A mechanism for the formation of lamellar systems in the plant cell has been 
proposed as a result of electron microscope observations of young and mature 
cells of Nitella cristata and the plastids of Zea mays in normal plants, 
developing plants, and certain mutant types. The results are compatible with the 
concept that lamellar structures arise by the fusion or coalescence of small 
vesicular elements, giving rise initially to closed double membrane Structures 
(cisternae). In the chloroplasts of Zea, the cisternae subsequently undergo 
structural transformations to give rise to a compound layer structure already 
described for the individual chloroplast lamellae. During normal development, 
the minute vesicles in the young chloroplast are aggregated into one or more 
dense granular bodies (prolamellar bodies) which often appear crystalline. 
Lamellae grow out from these bodies. In fully etiolated leaves lamellae are 
absent and the prolamellar bodies become quite large, presumably because of 
inhibition of the fusion step which appears to require chlorophyll. Lamellae 
develop rapidly on exposure of the plant to light, and subsequent development 
closely parallels that seen under normal conditions. The plastids of white and 
very pale green mutants of Zea similarly lack lamellae and contain only 
vesicular elements. A specialized peripheral zone immediately below the double 
limiting membrane in Zea chloroplasts appears to be responsible for the 
production of vesicles. These may be immediately converted to lamellae under 
normal conditions, but accumulate to form a prolamellar body if lamellar 
formation is prevented, as in the case of etiolation and chlorophyll-deficient 
mutation, or when the rate of lamellar formation is slower than that of the 
production of precursor material (as appears to be the case in the early stages 
of normal development).