236 ON THE SAP OF PLANTS. 



can it set up a current ? By the atttaction for each other of the 

 components of tissue, on the one hand, and on the other, of those 

 true compounds of water with even minute amounts of gases or 

 solids which we know to exist. Tissue-compounds and water- 

 compounds meeting, ordinary chemical changes occur, force, in the 

 form of heat, is absorbed, more complex compounds are built up, 

 and the great bulk of the water per se, or, it may be, water holding 

 in solution effete matters, is left to shift for itself, perhaps even 

 repelled, to escape in the direction of least resistance — by transpi- 

 ration in summer and by slower processes in spring. The water 

 exuding from my birch-branch may be such water, a portion of the 

 water that might ordinarily escape from the external surfaces of 

 the wood or otherwise, with the difference that it is carrying away 

 matter (sugar, etc.) not wanted just now, perhaps, but which un- 

 fortunately, or, at all events, as we believe, will be wanted pre- 

 sently. The warrant for the remarks respecting the highly aqueous 

 water-compounds is to be found in the known tenacity with which 

 water retains traces of gases, as shown by Groves, and in the 

 altered properties possessed by water when containing even traces 

 of saline or other matters. Finally, whence comes the large quantity 

 of chemical force or chemical attraction necessary for the binding 

 together of such numbers and such amounts of substances as occur 

 in the fully-formed wood of a forest tree ? From the heat-force 

 poured on to its leaves by the sun — force which will be re-converted 

 into heat when the wood is transformed, no matter whether slowly 

 by decay or quickly by combustion, into its constituent gases and 

 ashes. 



It would be unwise, however, to speculate further without those 

 confirmations and checks which experiment alone could afford, and 

 without those safe guides which experiment alone could furnish. 

 Here as elsewhere in all departments of knowledge earnest seekers 

 after truth are wanted, men possessing, for this enquiry, adequate 

 knowledge of physics, chemistry, and botany, and with the neces- 

 sary time and means for carrying on the work. To such men 

 would certainly, sooner or later, be accorded the honour of dis- 

 covering, or unveiling, one more of the laws by which all nature is 

 governed. Then we shall know the true cause or causes of the 

 movements of sap in plants. 



