ELECTRON MICROSCOPY 



1731 



or other changes in tlie physiological conditions in the cell. This led Leyoii 

 (1953-) to an electron micrographic study of the development of the 

 chloroplasts. He used leaves of VaUota speciosa growing in light, and 

 Taraxacum grown in darkness — the former containing much, the latter 

 little assimilation products. In young leaves he found "proplastids" 



Fig. 37A.18b. Same as in fig. 37A.18a, but sectioned normal to the large plane. 



showing a laminated structure. The continuous stratification was inter- 

 rupted, in these proplastids, only by irregularly occurring, unstructured 

 bodies of assimilates. The grana appeared in mature leaves. Menke, as 

 well as Strugger {cf. Strugger 1950) , had suggested that grana first arise as 

 nodules in continuous laminae (figs. 40b and 37A.2), and remain suspended 

 in the latter, even when mature; but Ley on, similarly to Steinmann, could 

 see no evidence of the existence of supporting laminae in mature granu- 

 lated chloroplasts. He suggested that, in a certain stage of de\elopment, 

 continuous laminae break up into packages of stacked discs — perhaps torn 

 apart by the pressure of the accumulated assimilates. 



Further studies of the development of the chloroplasts by Leyon (1954), 



