In roots of Phaseolus vulgaris plastid development takes place from the apical meristem both upwards into the root proper and downwards into the root cap. The maximum state of plastid development seems to be achieved in the cortical cells of that part of the primary root associated with mature root hairs. However in these cortical cell plastids the thylakoid system is very limited in extent and no true prolamellar bodies are formed. Farther from the apex of the root proper, in the zone of mature lateral roots, the plastids appear to have undergone dedifferentiation and to have lost whatever thylakoids they once contained. Both the rate of plastid development and the patterns of production of ancillary plastid structures, including starch and membrane–bound bodies, vary in different cell files. The differential distribution of these and other plastid features are summarized in the accompanying diagrams.