In this study the proplastid development in embryonic cells is described for the apical meristem of Elodea canadensis, embryo sacs from Lilies, and Begonia leaf buds. The formation of these cell organelles originates with submicroscopical particles which consist of a homogeneous stroma with a surrounding double membrane. When these proplastids reach an average size of 1 µ, the inner layer of the membrane begins to invaginate into the stroma. This process is comparable to tubuli formation in mitochondria.
Under growth conditions with sufficient exposure to light, the development of the grana and stroma lamellae proceeds without interruption. If the plants are kept in the dark, small vesicles are formed which accumulate in the prolamellar body of the proplastids. After illumination these elementary vesicles merge to form membranes which evolve into grana and stroma lamellae. The structural similarity of the early proplastid stages with the mitochondria seems to indicate that there exists some phylogenetic relationship between the two cell organelles.