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Since the division of the light reactions of photosynthesis into two separate photoacts by Hill and Bendall (1960), several investigators have attempted to identify morphological units within chloroplast membranes which correspond to the two separate photosystems . Arntzen et al. (1969) indicated that the membrane fragments enriched in photosystem Il (PSI) activity exhibited only small (110 A, diameter) particles on fracture faces while membranes enriched in photosystem II (PS II) activity showed primarily larger particles (175 A, diameter) on their fracture faces . Goodenough and Staehelin (1971) examined freeze-fractured replicas of wild type and mutant Chlamydomonas reinhardtii and found that the distribution of the large (160 f 10 A) particle was related to the stacking of the membranes and was not related to their photosynthetic capacity, We have observed chloroplasts that have "supergrana" consisting of many stacked membranes in tissue cultures containing a viruslike particle (Sjolund and Shih, 1970) . These superstacked chloroplasts contain normal levels of chlorophyll, but they are deficient in both CO2 incorporation and 0 2 evolution. Although these chloroplast membranes are aggregated into large stacks, freeze-fracturing reveals that they lack the 175-A particle found in control chloroplast membranes .

FREEZE-FRACTURE STUDIES OF PHOTOSYNTHETICALLY DEFICIENT "SUPERGRANAL" CHLOROPLASTS IN TISSUE CULTURES CONTAINING VIRUS-LIKE PARTICLES