IN THE MOTOR ORGANS OF LEAVES. 



59 



CHAPTER VI I L 



%\it nnciitropic mobments of particular leabes in relation 



the 



structure anft arrangement of the tissues in their ^totor organs. 



"\Lhc movements of the $>abes of (Cassia alata aiib 



Cassia Sumatrana. 



In the course of the preceding chapter, an attempt has been made to show that the 

 occurrence of nyctitropic movements generally is dependent on the presence of opposing 



masses of tissue situated in the motor 



organs 



and 



differing 



from ono another in 



it ought to be possible 



functional and structural strength. If this be actually the case, 



to show that some definite relation exists between the special movements exhibited by 



the leaves of particular plants and certain peculiarities of structural detail and arrange- 



ment in the tissues of their motor organs. 



In attempting to do this, the best course 

 appears to be to take a certain number of individual leaves and consider the peculiarities 

 of their movements and the structure of their motor organs in detail. 



The leaves of Cama alata afford specially favourable opportunities for study, because 

 of their large size and the highly developed and complicated movements which they 

 exhibit. Fully developed leaves in this species have from nine to twelve pairs of 

 practically sessile pinnae, the lowest pair being of relatively small size and inserted close to 



the base of the leaf and at a considerable distance from 



pair. During the period 



m 



the 



of a leaf in which movements are at a maximum, that is, in fully developed 



but still comparatively young leaves, the diurnal and nocturnal positions of 



pinnae 



are as 



foil 



ows 



— Diurnally their superior and inferior surfaces look directly upwards and 



downwards and 



approximately 



pi 



their midribs at the j-ame time diverging almost 



r 



with the upper face of 



■lit angles to it. 



raehis 



II, Fig 



In the early part of the 



on 



other hand, the upper and under surfaces of 



lamina? look directly inwards and outwards in a plane at right angles to the upper face 

 of the raehis, aud the midribs are close 



con versren t 



d directed downwards at an 



acute angle to it. 



(Plate II, Fig. 2.) It is clear that the movements which lead to the 



alteration of these positions must 



f alternating convergence and divergence of 



midribs of the pinnae 



bined with excessive rotation in opposite dir 



and a 



certain amount of elevation and dep 



atch the actual progress of the transition from the diurnal to the nocturnal 



we 



find that the first indication of displacement lies in a certain deg 



If Wi 



position, 



of elevation of the lamina, so that its upper surface slopes upwards from the plane of the 



raehis. At the same time the divergence of the midribs gradually decreases. The general 



of the lamina is 



ceded by steadily 





outer half over the level of the anterior or 



y 



which 



of its posterior or 

 necessarily implies 



corresponding rotation of the midrib. Rotation and convergence now advance simultane- 

 ously until the upper surface of the lamina has moved through a quadrant and the 

 midrib has converged so greatly that the upper surface of the opposing members in each 



Ann. Roy. Bot. Gard. Calcutta Vol. VI. 



