708 



PLANT GROWTH AND PLANT COMMUNITIES 



water through the cortex of an absorbing root are illustrated in Fig- 

 ure 12. 



A. The most obvious path, and indeed the only path usually con- 

 sidered in a cursory examination of the problem, is that directly 

 through the cell walls, cytoplasm, and vacuoles of the cells. 



B. The cell walls themselves ofiFer an attractive possibility. 



C. The third path, involving cytoplasm and cell walls, but not in- 

 volving the vacuoles, must be considered, especially in view of recent 

 evidence that the cytoplasm itself may be relatively permeable ( Kylin, 

 1960). 



Additional paths involving intercellular spaces may be available, 

 but they will not be considered here because such spaces are thought 

 usually to be air-filled and therefore unavailable for liquid-water trans- 

 port. If these spaces should be water-filled, however, their maximum 

 eflFect would be to approximately double the permeability of any path 

 involving the cell walls. Examination of the longitudinal and cross- 

 sections of a root reveals no continuous radial path available through 

 intercellular spaces alone. 



To decide which of the available paths might be taken by the 

 water, it is helpful to make estimates of the water permeabilities of the 

 various materials involved. 



Cell walls are composed of a framework of cellulose associated 

 with a variety of pectic and other substances; the latter sometimes ac- 



Figure 12. Schematic diagram of three possible flow paths through plant 

 tissue. 



