360 CHLOROPLASTS AND CHROMOPLASTS CHAP. 14 



(Weier 1938; Beauverie 1938), while in others, the grana become clearly 

 oiithned only after a special treatment (e. g., in clover, according to 

 Weier, after the leaves have been kept in darkness for 15 hours). Chemi- 

 cal treatment, which tends to make the protoplasmic structure coarser 

 (as, for example, treatment with 20% ethanol according to Wieler 1936), 

 often makes the granular structure more pronounced. Chodat (1938) 

 obtained a similar result by heating the starch-bearing chloroplasts of 

 Pellionia daveanana to 80° C. 



As a result of these varying observations, some authors, e. g., Heitz 

 (1936), consider the grana as obligatory structural elements of all normal 

 chloroplasts, while others (Weier 1936; Menke 1938) think that some 

 chloroplasts are granular and others genuinely homogeneous. 



Heitz (1936) found that the grana remain 

 intact in chloroplasts treated by fixatives, as 

 well as in dried cells, and can be therefore iden- 

 tified even in leaves from an herbarium. 



Additional evidence in favor of the granular struc- 

 ture of chloroplasts has been derived from experiments 

 on the reduction of silver nitrate by these bodies (the 

 Molisch reaction mentioned on pages 2.54 and 270). In 

 some of these experiments {cf. Fig. 40), silver deposits 

 were found to enclose lighter colored islands (Wieler 

 1936; Weber 1937; Dischendorfer 1937); and it was sug- 

 gested that these are identical with the chlorophyll grana. 

 According to Pekarek (1938), silver is deposited on the 

 boundary between stroma and grana, forming a netlike 

 pattern. However. Weber (1937) and Weier (1938^) 

 warned against uncritical identification of the silver- 

 surrounded islands with chlorophyll grana, pointing out 

 that silver patterns of a different type are often obtained 

 in the Molisch experiment, and that the location of silver 

 Fig. 40. — Granular sil- deposits around the chlorophyll grana has never been 

 ver deposits in chloroplasts directly demonstrated. The silver nitrate treatment kills 

 (after Weber 1932). the cells, and can thus produce all kind of artefacts. 



Some earlier authors, e. g., Timiriazev (1903) and Priestley and 

 Irving (1907), assumed that all grana are concentrated on the surface of 

 the chloroplast. More recently, Wieler (1936), too, thought the grana 

 to ])e embedded in the surface layer of the stroma. However, the photo- 

 graphs by Heitz (1936) and Doutreligne (1935) show the grana to be 

 distributed more or less uniformly throughout the chloroplast. Heitz 

 observed that sometimes the grana form several layers, so that the 

 chloroplasts appear striated when looked upon from the side (cf. Fig. 

 39c). Certain reagents (10% acetone, for instance) cause the grana to 

 clump together in one corner of the chloroplast (Wieler 1936). 



According to Heitz (1936), the grana are flat discs. This can best 



