









iy THE MOTOE ORGANS OF LEAVES. 



10 



rates under the influence of increased tran^iratory Hw. The only l,,dcal conclushm 



therefore which can be arrived at from the data is, 'that the differ,-,,,', s hi" the l 



of 1110 \ 



presenting themselves under the influence of different conditions of atmoi 

 pheric humidity are owing to oorrespondmg differences in the decree to whicl 



these conditions favour the occurrence of purely physical loss of water f. 



The phenomena are readily explicable on the theoty that t lie movements ha* a pureh 

 physical origin; they remain entirely mysterious on that which ,, siu fo ti) , m w tho 

 result of active protoplasmic contraction. 



But another phenomenon frequently manifests itself in connection with ih< 

 presence of excessive atmospheric humidity which is even moro in ex plica 1)1 6 on tho 

 theoiy of active protoplasmic contraction. In cases in which not only both air and 

 soil are saturated with moisture, but recent rainfall h; is occurred and loaded mam of 

 the leaves with adherent drops of water, phenomena which according to the contrac- 

 tion theory must be regarded as indicative of special irritability of tho prii 

 pulvini, frequently manifest themselves. In such cases, after amputation of the tip of 



•> 



>f the distal pinnules, either no movement at all occurs in the pinnules, or slow imperfect 

 movements occur in the injured pinnule alone, or in it and in ono or two neHi- 

 bouring ones, and yet rapid and profound depres ion of the primary petiole takes place 



try after the injury and often before any pinnular dis] 



begun to appear 



(vide Appendix A). On the theory that movements are duo to active protopl <mic 



contraction, this must be taken to indicate 



atmospheric humidity decreases 



the irritability and contractility of the tertiary pulvini, and increases those of the 

 primary one ; that is, that the same cause gives rise to different el -et§ in two parallel 

 sets of organs. But this is a conclusion for which no evidence exists save the very- 

 phenomenon for which it has to serve as an explanation. But on the phvsical theor\ 

 there is nothing remarkable about the coincidence. We know, that under any cir- 

 cumstances, the mass of tissue in the primary pulvinus which makes for elevation of 



the primary petiole has to struggle, not only with its local opponent, but .also wit!) 

 the depressant action of distal leverage; and it is clear that any accumulation of rain- 

 drops on the pinnse must increase the latter. But adherent rain-drops cannot serve 

 to increase the resistance which has to be overcome by the tissues in the tertiary 

 pulvini which make for depression of the pinnules, and as a rule they must tell 

 directly against their elevation. In the primary pulvini the masses of tissue which 

 make for depression, and which are structurally stronger than their opponents, are co-opera- 

 ted with by the adherent rain-drops in the attempt to depress the primary petiole; 

 in the tertiary pulvini the structurally feebler tissues, instead of encountering any 

 additional opposition under the circumstances, are aided in securing depression of the 

 pinnules. That is, under the circumstances, the tissues which make for the diurnal 

 position are relatively weakened in the primary pulvini and relatively strengthened in 

 the tertiary ones ; or, in other words, under the circumstances, the stability of the 

 equilibrium of position is decreased in the primary pulvini and increased in the 

 tertiary ones. But, if this be so, a fall in general fluid pressure incident on the 

 escape of liquid from the tissues of an injured pinnule, and which is incapable of 

 producing any but very limited effects in the site of increased stability, may readily 

 induce considerable effects in that of decreased stability. So far as transpiratory loss 

 is concerned, both primary and te.tiary pulvini are, under the circumstances, in a 



