of the outer half of the lamina. As yet there is very little 

 accumulation of green pulvinar tissue on the inner side of the keel, but a mass of it has 



begun to appear over the base of the upper surface of the laminae and to extend outwards 

 alon 



g the 



mg agencies: 



groove of the midrib. The diurnal position at this period is due to the follow 



presence of a certain amount of green 



The divergence is caused by the 



axillary pulvinar tissue, the partial unfolding to the commencing accumulation of pulvinar 

 tissue over the base of the upper surface of the lamina and along the groove of the 



id- 



rib, and the persistent elevation of the outer half of the lamina to the excessive develop- 

 ment of pulvinar tissue over the bases of the lower ribs on its under surface. As time 

 advances, diurnal divergence 



ment of axillary pulvinar tissue ; expansion of the lamina becomes complete as the 



mass of tissue on the upper surface of the base of the outer half of the lamina 

 increases in 



goes on steadily increasing with the increased develop- 



proportion to the earlier developed mass on the under surface, and 



permanent as the tissue over the upper surface of the mid lib 



and structural strength. For 



increases in amount 



a 



differ 



long time, 



:ences are present between the earlier and lat 



however, such structural and functional 



tissue, making respectively for the nocturnal and diurnal 



er developed masses of pulvinar 



nocturnal and diurnal fluctuatio 



positions as to give rise to 



induce extensive movements. After 



ns in their relative strengths of sufficient magnitude to 



a time the functional strength of the axillary 



strength simultaneously 



superior pulvinar tissue gradually decreases, whilst its structural 



increases, so that the differences originally existing between it and its oppone7t"progress- 



ively dimmish and ultimately disappear, and after this, although general tumescence 



