IN THE MOTOK ORGANS OF LEAVES. 



s:: 





relative strength is opposed by progressively diminishing distal leverage which is t juiv- 

 alent to its undergoing progressive and additional increments in relative strength. 



The structural features in the motor organs of Mimosa ptulica whi< h determine tin 

 occurrence of the various periodic movements are parallel to those which we have already 



found to be characteristic of those of other nyctitropic leaves. . i i the case of tli 



latter, the opposing musses of pulvinar parenchyma appear at a relatively late period 

 in the development of the leaf and are not develop d sinmltaneouslv, the development of 

 the masses winch make for the diurnal position la ging beliind that <>f tin ir opi>onenl 

 In the case of the primary pulvinus the antcced. nt development of tlie ina» of tissm 

 which makes for the nocturnal position is clearly indicated by iho fact that, f.-r a 

 considerable time ere periodic movements set in, a process <.f continuously increasing 

 divergence of the primary petiole from the line of the axis manin- s itself, win h 

 ultimately leads to its assuming a permanent position corresponding to the primary 

 nocturnal position of a later stage of development. At first sight it might appear a- 

 though the permanence of the position were anomalous, and as though movements <»f 

 nocturnal convergence similar to those occurring in young leaves of Pi f fa citoh ium ought 

 to attend the nocturnal loss in turgescence of the growing mass of parenchyma overlying 

 the vascular axis of the pulvinus. There is, however, no appreciable evidence of then 

 occurrence, and as a matter of fact there is no reason why they should occur. 



The conditions are not parallel in the two cases. In Pi ( hccolobium saman the ma<- 



of pulvinar tissue which leads to divergence has not only to overcome the resistant 

 of a relatively strong vascular axis, but also that of an antecedently developed mas* o 

 pulvinar parenchyma making for elevation; whereas in Mimosa pwlica, during the period 

 at which progressive divergence of the primary petiole advances continuously, the 

 accumulation of pulvinar ti-sue making for convergence has not yet begun to make 

 its appearance; and the axial cord of vascular tissue is very weak, and from it- 

 flattened form, specially flexible in a vertical direction, so that, so long as it i 

 unaided, it is incapable of producing any appreciable effect in resisting the pressure 

 of the axillary parenchyma aided by the rapidly increasing leverage of the distal 

 portions of the leaf bearing on the continuously elongating petiolar lever. 



In the mature primary pulvini the axis consists of a solid cord of soft flexible 

 wood, surrounded by a sheath of bast and containing no medullary tissue. It i- 

 compressed vertically, so as to present an oval figure with the long diameter horisontal, 

 and is surrounded by a thick mass of parenchyma (Plate VI, Figs. 2, bb). A- the outline 

 of the pulvinus as a whole is more circular, it necessarily follows that the mn- s of 

 parenchyma lying to either side of the fibro-vascular axis are thinner than those 

 which lie above and below it. The latter, moreover, are not of equal thickness, for 

 the superior one is only slightly thicker than the fibro-vascular bundle, while the 



nferior one is considerably so (Plate VI, Figs. 1, 2, 5*). The vertical measurements of 



the masses above and below the fibro-vascular bundle of the latter, and of the pu 

 as a whole, in the specimen illustrated in the figure were as follow 



Vertical thickness of entire pulvinus 



1 74 m.m 



H upper parenchyma ... ... u'->7 



fibro-vascular bundle ... ••• 0'5 1 



>» »» 



» » 



lower parenchyma ••• ••• 0'6G 



» 



»» 



>» 



Ann. Roy. Bot. Gakd. Calcutta Vol. VI 



