OF TENNESSEE. 57 



and which not (infrequently oprn into one another by their 

 applied end.s, and by that way form very fine capillary tubes 

 of considerable length. 



The mechanism by which this ascent is effected is of two 

 kinds ; there is a pull from above and a push from below. 

 The pull from above is the evaporation which takes place 

 at the surface of the plant, and especially in the air-passa- 

 ges of the leaves, where the thin- walled cells of the paren- 

 chyma are surrounded on almost all sides with air, which 

 communicates directly with the atmosphere through the 

 stomates./ The push from below is the absorptive action 

 which takes place at the extremities of the rootlets, 

 and which, for example, in a vine, before its leaves have 

 grown in the spring, causes a rapid ascent of the fluid (sap) 

 absorbed from the soil. A certain portion of the fluid thus 

 pumped up from the roots to the surface of the plant 

 doubtless exudes, laterally, through the walls of the vessels, 

 and, passing from cell to cell, eventually reaches those which 

 contain chlorophyll. The distribution of the compound 

 containing nitrogen and carbon, whatever it may be, which 

 is formed in the chlorophyll bearing cells, probably takes 

 place by slow diffusion from cell to cell. 



It also can hardly be doubted that all the living proto- 

 plasm of the plant undergoes slow oxydation, with evolu- 

 tion of carbonic acid, and that this process, alone, takes 

 place in the deeper seated cells. The supply of oxygen 

 needful for this purpose is sufficiently provided for, on the 

 one hand, by the minute air-passages which are to be found 

 between the cells in all parenchymatous tissues, and on the 

 other, by the spiral vessels, which appear always to contain 

 air under normal circumstances in the woody vascular 

 bundles. 



The replacement of the oxygen of the air thus absorbed, 

 and the removal of the carbonic acid formed, will be suffi- 

 ciently provided for by gaseous diffusion. 



From what has been said, it results that, in an ordinary 



