Development of oil and mucilage cells in Cinnamomum burmanni. An ultrastructural study

The development of oil and mucilage cells in the shoot apex and young leaves in Cinnamomum burmanni is described. Three arbitrary developmental stages are distinguished. At stage 1, the cells differ from the surrounding cells by the absence of osmiophilic deposits in the vacuoles and the presence of distinct small plastids with reduced thylakoids. At stage 2, a suberized layer is deposited in the cells. At stage 3, the oil or mucilage secretion has started in the cells. For both oil and mucilage cells, three intergrading stages (3a, 3b, 3c) are recognized. In oil cells, the subdivision is based on the thickness of the inner-wall layer, the composition of the cytoplasm and the extent of the oil cavity. The thickness of the inner-wall layer increases with development. The oil cavity, enclosed by the plasmalemma and attached to the wall by a cupule, enlarges. Oil formation presumably takes place within the plastids and apparently results in the disintegration of the plastids. In mucilage cells, the substages are distinguished based on the amount of mucilage deposited between the suberized layer and the plasmalemma, and the composition of the cytoplasm which contains an increasing amount of dictyosome vesicles. For the first time, cupule-like wall structures are reported in mucilage cells. Therefore, the number of similarities in both oil and mucilage cells increases to four: the presence of a suberized layer, an extraplasmatic accumulation of the secretory product, and the presence of a cupule and characteristic plasmodesmata. These resemblances strongly support their hypothesized homology.