1718 CHLOROPLASTS, CHROMOPLASTS AND CHROiMATOPLASM CHAP. 37A 



grana. We mentioned above the finding of Strugger (1950, 1951j that when 

 the grana are arranged in columns the areas between the cohimns appear 

 colorless. Fluorescence-microscopic observations of Strugger (1951) and 

 Diivel and Mevins (1952) confirmed the report of Metzner (cf. p. 362) that 

 (contrary to some earlier findings of Lloyd) the chlorophjdl fluorescence is 

 restricted to the grana. We will see in section 2 that electron microscopy 

 supplies an indirect support for the assumption that all chloiophyll is lo- 

 cated in the grana. 



2. Electron Microscopy 



While light microscopy permits observation of the chloroplast struc- 

 ture under more nearly natural conditions than electron microscopy, it 

 pushes the capacity for visual discrimination to its limit, where inter- 

 pretations tend to become subjective and generalizations uncertain. Elec- 

 tron microscopy, on the other hand, gives objectively precise shapes of 

 even much smaller structural elements, but does it only after the cell has 

 been destroyed mechanically and subjected to drying in vacuimi. 



(a) Structure 



In Volume I on pages 363-364 are two early pictures of chloroplasts 

 under the electron microscope — a photograph of the edge of a chloroplast 

 from a mosaic-diseased tobacco leaf, })y Kausche and Ruska (1940), show- 

 ing the presence of large, thin lamellae (thickness 0.1 ix, diameter -^1 y); 

 and a photograph by Roberts (1944) of a single chloroplast disintegrated 

 into a conglomerate of grana of various size, from 0.02 to 0.5 /x in diameter. 



Much more detailed and revealing studies of chloroplast structure have 

 been made since. Granick and Porter (1947) used spinach. Its saucer- 

 shaped chloroplasts were found to swell in distilled water, their content 

 became clearly granular, and flat protrusions or "blebs" — probably related 

 to Kausche and Ruska's "lamellae" — appeared on their surface, the larger 

 ones on the conca^'e side of the saucer. No swelling occurred in 0.5 M su- 

 crose, suggesting that the chloroplasts were enclosed in a membrane. Ma- 

 terial most clearly showing the grana could be prepared by macerating tur- 

 gid leaves in a Waring Blendor in 0.05 M phosphate solution (?)H 6.5), at 

 5° C. The suspension was filtered through cheesecloth, and the filtrate 

 centrifuged at 200 g. to remove whole cells, crystals, and other heavy ma- 

 terial. Sedimentation at higher speed was then used to precipitate chloro- 

 plasts and their fragments, containing grana. 



It was noted that, when chloroplasts were separated from the plasma by 

 this procedure, the absoiption peak of chlorophyll shifted from 681 to 679 

 m)u, and fluorescence intensity increased. This may indicate that a change 



