TRANSPORT PROCESSES IN THE SOIL-PLANT SYSTEM 717 



water in equilibrium with 12 per cent oxygen. The equation indicates 

 that the oxygen concentration at the root surface will depend on both 

 the oxygen consumption rate and the diflFusion characteristics of the 

 surrounding flow path. Hence the phenomena of root aeration and 

 water movement to an absorbing root are similar, in that both are rate- 

 dependent and also are strongly affected by the transmission charac- 

 teristics of the adjacent soil. 



Gas transport in plant tissue 



The two gases with which the plant is concerned— carbon dioxide 

 and oxygen— are intimately related quantitatively, because the chemi- 

 cal process of the plant usually involves both gases in equal amounts. 

 But the solubilities of the two gases, and their normal concentrations 

 in the atmosphere, are so different that each gas presents a unique 

 transport problem. 



Because of the limited solubility of oxygen in aqueous media, the 

 supply of respiratory oxygen to the center of the root is limited. It is 

 doubtful that diffusion could provide , sufficient transport of oxygen 

 from the outside of a root larger than 2 mm. in diameter, regardless of 

 which of the three transport paths were taken. But movement in tissues 

 is not all by diffusion. Protoplasmic streaming could account for con- 

 siderable oxygen movement, and, indeed, transport of oxygen may be 

 the principal function of such streaming. It is true that there are nu- 

 merous longitudinal intercellular spaces in roots, and that these spaces 

 may be interconnected to form long tubes, so that some root aeration 

 could be by diffusion of oxygen through these tubes. Certain aquatic 

 plants have specialized conducting tubes which allow respiration of 

 submerged roots, but calculations show that the possible oxygen supply 

 through intercellular spaces to an ordinary root in soil would be rather 

 limited. If 10 per cent of the root cross-sectional area were taken up 

 by the intercellular spaces, and if the root were respiring at the rate 

 of 0.5 ml, of oxygen per c.c. per hour, oxygen diffusion from free air 

 at one end of the root would be sufficient for a root about 5 cm. long. 



Oxygen itself need not necessarily all be transported in plant tis- 

 sue. It may be that some electron carrier is transported, and that the 

 final biochemical utilization of oxygen takes place only near the sur- 

 faces of plant tissues. 



The solubility of carbon dioxide in aqueous media is about 25 

 times as great as the solubility of oxygen, and some carbon dioxide may 

 be transported as the bicarbonate ion, so the transport of CO2 as a 

 respiratory product is no great problem. That is, if oxygen can get into 

 a tissue, carbon dioxide can get out. But, because of its low concen- 

 tration in the atmosphere, the movement of carbon dioxide into the leaf 



