Seedlings of Chenopodium rubrum were grown on a fully defined medium and induced to flower by exposure to cycles of 8 hr light/16 hr darkness. The development of the anthers was studied using histochemical techniques. Particular attention was paid to the delayed development of the endothecium or fibrous layer and to the chemistry of the endothecial bars of thickening. These bars were found to consist mainly of α-cellulose as defined by insolubility in 17.5% NaOH. In contrast, most of the wall material in the other tissues of the another was low in oriented α-cellulose and consisted of pectic substances (0.5% ammonium oxalate soluble), hemicelluloses (4% NaOH soluble) and non-cellulosic polysaccharides (17.5% NaOH soluble); callose (defined as positive to lacmoid) was also detected as a transient material. Lignin was not detected in the endothecial bars of thickening. The concept of programmed control of tapetal and sporogenous tissues invoked in a paper by Heslop-Harrison (1964) has been extended to sugge...