Chloroplast development in both coiled and uncoiling croziers of Phyllitis scolopendrium has been studied. An interesting stage of chloroplast ontogenesis appears to be that characterized by the presence of very long thylakoid stacks. It resembles the final stage of organization attained by the chloroplasts of many green algae at maturity even if, in P. scolopendrium, it represents only an intermediate stage in thylakoid system ontogenesis.

Growing the plants in the dark has a pronounced effect on the morphogenesis of the frond and on plastid differentiation. In both dark- and light-grown plants we could observe, even if in different amounts, prolamellar body-like structures interconnected with the thylakoids. The presence of such structures is discussed, especially in view of the fact that ferns are able to synthesize chlorophylls in the dark.