152 KEITH R. PORTER 



evident during secondary wall formation (Fig. 12). As before, the ER is 

 represented by small profiles at the cell surface, a reflection of the smaller 

 units comprising the system at this level. At a phase in active wall forma- 

 tion, presumably depicted in Fig. 12, a layer of the cell cortex is externalized 

 and the peripheral elements of the ER with it. In this process of ecdysis, as 

 pointed out in the figure-legend, a new plasma membrane appears to form 

 back of the margin. Again the morphological involvement of the ER is 

 taken to suggest functional involvement and one would gather from the 

 micrographs that parts of the ER are used up in the process [46]. 



If now these peripheral elements are functionally significant, let us say 

 in cellulose polymerization, any patterns in their distribution at the surface 

 should find some reflection perhaps in the thickness of the wall. One can 

 therefore with profit and interest explore cells where one finds ring or 

 spiral thickenings, as in primitive tracheids. These represent the final stage 

 in the difl^erentiation of these cells and they contain only a small residue of 

 cytoplasm. This and its properties make fixation diflicult. Nonetheless 

 when, after sectioning, these cells are examined, it is found that residual 

 elements of the ER are distributed in a pattern relative to the secondary 

 thickenings (Fig. 13). Earlier stages in the formation of these rings have 

 not been observed as yet, so the character of the ER which anticipates their 

 location cannot be depicted or commented upon. 



The question of organization in the ER and its role, if any, in the 

 determination of cell form and polarity and the disposition of secondary 

 products of differentiation, is of course a large one and not to be solved 

 with a few electron micrographs. It does seem, however, that the form the 

 E.R adopts is determined in part at least by the molecular architecture of 

 the system itself and more remotely by genie information. In determining 

 through its morphology the spatial intracellular disposition of biochemical 

 reactions, it becomes an instrument for the nuclear control of these 

 phenomena. 



References 



1. Porter, K. R., Claude, A., and Fullam, E.,7. exp. Med. 81, 161 (1945). 



2. Hartmann, J. F.,jf. comp. Neurol. 99, 201 (1953). 



3. Watson, M. L.,^. biophys. biocheni. Cytol. i, 257 (1955). 



4. Porter, K. R., J. exp. Med. 97, 727 (1953). 



5. Hagunau, F., Int. Rev. Cytol. 7, 426 (1958). 



6. Palade, G. E.,^ biophys. biocheni. Cytol. 2 suppl., 85 (1956). 



7. Porter, K. R., in "The Cell", eds. J. Brachet and A. E. Mirsky, Vol. II. 

 Academic Press. 621 (1961). 



8. Brachet, J., Arch. Biol. 53, 207 (1941). 



9. Caspersson, T., " Cell Growth and Cell Function." New York, Norton (1950). 

 10. Garnier, Ch., Contribution a I'etude de la structure et du functionnement des 



cellules glandulaires sereuses. Du role de Tergastoplasm dans la secretion. 

 Thesis, Nancy, No. 50 (1899). 



