The Chloroplasts of Equisetum Telmateia Erhr.: A Possible Developmental Sequence

S ummary Parenchyma cells in the leaf sheath of Equisetum telmateia form linearly arranged series of maturing and senescing cells. These series of cells contain chloroplasts showing a progressive sequence of developmental stages easily recognizable because of the spatial relationships involved. The cell sequence may be classified into four successive zones, characterized by the typical structure of the chloroplasts present in the cells of each zone. Zone I. Meristematic cells contain immature chloroplasts with little internal lamellar structure. Zone II. Adjacent to the above are cells containing mature chloroplasts with a well‐developed internal lamellar system. Zone III. Next in sequence are cells in which the chloroplasts retain a mature internal lamellar system but are distinguished from the previous group by the presence of irregular osmiophilic deposits around the outer double membranes. Zone IV. Finally there occur cells with usually rounded chloroplasts which show the osmiophilic deposits described above, but in which the outer double membrane is often difficult to distinguish. Internally the lamellar system is highly sinuous, and contains many swollen locules. In spite of a general similarity in appearance between chloroplasts of Equisetum and those of angiosperms, Equisetum plastids are characterized by a distinctive feature more commonly associated with algae than with higher plants. Young chloroplasts are almost completely surrounded by rough endoplasmic reticulum. This very close relationship between reticulum and chloroplast is retained as the chloroplast matures, though the degree of envelopment becomes less with increasing age.