THE CHLOROPLAST 95 



the chloioplnll molecules are also turned inward, as well as being 

 oriented on the surface (Hodge ct ah, 1955). Calvin has recently 

 suggested a similar model in which the porphyrin heads lie at an 

 angle of 45°; one protein layer has CO^-reducing enzymes and an- 

 other protein la\'er has Oj-evolving enzymes (Calvin, 1958, 1959a, 

 1959b). 



There are several possible ways in which the chlorophyll mole- 

 cules could be oriented in the lamellae. If the porphyrin heads of 

 the chlorophyll molecules lav at 0° as flat plates as indicated in Fig. 

 5(b), their greatest cross-section would be available. However, if 

 they were oriented within the lamellae at increasing angles up to 

 90°, the cross-sectional area available would be decreasing, as would 

 the photosynthetic efficiency. Studies of chlorophyll monolayers on 

 various liquid surfaces suggested that the chloroph\'ll molecules 

 would probably lie at an angle of 35° to 55° within the chloroplast, 

 thus reducing the above calculation for the cross-section of the 

 chlorophyll molecule to 100 A- (Trurnit and Colmano, 1958). It is 

 very likely that the absorption oscillators of these pigment molecules 

 are arranged with an orderly orientation in a way that a maximum 

 absorption will occur for an incident light polarized in a given direc- 

 tion. 



The question of how the chloroplast evolves from a rudimentary 

 proplastid into an organized structure is a difficult one to answer. 

 Euglenas grown in darkness for long periods become colorless. We 

 have noted that there is an obvious change in the chloroplasts; they 

 fragment, and their lamellar structure is no longer recognizable. 

 Such dark-adapted euglenas, brought back to the light and fixed for 

 electron microscopy at regular time intervals, showed in electron 

 micrographs, after as little as 4 hours' light exposure, recognizable 

 elongated bodies with the characteristic laminations of chloroplasts. 

 At the beginning, their lamellae were very thin and few in number 

 and were not tightly or regularly packed. The lamellae grew in size 

 and number progressively with continuing light exposure along with 

 the increase in the synthesis of chlorophyll, and, by 72 hours of 

 light exposure, the chloroplasts had the form and organization 

 described for actively photosynthesizing euglenas (Wolken and 

 Palade, 1953). As soon as we could detect chlorophyll spectrophoto- 

 metribally, the chloroplasts were already lamellar. The alga Chlamij- 

 domonos, whether grown in light or in darkness, shows a chloroplast 

 organization of lamellae, pyrenoid, and eyespot within a limiting 



