26 Henry H. Dixon. 



the tracheae and the accumulation of air must infallibly soon bring 

 the whole process to an end. 



From all points of view the introduction of the idea of a J a m i n 's 

 chain into the discussion of the problem of the ascent of sap is as 

 unwarrantable as it is needless. No assistance in the solution of the 

 problem can be obtained from its properties, while, as suggestive of 

 false analogies, the comparison of the water in plants to that of a 

 Jamin's chain can only be regarded as most unfortunate. It is 

 interesting to note that although this configuration is fully described 

 by Jamin, he does not introduce it as part of his own theory^) of 

 the ascent of sap, wliich simply depends on the capillary properties 

 of porous bodies and the difference in tlie state of saturation in the 

 space round the leaves and that round the roots, and in no way 

 demands the presence of Jamin's chains in the tracheae. 



Various writers have endeavoured to explain the rise of the 

 transpiration stream by appealing to the action of osmotic phenomena. 



In almost all the vital theories the hypothetical pumping actions 

 of the cells are supposed to be effected by osmosis. Consequently the 

 osmotic hypothesis, so far as the osmotic actions in question are 

 supposed to be manifested in connection with the cells of the wood, 

 has been discussed with those theories. It is in fact hard to see how 

 osmotic pressures can be generated in the conducting tracts apart 

 from these cells. In them only are to be found semipermeable or 

 approximately semipermeable membranes. L armor's suggestion^) 

 that an upward movement is determined b}^ a gradient of concen- 

 tration in the solutions contained in the tracheae from below upwards, 

 could only apply to an ideal state of things and is negatived by the 

 facts: (1) The membranes of the tracheae are freely permeable to 

 dissolved substances; (2) No such differences in concentration are 

 found; (3) The resistance to flow upwards and downwards is the same. 



Sachs' imbibition hypothesis must be classed among those 

 phj^sical hypotheses which appeal to the inflow of energy at the 

 leaf as directly responsible for the lifting of the water. The assumed 

 passage of imbibed water in the walls of the tracheae could only be 

 compared to the diffusion of a salt in a solvent. The motion upwards 

 is due to the reduced water pressure in the walls above. The initiation 

 and maintenance of this reduced pressure is the work of evaporation 

 kept up by the inflow of heat at the evaporating surfaces. The fatal 



') J. Jamin, Mémoire sur l'équilibre et le mouvements des liquides dans les 

 corps poreux. Compt. rend., 50, 1860, pp. 172, 311, 385. 



^j J. Larmor, Note on the Mechanics of the Ascent of Sap in Trees. Proc. 

 Roy. Soc. Lond., Vol. 76, B, 1905, p. 460. 



