710 PLANT GROWTH AND PLANT COMMUNITIES 



found for use in evaluating path C. If the resistance to flow is the 

 same throughout the cytoplasm, or if the resistance of the external 

 membrane is the same as that of the vacuolar membrane, very little 

 flow might be expected in path C, due to its limited cross-sectional 

 area. However, if the major cytoplasmic resistance to flow is in the 

 vacuolar membrane, it may well be that substantial flow occurs in 

 path C. For the moment, flow in this path will be neglected. Based on 

 the rough calculations of the transmission characteristic of paths A and 

 B, it is concluded that, in spite of the relatively small area presented by 

 the cell walls, the major portion of the water flow may occur in them 

 rather than through the cells themselves. It may also be concluded that 

 the root cortex presents two parallel paths for water flow, one path 

 being independent of the cytoplasm and one being through the cyto- 

 plasm. Therefore any changes in the permeability of the cytoplasm 

 might be expected to change the total root permeability to a slight ex- 

 tent, but the water flow through the root is probably not completely 

 controlled by the cytoplasmic permeability. 



The endodermis is an enigmatic tissue, but it seems usually to be 

 interpreted by plant physiologists as a structure in which all transport 

 must necessarily take place via the cytoplasm and vacuole. If this is 

 true, the endodermis may be the site at which all material entering or 

 leaving the root becomes subject to the restrictions imposed by living, 

 differentially-permeable membranes. It seems likely, therefore, that the 

 endodermis is a salt barrier rather than a water barrier, and it will be 

 treated as such in this paper. 



Recent work has tended to indicate that the vacuolar membrane, 

 rather than the external membrane or the cytoplasm of a cell, might be 

 the cell's salt barrier. If this is true, one might reasonably assume that 

 the greatest resistance to water flow also resides in the vacuolar mem- 

 brane, and that the cytoplasm itself is relatively permeable to water. 

 In this connection it should be pointed out that the figures used in the 

 calculation of the cell-wall permeability are for rather permeable cellu- 

 lose, but that normal cell walls contain considerable pectic material, 

 which might decrease the wall permeability. Therefore, if only moder- 

 ate potential drops exist across the cortex, it might be reasonable to 

 assume that most of the water is flowing through path C. Insufficient in- 

 formation is available at present to justify a definite statement as to 

 whether path B or path C is the principal water path. However, there 

 is some evidence that the permeability of roots to water can change 

 rapidly, in a direction opposite to that which might be expected from 

 purely physical considerations, and that therefore a significant part of 

 the water ffow must be through cytoplasm. Rufelt (1959) found that 

 when the roots of transpiring plants were moved from a warm to a cold 

 solution, there was an initial increase in transpiration, followed by a 



