THE ENDOPLASMIC RETICULUM 1 55 



of synthesis would be designed to accept miscellaneous components of the 

 environment. 



Jagendorf : I think there is one type of cellular synthesis which is not associated 

 with these membranes. Dr. Beer now at Johns Hopkins has looked at steni 

 collenchyma cells which continue to grow even after being secondarily thickened 

 and his impression is that the new cellulose is deposited very much removed from 

 the cell membrane. 



Porter: This is a possibility, of course, and I suppose there is no reason why 

 cellulose precursors should not diffuse away from the cell surface before poly- 

 merization. 



Hestrin : When you see a vesicle in the vicinity of a cellulose fibre, the possi- 

 bility must of course be considered that the fibre arose from the vesicle. This is not 

 however the only possible explanation. In the case of a bacterium Acetobacter 

 xylinutn, cellulose occurs, at a distance from the cells and does not appear to be in 

 necessary physical connection with the cells. It seems necessary to assume for this 

 case that cellulose was excreted in the form of a diflfusible particle and that fibre 

 formation occurred in the medium by a process akin to crystallization, one might 

 be allowed on a basis of analogy to speculate that within plant cells cellulose can 

 also exist in a diflfusible form. 



Porter: Well, in the first instance I should have emphasized, if I failed to, that 

 this is strictly a morphological association which requires or asks for further 

 investigation. I think you may agree, however, that the association of the ER with 

 the cell surface is a selective one and is probably not without some significance. 

 W'hat it really means is, of course, not evident at this time. Now, in regard to the 

 other point, I recognize that cellulose polymerization or crystallization can take 

 place at points removed from the cell surface, but it was my impression that the 

 polymerization is influenced by a factor that is heat-labile and probably enzymic 

 and produced by the cell. One might expect it to act at the cell surface where 

 liberated. Am I wrong in this ? 



Hestrin: No other assumption can reasonably be made. 



