ELECTRON MICROSCOPY 



1727 



micrograph of a fixed and sectioned Chlorella cell, showing that the ciip- 

 shaped chloroplast consists of 4-6 dark shells, each about 35 m,x thick, 

 separated by lighter layers of about the same thickness. Each dark shell 

 seems, in its turn, to consist of several (usually four) thinner shells each 

 about 5 m/x thick. No grana are visible in the picture. 



On the other hand, stacks of disc-shaped lamellae (of the type of those 

 shown in fig. 37A.8) were obtained, in the same study by Steinmann, from 



ca. 



5000 i 



ca.lOOA 



Fig. 37 A. 15. Idealized cross section of a granum disc, consisting of two 

 monolayers of macromolecules (after Frey-Wyssling and Steinmann 1953J. 

 As observed under the electron microscope, the discs are only 50-70 A. 

 thick because of shrinkage. 



Aspidistra chloroplasts; they contained up to 40 thin discs of uniform diam- 

 eter. Figures 37A.18a and 37 A. 18b show the cross section of lamellae in 

 fixed and sectioned chloroplasts of a higher plant according to Palade 

 (1953). 



Steinmann drew attention to the occurrence of lamelhir structure in visual rods, 

 and suggested that this similarity may have something to do with the common purpose 

 of the two cellular structures— utilization of light energy. 



In another note, Steinmann (1952^) showed that single thin round 

 protein discs can be obtained from granular chloroplasts {e. g., those of 

 tulip, or Aspidistra) by sonic disintegration (fig. 37 A. 13). Electron micro- 

 graphs of sections through fixed tulip chloroplasts clearly showed that such 

 discs are the structural elements of intact grana. Incidentally, these elec- 

 tromicrographs— c/. fig. 37A.14— also confirmed the layered arrangement 

 of the grana in each chloroplast, first noted by Heitz on ultraviolet micro- 

 graphs (cf. fig. 39c in Vol. I). 



No carrier lamellae which support the grana in a layer, accordnig to 

 Strugger (1951), are visible on fig. 37A.13. Leyon (1954') noted, how- 

 ever, that in Aspidistra elatior, the lamellae within the grana extend from 

 it into the surrounding stroma, the grana being merely regions of accentu- 

 ated lamination. 



Frey-Wyssling and Steinmann (1953) suggested that the discs (thick- 

 ness, 7-10 mM) obtained by disintegration of grana from the chloroplasts of 



