XVI.] THE BEAN PLANT. 469 



water they take up passes to the parenchymatous cells of 

 the cortex of the root, and is thence pressed into the vessels. 

 In a vine, for example, before its leaves have grown in the 

 spring, this process, called "root-pressure," 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 

 pits 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 

 carbohydrates formed in the chlorophyll-bearing cells, pro- 

 bably takes place by slow diffusion from cell to cell. The 

 proteid compounds are in all probability conveyed through 

 the sieve-tubes of the vascular bundles. 



There is no doubt that all the living protoplasm of the 

 plant undergoes slow oxidation, with evolution of carbonic 

 anhydride. In the green parts, and in daylight, this process 

 of respiration is disguised by the more conspicuous one of 

 assimilation, in which carbonic anhydride is decomposed 

 and oxygen given off. In the deeper seated cells, and in 

 all parts of the plant when light is absent, respiration alone 

 goes on. The supply of oxygen needful for this purpose 

 is sufficiently provided for, by the air-passages which are to 

 be found between the cells in all parenchymatous tissues. 

 The replacement 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 



