15 THE MOTOR ORGANS OF LEAVES. 



79 



The position of the variou- parts of very young 



loaves, in which periodic movements 

 have not yet begun to manifest themselves, is as follows:— The primary petiole ii 



extremely divergent from the line of the axis; so that, where the lath/ is vertical, 



it is either slightly ascending, horizontal, or slightly depressed (Plate I, Flgg, 11 



12, 18 ; Plate V, Fig. 14). The pinnae are clone ly convergent and bent ba< k wards 



at an acute angle over the upper surface of the primary petiole, the radices of the 



lasal paii- Usually having their upper surface- directed more or I s inwards, so 



that the edges of the closely- folded pinnules are directed obliquely outwards and 



inwards in place of directly upwards and downwards, and the distal pinn are 



overlapped and partially concealed. The pinnules, which from the out «t are fully 



expanded, are closely convergent, and have their midribs directed upward- and 



forwards at an acute angle to the line of the secondary rachises. As time goei on, the 



primary petiole lengthens, but retains its previous p. ition to the axi-, and the pinna- 



gradually become first erected and then bent backwards, so as to lie in the same plane as 



the primary petiole or slightly ascending from it, and at the same time become slightly 



divergent (Plate I, Figs. 11, 12, 18). When periodic movements first appear they are for 



a time confined solely to the primary petiole, which during the day und< rgi es elevation < r 



convergence towards the axis, and noctumally returns to its original position of extrcm 

 divergence or slight deflection. The amount of diurnal displacement 10 



at first very small 



mov 



determini'iiLT diurnal i\i 



. ...... ....ergence 



but undergoes rapid increase; and, with this, 



and nocturnal convergence of the pinnae begin to manifest themselves. Finally, movements 



appear in the pinnules also, leading to steadily increasing diurnal divergence and 



depression and 



of rotation downwards of the posterior or outer half of 



the lamina alternating with nocturnal convergence, elevation, and reverse rotation. When 

 movements are at a maximum, the diurnal and nocturnal positions of the >an ns parts 

 of the leaves are normally as follow:— Diumally the primary petiole usually lies at an 

 acute angle to the axis, but the degree of convergence varies greatly in different instances 

 and under differing conditions, owing to the influence of variations in the amount of 



£ tvuuHwnu, v.. ". 



the leverage exerted by the distal parts of the leaf. The pinnae lie almost in the same 

 plane as the primary petiole or slightly ascending from it, and are widely divergent; the 

 divergence of the basal ones being frequently so great as to exec d a right angle, 



SO 



they are directed slightly backwards (Plate V, Fig. 12). The pinnules lie with their upper 

 surfaces facing directly upwards and their midribs widely divergent (Plate V, Figs. 2 

 The divergence varies in different parts of the pinna, the lower pinnules being frequently 

 directed somewhat backwards, tie central ones at right angles to the secondary rachis, and 

 the distal ones inclining forwards. As the pinnules are set on very closely, the intervals 

 between them falling very considerably short of the breadth of individual lamina? 



is 



y seldom that the entire surface of any pinnule is entirely exposed, and, as a rule, a 



amount of the anterior half of each successive lamina is concealed by the posterior 



half of the succeeding one (Plate V, Fig. 3). In tic maximal normal nocturnal 



position determined solely by absence of photic stimulation, the primary petioles 

 lie nearly at right angles to the axis; that is, in cases where the latter is vertical, 

 some are horizontal, others slightly ascending and still others slightly d< sending. The 

 secondary rachises are closely convergent and slightly elevated, and the lower pair are 

 rotated, so that their upper surfaces are directed more or less inwards. f J he pinnules are 

 closely convergent, so that the two rows are in contact with one another, the upper sur- 

 faces of the laminae facing directly inwards and the lower ones directly outwards (Mate V, 



