IS THE MOTOR ORGAS8 OF LEAVES. 



110 



disturbance in the relations bet 

 contact is capable of determining. 



loss and supply of 



liquid 



th 



any 



mo 



There can be no question that rupture of the continuity of the tissues by incision 



mere contact 



is a much stronger determinant of the diflusicn of movement than 



impulses are, but in this respect it falls far short of injuries effected by means of In at 



application of an open flame, of highly heated metal, 



or 



The movements attending the 



of the carefully focussed rays of the sun to individual pinnules un never of a «■ rictly 



localised nature. Even under conditions of ma imnl humidity a certain amount of 



diffusion of movement occurs; under circumstances wlnn incision is followed 



move 



ments extending over the entire secondary rachis to which tho primarily injured 

 pinnule belongs, the application of heat gives riso to movements 



extending 



over 



the entire leaf, and when incision is followed by movements extending over the 



followed not meiely by these, but ly movements in a varying 



entire leaf, heating is 



leaves along the course of the axis to which 



as though it 



but sometimes considerable number of 



the primarily affected one is attached. This may at first sight appear 

 were adverse to the physical theory of movement, and only explicable as tlie result 

 of the presence of differences in degree of specific protoplasmic stimulation, but it is 

 only to casual observation that it does so. The evidence of the occurrence of any 

 considerable exudative loss of liquid from the tissues is certainly not as a rule so 

 conspicuous as it is, in cases where the continuity of the tissues is interrupt! 1 by 

 means of incision. This is, however, in great part explicable as the iesult of the 

 different conditions to which any exudation is exposed in the two cases. In cases 

 of incision we are dealing with mere rupture in the continuity of tho tisguos, and, so 



as atmospheric humidity is not excessively low, any liquid which escap 



long 



Hut 



naturally tends to accumulate locally, and therefore to appear conspicuously, 

 where the tissues are injured by the application of heat, localised conditions for excessive 

 evaporation are necessarily present, and therefore an absence of local accumulation 

 of liquid does not imply an absence of local exudation. AY here a considerable n tss of 

 soft succulent tissue is present, as in the case of young axes, the local application 

 of heat is often followed by conspicuous indications of exudation even in spite of 

 the attendant excessive evaporation; but when the mass of tissue is limited in 

 relation to the heat, as it is in the case of pinnules, the rapid evaporation masks 



the discharge. 



to the spot to w 



a 



The rise in evaporation, moreover, is not of course strictly localised 

 hich the healing agent is directly applied, but must extend over 

 certain area beyond this, so that there is not mere local exudation and evaporation 

 from the actually injured tissues, but a cause for sudden increase in transpiratory loss 

 in neighbouring and intact ones. This in itself is sufficient to account for the fact 

 that heat should be a more efficient determinant of movement than incision is. In cases 

 of incision the primary alteration in the condition of the tissues is a strictly localised one; 

 whilst, in cases where heat is applied, there is not merely intrinsic local change in the 

 tissues in the immediate site of application, but an alteration in the extrinsic conditions 



exposed. But another circumstance also comes in to 



to which neighbouring tissues 



are 



render heating a more 



efficient cause of extensive movement than simple rupture 



of the continuity of the tissues 



is. 



The liquid which exudes from the tissues on 



_ very rapidly evaporates and condenses when exposed to the air, and goon forms 



protective stratum precluding farther escape from the divided surfaces; and as the 



txticmely limited, ihe site of incision ceases to be a weak 



incision 



a 



lecal injury to tissue is 



