Chapter VI —51— The Plastids 



stratum for the pigments. Schmitz attributed a fibrillar structure 

 to algal chloroplasts and Schwarz considered the chloroplasts of 

 higher plants to be composed of two substances, the one, chloro- 

 plastin, formed of basic filaments, lying closely together, containing 

 chlorophyll grains, and the other a colorless substance, metaxin, 

 interposed among the filaments. 



SCHIMPER and Meyer held that chlorophyll in chloroplasts was 

 found as inclusions, the grana, so small as to border on the limits 

 of visibility. These grana they found to be very numerous and 

 diflficult to distinguish, giving the impression that the piginent is 

 found in a diffuse state in the plastidial substratum. According to 

 these two investigators, xanthophyll is distributed in the same way 

 in chromoplasts and only carotin, when it is not in a crystalline 

 state, exists in the form of clearly differentiated grana. 



Various arguments of a theoretical nature next led physiologists 

 to think that the chloroplasts must have a homogeneous structure. 

 A comparative study of the spectra of a chlorophyll solution and 

 of a living green leaf show, indeed, that the absorption bands do 

 not occupy the same positions in the two cases, and that those for 

 the living leaf coincide with those shown for a colloidal solution of 

 chlorophyll. It is known that solutions of chlorophyll break down 

 rapidly in light in the presence of oxygen. There is then an 

 oxidation of chlorophyll. In the chloroplast, on the contrary, the 

 chlorophyll manifests a great stability which could only be shown 

 by a colloidal solution of chlorophyll united to other colorless col- 

 loids. Certain facts, furthermore, lead to the supposition that 

 chlorophyll may be combined in the plastidial substratum either 

 with proteins or with lipides. Hence it has been thought that 

 chlorophyll must be uniformly distributed throughout the entire 

 mass of the chloroplast and that the latter is constituted of a 

 hydrogel formed of various colloids, some colorless, others colored 

 (Siebold), or formed of colloids containing chlorophyll chemically 

 combined with colorless colloids. According to Ponomarew, Le- 

 PESCHKIN, and Kuster, the gel of the chloroplast is in a semi-fluid 

 state and the variations of form undergone by this organelle may 

 be explained by a too weak tension between the cytoplasm and the 

 chloroplast. Investigations of Price, Scarth, Lapicque, Guillier- 

 MOND and Mangenot have shown, moreover, that chloroplasts, al- 

 though sometimes giving the impression in direct light of having 

 structure, are always optically empty under the ultramicroscope 

 and are not seen except for their color and their faintly luminous 

 contours. This confirms the opinion stated above. Finally, mito- 

 chondrial technique, which best conserves the chloroplasts and 

 makes their lipides insoluble, always reveals these organelles as 

 absolutely homogeneous, while those techniques which dissolve 

 lipides bring out a heterogeneous structure which may be supposed 

 to correspond to an alteration. 



More recent cytological work, however, would tend to prove, on 

 the contrary, that the chloroplasts do have a structure. According 

 to ZiRKLE, they are formed of a chlorophyll-containing hydrogel 



