

IN THE MOTOR ORGANS OF LEA YES 



69 



rotation like that which occurs in those of the secondary pulvini of the Cassia, 

 because they are not related to the presence of any structural arrangements corre- 

 sponding to the laminar expansions of pulvinar tissue, which are present in the latter. 



It is the veiy peculiar form of the vascular axis in the secondary pulvini that is 

 the principal determinant of the extreme convergence and depression of the secondan 

 petioles during the night (Plate IV, Figs. 4-5). The vascular tissue, in place of forming si 

 more or less rounded cord, as it does in the case of the secondary pulvini of Mwwta pvrfica, 

 or a tube filled with medulla as it does in those of Leu < ma glauca, takes the form of a 

 broad, flattened, slightly- curved band. This naturally implies corn ponding modification in 

 the resistance which it will present to horizontal and vertical displacement from its normal 

 passive position. It is morphologically adapted to aiford very considerable resistan > to 

 the divergence which is determined by the axillary portion of the inferior mass of pulvinar 

 parenchyma; and consequently,, when the latter loses turgesconce with the removal 

 of solar stimulation, it acts powerfully in producing convergence. Put its resistance to 

 depression is very feeble, and the distal leverage to which it is exjwsed is very great 

 the pulvinus only weighs from 4 to 7 per cent, of the entire secondary petiole and pinnules 

 and consequently when it loses the support afforded to it by the diurnal excess of 

 turgescence in the inferior mass of pulvinar parenchyma, it yields to the leverage aided 

 by the action of the relatively feeble superior pulvinar parenchyma. The central 



portion of the inferior pulvinar pad is not only thicker, but also much richer in 



chlorophyll, than that of the superior one is; and consequently, when in a condition of 

 diurnal turgescence, it is able not only to overcome it, but to overcome the action 



of distal leverage on the vertically flexible vascular axis of the pulvinus. The lateral 

 portions are not conspicuously thicker than those of the superior pad; but the inner 

 of them, when in a condition of diurnal turgescence is, from its axillary position, 

 enabled to effect divergent displacement of the vascular bundle in spite of the rela- 

 tively great resistance which it opposes to horizontal flexion. (Plate IV, Fig. 5.) 



In the secondary and specially in the tertiary pulvini, during the period in the 

 life of the leaf in which extensive movements are regularly carried out, the masses of tissue 

 which make for the diurnal position are conspicuously distinguished by their colour 

 from those which make .for the nocturnal one. In both cases they are of a deep 

 green, whilst their opponents have a much paler ochreous green tinge (Plate IV, Figs. 3, 



5 — 9). Consequently, in the case of the secondary pulvini, it is the inferior mass of 



parenchyma which is green and the superior which is ochreous; whilst in the tertiary 

 ones precisely the reverse holds good. 



The opposed masses of parenchyma in the primary pulvini do not show such con- 

 spicuous and readily appreciable differences in colour as the corresponding tissues in the 

 secondary and tertiary pulvini do, but the steady and progressive increase in relative bulk 

 which takes place in the superior one coincidently with diminution in and final 

 abolition of the assumption of the nocturnal position by the primary petiole is very 

 striking. (Vide Plate I, Figs. 14, 15; Plate IV, Figs. 1, 2.) In leaves in. which petiolar 

 movements are at a maximum, the superior mass is distinguished merely by the 

 fact that it is vivid green almost throughout its entire thickness. Whilst, in the 

 inferior mass, the tissue nearest the wood is almost colourless, and the superficial 

 band of green has a somewhat olive tinge. Microscopic examination, however, shows 

 that the tissue of the superior mass is composed of cells which are of considerably 



smaller size and much more richly provided with green chromatophores than those 



«v 



