IN THE MOTOR ORGANS OF LEAVES. 



117 



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the pinnules originating in connection with contact is much less liable to be diffused 

 to any considerable extent centripetally than cent rifu gaily, seems to indicate that any 

 such removal of pressure is a factor of no considerable importance; and it appears 

 probable that the spread of centrifugal movement is aided by, and that of contrijxjtal 

 movement mainly dependent on, the action of an agency of a perfectly distinct 

 nature. 



So long as transpiratory loss is low and root-supply of water is abundant, the 

 primary filtrative discharge of fluid incident on the local increase in pressure caused 

 by contact implies a mere local redistribution of water and no diminution in the 

 stock available for the maintenance of turgescence. Tho mechanical disturbance ensures 

 an increased activity of filtrative discharge and corresponding decreaso in turge ence 

 in certain masses of tissue; but it does not interfere otherwise with tlio asshmlatory 

 activity of the protoplasts on which the osmotic capacity of tho cell-sap and consc 

 quently the turgescent capacity of the cells depends. On tho cessation of the 

 tiltrative disturbance, they will, therefore, at once begin to regain their normal state of 

 turgescence, and, owing to the abundance of water which is available, they will readily 

 be able to do so without in any way interfering with tho turgescenco of neighbouring 

 elements. But where conditions of humidity are such that turgescence is on 

 maintained as the result of an exact equilibration between root-supply and transpiratory 

 loss, this is no longer the case. A certain amount of tho water discharged from 



the turgid elements of the active tissues, all of it, at all events, which passes into 

 the intercellular system of spaces, is liable to bo rapidly removed under the influence 

 of continued active transpiration, and this, under the circumstances, implies an absolute 

 diminution in the entire stock available for the up-keep of turgescence. The tissues 

 which primarily lost turgescence in connection with increased filtration remain as 

 active as ever. They are just as eager to satisfy their osmotic capacities as they 



before and in their efforts to do so they must tend to interfere with tho 



were 



pply of neighbouring tissues just in proportion to the amount of actual loss in tho 

 general stock of water which was induced by the primary filtrative discharge. The 

 spread of movements along the course of a rachis must certainly be favoured by such 

 a disturbance in the supply of water; and, as in normal leaves it is very difficult to 

 secure that any mechanical disturbance has been primarily strictly localised to one 

 or other of the individual pinnules, it is little wonder that, in tho presence of e t.rnai 

 conditions implying maximal instability of position, a certain extension of movement 

 should occur even in cases in which the direct mechanical action of the moving pinnule 

 upon their neighbours cannot fully account for the phenomenon. In any case, the move- 

 ments following contact have a very limited extension and, under circumstances in which 

 the evidence of other so-called stimuli is followed by more or less extensive propagation 

 of movement, they are normally strictly localised to the points primarily affected. They 

 do not in themselves necessarily involve any actual loss of liquid, but mere temporary 

 redistribution of it, and hence they are not in themselves efficient causes of widely 

 diffused disturbance in the relations of supply and loss of liquid in the actively turgescent 



tissues. 



This is no longer the case when the primary agent in determining the occurrence 

 of movement is a rupture in the continuity of the tissues Incision of the osis or petiole, 



or amputation of portions of the luminal of pinnu!*, is followed not merely hy movements, 



