140 TRANSLOCATION IN PLANTS 



through surface membranes, whereas a surface membrane 

 would interfere with such movement between the same 

 cells and nonliving, water-conducting cells or any sur- 

 rounding solution. This may explain the seeming contra- 

 diction that living cells are only slightly permeable to 

 sugars and yet sugars must move readily and rapidly from 

 cell to cell within the plant. 



On the other hand, Weevers (1931) has recently claimed 

 that the low permeability to sugars of the outer plasma 

 membrane has been overstressed. He thinks that the 

 low permeability is chiefly restricted to the tonoplast or 

 vacuolar membrane and that methods used which show 

 low permeability involve permeabiUty of this inner mem- 

 brane and not of the outer one. The rapidity with which 

 yeast absorbs and ferments sugar and the readiness with 

 which algae and leaf parenchyma tissues store starch when 

 floated on sugar, he cites as evidence of rather high per- 

 meability of the outer membrane to sugar. Fungi and 

 bacteria obviously must necessarily receive all their 

 organic solutes through surface membranes and may have 

 a different type of surface membrane, but with chlorophyll- 

 bearing organisms this surface membrane seems to be 

 much less permeable. It is true that parenchyma tissues 

 can absorb sugar from bathing solutions or lose it to them, 

 but the indications are that the rate of this absorption 

 or loss through the surface membrane is much less than 

 the absorption by storage tissues connected by living 

 cells to supplying tissues, or the loss from the supplying 

 tissues to receiving ones. 



It is possible, on the other hand, that the differences in 

 rates of exchanges between living cells and those between 

 living cells and a nonliving environment are not related 

 to differences in permeability but to relative surfaces 

 exposed. There is evidence also that the conditions of 

 aeration may partly account for slow movement to or from 

 a bathing solution. Puriewitsch (1898) and Griinfeld 

 (1926) have found that emptying and refilling of endo- 

 sperms and cotyledons are dependent upon favorable 



