TRANSPORT PROCESSES IN THE SOIL-PLANT SYSTEM 709 



counts for as much as half of the dry weight of the cell wall (Jensen 

 and Ashton, 1960 ) . Direct measurement of the permeability of the cell 

 wall is made difficult by the cytoplasm, which is closely associated with 

 the wall, but the permeability of artificial cellulose membranes may be 

 taken as an approximation. Measurements of the permeabilities of such 

 membranes vary greatly, but a value of 0.001 cm- per hour per at- 

 mosphere of pressure is reasonable as the maximum permeability that 

 might be shown by cellulose similar to that of the root cell walls. 



The best available measurements of the water permeability of 

 plant cytoplasm are those made by plasmolysis and deplasmolysis of 

 onion-scale protoplasts (Levitt, Scarth, and Gibbs, 1936). The measure- 

 ments involve the movement of water not only into and through the 

 cytoplasm but also through the entire path from the outside of the 

 cytoplasm to the vacuole; hence it is not clear whether the major re- 

 sistance to flow lies in the external membrane, the cytoplasm itself, or 

 the vacuolar membrane. The linear rate of flow of water into such iso- 

 lated protoplasts is about two thousandths of a centimeter per hour per 

 atmosphere. 



To evaluate the importance of the various available pathways, the 

 cross-sectional areas presented by each of the components of the root 

 must also be considered. In the cortex of small absorbing corn roots the 

 relative volumes (and therefore relative areas presented) are approxi- 

 mately as follows: intercellular space, 10 per cent; cell wall, 4 per cent; 

 cytoplasm, 4 per cent; and vacuole, 82 per cent. From the preceding 

 data it is possible to estimate the resistance to flow of the three alterna- 

 tive paths through the root. 



Path A: Assuming a path length six cells thick from epidermis to 

 cortex, involving twelve layers of cytoplasm, the total cytoplasmic rate 

 of water flow would be 2 X ^0'^ cm. per atmosphere per hour divided 

 by twelve, or 1.7 X 10"* cni. per atmosphere per hour. The permeabil- 

 ity of the cell walls in series with the cytoplasm can be calculated by 

 dividing the specific permeability of cellulose by the total cell-wall path 

 length of 8 X 10"* cm. The resulting figure of 1.25 cm. per atmosphere 

 per hour indicates that the resistance of the cell wall in path A is so 

 much smaller than that of the cytoplasm that it can be neglected, and 

 the value of 1.7 X 10"* cm/atmos/hr, multiplied by the relative area of 

 82 per cent, gives 1.4 X 10"* cm/atmos/hr as the over-all path- A 

 permeability of one square centimeter of root surface. 



Path B : Assuming that the cell-wall path through the six-cell layer 

 from the epidermis to the endodermis is 0.15 mm. long, a hnear rate of 

 water flow in path B of 1 X lO-'^cmVatmos/hr, divided by 1.5 X 10"^ 

 cm., gives 7 X 10 cm/atmos/hr, which, multipHed by the relative area 

 of path B of 4 per cent, gives 2.8 X 10'^ cm/atmos/hr. 



Path C: No figures for the permeability of cytoplasm itself were 



