IN THE MOTOR ORGANS OP LEAVES. 



1 





direct sunshine normally occasions only slow insensibly progressive movements. Whv 

 should exposure to the sun's ray* only give rise to movements winch, judging from 

 the parallel phenomena occurring in non-motile leaves under similar circumstance., an 

 unequivocally dependent purely on increased transpiratory loss, so long as the leave, 

 retain their normal connection with the axis, and to movements dependent on ac-tiv." 

 protoplasmic contraction when they no longer do so? This, moreover, is not the only 

 problem which has to be accounted for; for, if exposure to the *un's rays primarih 

 occasions active contraction of the protoplasts of the motor organs, why should it .aae 



do so? why should continued exposure bo accompanied hy partial recovery of the 



diurnal position? These arc questions which the tlieory apparently ,an anaw- r onh 

 by means of a series of arbitrary assumptions. 



But if this theory be incapable of accounting for the phenomena, can that whiel 

 ascribes the occurrence of rapid movements to purely physical causes do so any mor. 

 satisfactorily? When a leaf is suddenly detached from the axis, an abrupt am- 

 is of course in any case put to any further root-supply of water. But in Miuim pudim. 

 owing to the high liquid tension throughout the tissues, this is accompanied by active 

 exudative discharge of a relatively large mass of liquid from * the extremity of thai 

 portion of the petiole which remains attached to the distal portion of the leaf. The 

 liquid which escapes may be mainly derived from particular tissues or it may not; but to 

 any case, the effect must be a general loss in liquid tension throughout the entire leaf 



and consequently a tendency to increased drain upon the active tissues. Jn cases win i 

 the tissues are excessively saturated as the result of antecedent conditions of atmosph. n< 

 and telluric humidity, and where at the time of separation from tlio axis tra 1 1 s p i ratory los* 

 is very low, the intrinsic water-conducting tissues contain an amount of fluid more 01 

 less completely sufficient to make good the actual discharge of liquid attending separation 

 and, for the time being, the arrest in root-supply, and hence either no movements a 

 all occur, or any which do occur are slight and partial. With any diminution in th« 

 amount of store- water, or any increase in the activity of transpiratory loss, the condi- 

 tions providing for the occurrence of movement will be present in progreiiivel I 

 increasing degree, and a point must eventually be reached at which separation from the 

 axis implies immediate active drain upon the turgid elements of th<3 motor organs. 

 But this drain will naturally tell most heavily and rapidly on those masses of tissue 

 presenting the greatest filtrative facilities and, therefore, on those which make for the 

 diurnal position, and hence movements corresponding to a weakening of these will occur. 

 The masses of tissue in the tertiary pulvini which make for the diurnal position oi 

 the pinnules afford great structural facilities for rapid filtration; both from their inherent 

 delicacy and from the great excess of structural strength in their opponents, and hence 



* 



the pinnules tend to pass on rapidly into the nocturnal position. In t\ie secondary 



pulvini the opposing masses of tissue do not present nearly such considerable differences, 

 and hence movements in the secondary rachises are often almost or entirely absent 

 when the pinnules exhibit conspicuous movement, and under normal circumstances they 

 never exhibit the same activity as those of the pinnules do. If transpiratory loss be very 

 active, any liquid which escapes from the turgid pulvinar tissues on the sudden disturb- 

 ance of equilibrium of general fluid pressure on separation from the axis and any Btore 

 of water within the water-conducting tissues will be rapidly removed and the nocturnal 

 position of the pinnules will remain permanent, and hence no phenomena of recovery 

 of the diurnal position manifest themselves in leaves which are detached from the axis 



Ann. Roy. But. Gakd. Calcutta Vol. VI. 





