IN THE MOTOR ORGANS OF LEAVES. 



1 



of the water-conducting system concerned in the supply of the leaf which moves. They 

 occur only when the point of application is immediately above the site of origin oi 

 a petiole, and they never extend beyond that leaf. 



Not only are the effects which are produced by so-called " stimuli " in giving 

 rise to movements in the leaves of Mimosa pudica profoundly modi lit 1 by the present 



of external conditions affecting supply and loss of Water, but the effects which 

 follow the action of individual "stimuli" differ in connection with the dogree to 

 which the latter are of a nature to give rise to disturbances in the relations of 

 general supply and loss. At any given period it will bo found that ample contact - 

 disturbance gives rise to less effect than section or other interruption in the continuity 

 of the tissues does, and that the latter, in its turn, is less efficient in giving rise 

 to movement than the application of strong heat is. 



When soil and air are alike humid, contact-" stimulation" occasions purely localised 

 effects. If pressure be applied to the under surface of the primary pulvinus or the 

 upper surface of the primary petiole, full depression of the latter takes place, but the 

 distal portions of the leaf show not the faintest traces of movement, the secondary 

 rachises remaining fully divergent, and the pinnules in the maximal diurnal position 

 of expansion. Under similar circumstances the movements of individual pinnules 

 which have been subjected to contact are equally purely localised, so long as displace- 

 ment of one does not imply mechanical disturbance of another. The movements 

 of individual terminal pinnules are, of course, not necessarily liable to give rise to 

 any considerable mechanical disturbance elsewhere, but, in the case of all the others 

 a disturbing influence is introduced by the fact that they are ordinarily set upon the 

 rachis at such narrow intervals that, when in the diurnal position, the posterior hall 

 of each lamina overlaps the anterior half of the lamina of the pinnule situated 



behind it on the rachis (Plate V, Fig. 3). Owing to this arrangement, the sudden 

 elevation of any pinnule must necessarily give rise to a certain amount of upward 

 pressure on the one in front of it. In spite of this, however, purely localised 

 pinnular movements may very readily be evoked even in perfectly normal leaves 

 when conditions of telluric and atmospheric moisture are very high. Where conditions 

 of moisture are not so excessive, movements in normal leaves are liable to extend 

 beyond the primarily affected pinnule for a certain distance along the rachis in . 

 centrifugal direction. Under these circumstances, in order to obtain perfectly localised 

 pinnular movements, it is necessary to select particular leaves for experiment. It is 

 not at all unusual to meet with leaves which deviate so far from the normal standard 

 to have their pinnules set on so far apart that, when in their maximal diurnal 



as 



position, they do not overlap one another, but stand quite free and apart as those of 

 Leucaena glauca normally do. In such leaves it is quite easy to show that, even 

 when conditions of humidity are only moderately high, pinnular movements evoked 

 by contact are purely localised. In such cases it is easy to cause full displacement 

 of alternate pinnules all along one or both sides of the rachis without inducing the 

 slightest movement in the intermediate ones which are not directly subjected to 

 contact. The displacement may be caused either by pressure on the upper surface 

 of the tertiary pulvinus or on the under surface of the lamina: in both cases alik. 

 the movement remains strictly localised. When atmospheric and telluric humidity 



Bot. Gabd. Calcutta Vol. VL 



