118 MINERAL SALTS ABSORPTION IN PLANTS 



the transpiration stream through wet cell walls in the more mature 

 regions of the root, via "passage" cells in the endodermis, where the 

 Casparian band (p. 15) is absent or incomplete, and directly into the 

 cavities of the conducting elements. The possibility that salts and 

 water traverse the cortex passively through the cytoplasm has also 

 been suggested (Kramer, 1957). If cytoplasm forms part of the 

 free space of a tissue, it may represent a significant channel for 

 passive ion movement, but present opinion is against such an idea 

 (see Chapter 6, p. 100). 



There are two possible routes along which salts are transported 

 actively into the stele. In the first, ions may be taken up into the 

 cytoplasm of the surface cells of the root, transported from cell to 

 cell via the plasmadesmata, and released from the cytoplasm adjoining 

 the conducting elements into the xylem sap. In the second, they 

 may move passively in cell walls as far as the endodermis and be 

 transported actively from this point via cytoplasm into the stele. 

 The relative importance of the two pathways is not certain. Measure- 

 ments of the apparent free space of roots (Hope and Stevens, 1952; 

 Butler, 1953; Epstein, 1955) indicate that a considerable volume of 

 the root (10-35 per cent) is passively penetrated by salt. This 

 probably means that the surface layer of cells does not present an 

 impermeable barrier to diffusion and mass flow, and that ions can 

 move relatively freely in the walls of the cortex cells, as far as the 

 endodermis, where further penetration is probably retarded by the 

 lipid-impregnated Casparian band. That the root surface does, 

 however, present some resistance to passive movement of salts is 

 indicated by an observation of Sandstrom (1950) that when an outer 

 layer of cells is removed by immersing roots for a short time in di- 

 n-amylacetic acid, absorption of salt subsequently occurs at an 

 increased rate. 



The amount of salt which penetrates passively through cell walls 

 as far as the endodermis probably depends on the absorptive activity 

 of cells located nearer the surface, and on the concentration of salts 

 in the medium. When the external concentration of salt is low, it is 

 possible that most of the salt has been extracted from the trans- 

 piration stream before it reaches the boundary of the stele. Ions 

 absorbed into the cytoplasm of cortex cells, either directly from the 

 medium, or from the transpiration stream, are then either transferred 

 into central vacuoles or transmitted via the plasmadesmata towards 



