106 THE BACTERIAL PHOTOCHEMICAL APPARATUS 



system, the detailed structure of which is not yet fully interpretable; 

 we have the impression that it may not be organized in the same 

 fashion in every strain. Immediately within the inner layer of the cell 

 wall, there are either one or two unit membranes. Parallel and adjacent 

 to this unit membrane system is a thinner, electron-dense membrane, 

 some 50 A thick. This thin membrane appears to surround and enclose 

 the series of large, clear oblong areas which line the cortex of the 

 cytoplasm between the surface membrane system and the ribosomal 

 region. Every strain that we have examined contains these character- 

 istic oblong structures, the shape and disposition of which are particu- 

 larly well shown in tangential sections (Figs. 13 and 14). We shall 

 term them chlorobium vesicles. They are relatively large, and of 

 somewhat variable dimensions, 1000-1500 A long and 300-400 A wide. 

 In vestopal- embedded cells (Figs. 13-15), the chlorobium vesicles are 

 transparent to the electron beam, and hence might be interpreted as 

 empty. However, in epon-embedded cells, they appear much denser 

 than the adjacent cytoplasm. 



All the strains of green bacteria that we have examined also contain 

 large, conspicuous, and complex membranous intrusions similar in 

 structure and derivation to the so-called "mesosomes" found in a 

 variety of nonphotosynthetic true bacteria. There are relatively few 

 (1 to 3) in the cell; they commonly lie deep in the cytoplasm, some- 

 times intruding into the nuclear region. In favorable sections, the 

 mesosomal membranes can be seen to connect with the inner layer of 

 the cytoplasmic membrane (Fig. 14), 



In this anatomical labyrinth, where is the photosynthetic apparatus 

 located? We are not yet prepared to give a categorical answer to this 

 question, and will simply describe a few observations which bear on it. 

 The first point to be emphasized is that all the strains of green bac- 

 teria we have examined have a very high specific chlorophyll content- 

 far higher than that of purple bacteria growing at the same light in- 

 tensity. This is shown by the data in Table 2, In view of our cytological 

 experience with purple bacteria, it seems reasonable to assume that 

 the photosynthetic apparatus of a procaryotic organism with such a 

 high chlorophyll content should occupy a substantial volume of the 

 cytoplasmic region of the cell. For this reason, it is unlikely that the 

 mesosomal elements constitute the sole, or even the major, site of 

 the photosynthetic pigment system. 



The cells of green bacteria can be readily broken in the French 

 pressure cell, or (in the case of cultures in the stationary phase) by 

 osmotic lysis, following treatment with lysozyme and versene. After 

 osmotic lysis, followed by DNAse treatment to reduce the viscosity of 

 the lysate, all the chlorophyll and carotenoid in the extract is sedi- 

 mented at low gravitational fields, in association with the lysed 

 spheroplasts. Such behavior would not be expected of a holochrome with 

 a molecular weight of 1,5 million. In another experiment, cells were 



