IN THE MOTOR 0]iOA>'S OP LEAVK8. 



15 



is primarily locally applied is readily soluble in water, and therefore liable to be 

 conducted by the stream of fluid traversing the vascular tissues. 



rr 



n 



Experiment X. — Two leaves of Cassia sttmatrana were taken and set with the 

 bases of their petioles immersed in water and subaqueously divided in order to permit 

 of freo absorption. One of them (a) was now along with its water- vessel introduced 

 into a beaker containing a little ammonia, the mouth of the leaker being closed by a 

 cork securely luted down and around the rachis, so that whilst the lower portion 

 of the latter and the basal pinme were enclosed in the beaker and exposed to the 

 ammonia vapour, the upper portion and the distal pimue projected free into the air 

 and bad no immediate relation to the reagent. The other leaf [b) was similarly 

 treated, only in this case the beaker contained chloroform in place of ammonia. The 

 pinnae of (a) which were within the beaker rapidly acquired a dark colour, whilst 

 drops of black fluid began to exude from the surface of the rachis, and shortly 

 afterwards similar changes began to manifest themselves in the part of the leaf outside 

 the beaker, the pinnae drooping and darkeuing in colour especially along the neighbour- 

 hood of their midribs, and exudation beginning to appear on the rachis. Twenty-four 

 hours later the leaf was wilted throughout, the pinme greatly blackened and the 

 rachis thickly studded with drops of black fluid. The portions of (b) which were 

 included in the beaker were also very rapidly affected, the pinnae becoming flaccid 

 and of a brownish olive colour and the rachis exuding drops of pale yellowish fluid, 

 but there was no extension of these changes to the portion of the leaf beyond the 

 cork, the pinnae retaining their original colour and turgidity and the rachis showin 

 no signs of exudation. On the following day the free portions of the leaf remained 

 entirely unaffected, and the pinnae were fully expanded, turgid and of their original 

 bright green colour, while the petiole, even down to the surface of the cork, showed 



no signs of any change. 



Here, in both leaves alike, the tissues which were directly exposed to the influence 

 of the reagent were rapidly killed and rendered flaccid by loss of turgidity, but 

 whilst in one case these changes extended rapidly and completely throughout the 

 entire leaf, in the other there was no propagation of them to the protected parts. 

 Both reagents rapidly induce death in tissues directly exposed to their influence; but 

 whilst chloroform is only very slightly soluble in water, ammonia is excessively so, 

 and therefore is readily conveyed to distal areas. Such experiments afford no evidence 

 of the transfer of any influences by means of the protoplasmic continuity of the 

 tissues, the complete death of great masses of tissue failing to give rise to any 

 appreciable effects in immediately contiguous parts so long as the water-supply of 

 the latter remains unaffected, but they unequivocally show how rapidly changes may 

 be propagated to distal areas by means of alterations in their water-supply. This 

 is a question which will be recurred to subsequently, and it is merely alluded to 

 here because the phenomena of the experiment appear so clearly to suggest that it 

 is to the water-conducting system and not to any system of continuous protoplasts 

 that we must look for an explanation of the propagation of effects from one area 

 to another in the organism of any of the higher plants. 



The above experinlents have demonstrated very clearly that exposure of vegetable 

 tissues to excessive heat gives rise to effects similar to those which follow their exposure 



