Dixon — Note on the Role of Osmosis in Transpiration. 771 



varying from 3-21 atmospheres have been demonstrated in various 

 positions in plants by different experimentors.^ 



Besides the arguments deduced from experimental evidence, there 

 are also reasons to believe that the raw sap is drawn out of the upper 

 extremities of the water conduits by the osmotic action of the neigh- 

 bouring cells, founded on anatomical considerations. The elements of 

 the tracheidal system in the leaf and elsewhere have their walls 

 strengthened by thickening layers, in addition to the fact that their 

 walls are of a more resistant material and thicker than that composing 

 the walls of the neighbouring cells- We may argue almost with 

 certainty that if the cells of the mesophyll had to withstand the same 

 internal tension or tendency to collapse as the elements of the trachei- 

 dal tissue that they would have equal provision to protect them from 

 collapsing. On the other hand, their highly elastic and flexible 

 membrane unfurnished with any form of rigid support is just what is 

 suitable to take up the internal pressure generated by the osmotic 

 activity of the cell. For the sake of comparison a few measurements 

 are given here, made on the cells and the tracheidal elements of the 

 leaf of Helianthus multiflorus : — 





Thickness of wall. 



Diameter of lumen. 



Cell of mesophyll, . . 



Cell of bundle-slieath, 



Tracheidal element of a fine 

 ramification of a bundle, . . 



1-0-0-5/x 

 1-2-3-Oai 



10^ 

 10-20^ 



6-8m 



From these measurements we see that the strength of the trache- 

 idal element to resist collapse is much greater than that of the cells of 

 the leaf both by its thicker wall and by its smaller internal diameter, 

 and consequently we may with confidence conclude that the cellular 

 elements of the leaf are not exposed to the same crushing forces as 

 those of the tracheidal system. 



Of course ultimately, even under normal conditions, the inflow of 

 heat causing the evaporation from the surface films formed in the cell- 

 membranes of the transpiring cells is what supplies the energy needed 

 to raise the water of the transpiration-current, for it is by this that 



^ Cf. H. de Vries, Jahrb. f . wiss. Bot. xiv. ; "Wieler, Jahrb. f . wiss. Bot. xviii. ; 

 Westermaier, Berichte d. deutsch. Bot. Gas., 1883, p. 371. 



