VIII.J THE BEAN-PLANT. 



to 



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 diff"usion 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 

 carboiiic 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 

 normal circumstances, in the woody bundles.- The replace- 

 ment of the oxygen of the air thus absorbed, and the removal 

 of the carbonic anhydride formed, will be sufficiently provided 

 for by gaseous diffusion. 



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

 plant, growing in damp earth and exposed to the sunshine, 

 a current of fluid is setting from the root towards the surface 

 exposed to the air, where its watery part is for the most 

 part evaporated ; while gaseous diffusion takes place, in the 

 contrary direction, from the surface exposed to the air, 

 through the air passages and spiral vessels which extend 



