viii.] THE BEAN-PLANT. 77 



The mechanism by which this ascent is effected is of two 

 kinds ; there is a pull from above, and there is 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- 

 passages of the leaves, where the thin-walled cells of the 

 parenchyma 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 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 (the thin 

 places which give rise to the dots on the walls of these 

 structures especially favouring this process), 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. 



The supply of air, containing carbonic anhydride, to the 

 leaves and bark is effected by the abundant and large air- 

 passages which exist between the cells in those regions. 

 But it can hardly be doubted that all the living protoplasm 

 of the plant undergoes slow oxidation, with evolution of 

 carbonic anhydride; 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 

 norma 1 circumstances, in the woody bundles. The replace- 



