TRANSPORT PROCESSES IX THE SOIL-PLAXT SYSTF.M 719 



upon the potential difference between the external medium and the 

 cell vacuole, which is difficult to measure over the course of an experi- 

 ment. Many investigators have assumed, however, that a carrier system 

 or "ion pump" may be functioning, using respiratory energy, regardless 

 of whether the ions are being pumped "uphill" or "downhill." Althougli 

 subject to numerous technical difficulties (measurement and respira- 

 tory quotients), studies of the relation of nutrient accumulation to 

 respiration have yielded the information that the f(nu- theoretical mono- 

 valent ions can seemingly be accumulated for each ox)'gen molecule 

 used in respiration. 



Because ions accumulate in the vacuoles, and because respiratory 

 energy is needed for this transfer, regardless of the actual potential 

 gradient, it may be that path A would require too much energy for 

 the transfer of ions across the cortex. Thus one oxygen atom might be 

 needed for the transfer of one mono\alent ion for each passage from 

 vacuole to cytoplasm or cxtoplasm to vacuole. In this case the effi- 

 ciency of the process \\()u]d be prohibitively low for a path several 

 cells long, imless there was some mechanism by which the energy re- 

 leased by an ion moving "downhill" would become available to move 

 another ion "uphill." 



Cellulose is permeable to salts, so tliere is a possibility that some of 

 the ions taken into the plant flow passively with the transpiration 

 stream through the cell walls ( path B ) . The same may be said for the 

 flow through the cytoplasm (path C) if the cytoplasm is indeed per- 

 meable to ions. The permeability of the cytoplasm is now being investi- 

 gated at several laboratories, with some investigators reaching one con- 

 clusion while other investigators reach the opposite conclusion ( Haap- 

 ala, I960; Kylin, 1960). 



For the present, the following picture may suffice to explain the 

 ni()\ement of ions in the cortex. Water flows through the cell walls, and 

 possibly the cytoplasm, drawn by the evaporation at the leaf. This 

 water carries ions with it. Tlie cortical cells act as ion sinks and sources, 

 and tliey lie adjacent to the path of transport, withdrawing ions from 

 the transpiration stream or secreting ions into the stream in response 

 to metabolic regulatory mechanisms. 



At the endodermis, as was mentioned before, the ions become sub- 

 ject to the influence of a differentially permeable membrane, and it may 

 well be that an osmotic potential at this membrane is responsible for 

 the well-known root pressure of plants at low transpiration rates. The 

 endodermis need not be completely impermeable to salts to cause root 

 pressure, as the root could be acting as a "leaky osmometer" (Philip, 

 1958), with merely a greater resistance to ions than to water in the 

 endodermis. From the inside of the endodermis upward through the 



