The development of the chloroplast in the caulerpalean algaeHalimeda cylindracea, H. discoidea andH. macroloba has been studied by electron microscopy. The chloroplast develops from a proplastid via a starch containing young plastid. This young plastid may also develop into an amyloplast. The plastids contain a unique concentric lamellar system at one end. This has been called the thylakoid organising body (TOB) as it is at the base of this body that thylakoid formation is initiated. The TOB persists throughout the life of the chloroplast