IN THE MOTOR ORGANS OF LEAVES. 



89 



parenchyma, are slight excess in bulk (Plate VI, Fig. 7), slight excess in the thickness of 

 the walls of the constituent cells, and slight excess in chlorophyll content. Tin -re are no 

 intrinsic structural features in the secondary pulvini implying such instability in quilibrum 

 of position as there are in the primary ones; and, in addition, so long as the priman 

 petiole is not deeply depressed, distal leverage does not come into play as an e tonne 

 factor of disturbance, and with this we find much less liability to the <.<•< nrrence of 

 extensive and especially of rapid extensive movements in the secondary rachises than in 

 the primary petiole. Very considerable periodic movements of divergence and 00nvenrenc< 



are regularly carried out as in other nyctitropic leaves in connection with the incidene. 

 or removal of solar stimulation; but there is very much less tendency to the occurrence 

 of extensive and rapid movements under the influence of other factors than there is in 

 the case of the primary petioles and pinnules — conditions wluch determine the fullest 

 depression of the primary petioles and maximal movements of the pinnules often leaving the 

 secondary rachises almost entirely unaffected (Plate V, Figs. 12, 13); and, if the intrinsir 

 structural peculiarities of the tissues of the secondary pulvini alone be taken into account. 



7 



this is not so much a matter for surprise as that extensive movements should occur at all 

 The opposing masses of pulvinar parenchyma acting on the relatively strong iibro- vascular 

 cord differ so slightly in character from one another that, if attention he limited to them 

 it would be hard to explain the occurrence even of regular nyctitropic movements of an 

 extensive character. There are no intrinsic structural peculiarities present in the pulvini 

 to justify us in supposing that any very considerable fluctuations in the relative strengths 

 of the axillary and external masses of pulvinar parenchyma take place; and, in unite of 



this, very extensive movements of convergence and divergence regularly succeed one 

 another every evening and morning. When, however, the relations of the opposing Diasni 



f parenchyma to neighbouring parts are tak 



pparcnt anomaly dis 



appears (Plate VI, Figs. 9, 10, 11). The base of the outer mass of parenchyma is in 

 immediate contact with a mass of tissue projecting in bracket-fashion from the axis, and 

 composed of very delicate cells poor in protoplasm and specially poor in chromatophores. 

 The lower portions of the axillary masses, on the other hand, are either in close contact 

 with the side of the rachis, which presents little compressable tissue, or in the case of 

 terminal pulvini, with the corresponding mass of the neighbouring pulvinus. The bracks t 

 or cushion on which the base of the outer mass rests is composed of tissue which is weak 



com 



alike structurally and functionally, and consequently presents little resistance to 

 pression, so that, when turgescence rises in the axillary parenchyma, displacement i- 

 effected not so much by compression of the tissue of the outer parenchyma as by cone 



pression of the bracket upon which it rests. There is probably only an inconsiderable 

 excess in rise in turgescence in the axillary mass as compared with the outer one under 

 the influence of solar stimulation ; but the rise that does occur acts at an advantage in 

 effecting displacement because it has a resistent basis to work upon. The turgeseent 

 tissue of the axillary parenchyma is supported by the resistance of the lateral surface of 

 the rachis or by the equally turgeseent tissue of the neighbouring pulvinus, wherea 

 the external parenchyma rests on an insecure foundation, and readily sustains displace- 

 ment. From its relations to neighbouring parts, the axillary tissue, when its turgescence 

 rises under solar stimulation, is able to overcome the resistance of the relatively strong, 

 passively convergent fibro-vascular bundle, which receives little support from the external 

 parenchyma owing to the insecure foundation on which the latter rests. The struggle 



Ann. Roy. Bot. Gaud. Calcutta Vol. VI. 



