ELECTRON MICROSCOPY 



1729 



become autotrophically active when exposed to light, but switch to hetero- 

 trophic metabolism in darkness. The organisms used were Euglena gracilis, 

 which, in the active state, measures about 70 X 20 ^ and contains 8-12 

 chloroplasts, and Poteriochromonas stipitata, a much smaller, round cell, 

 with usually only two chloroplasts. 



The cells were fi.xed with 1% OSO4 at pH 7-8, swelling V)eing prevented 

 i)y the addition of sucrose. The preparations were embedded in plastic 

 and sectioned to 0.1 n thickness, or thinner. The chloroplasts of active 



Fig. 37A.17. Sectiun of an Aspi({i)<tra cliloroplast hxvd in I'ormaklehytle solution 

 ing continuous laminar structure inten-uptcd by starch grains (Lcyon 1!)53V 



■;ho\v- 



Euglena appeared as cylindrical bodies, 0.5-1.5 m by 5-10 m in longitudinal 

 cross section. They showed faint lamination luider the phase contrast 

 microscope, and double refraction in the polarization microscope. Electron 

 microscopy showed a laminated structure of the chloroplasts as a whole, 

 without subdivision into grana. The lamina were 18-32 mfj. thick — con- 

 siderably thicker than the "discs" into which grana-bearing chloroplasts 

 disintegrate (< 10 myuj. The lamina were separated by interstitial spaces 

 30-50 m^i in width. The layers were even thicker in Poteriochromonas; 

 an indication of a further substructure (a central layer of lighter mate- 

 rial, and two denser outside layers) could be discerned in them. 



After 7 days dark gi'owth, the Euglena chloroplasts vanished altogether, while 

 those of Poteriochromonas appeared to have shrunk and lost their stiucture. Laminated 



