Characteristics of the spore walls developing during the tetrad period have been studied in Lilium longiflorum using cytochemical methods and scanning electron microscopy. Following upon meiosis II, each spore forms a separate wall, the primexine. The primexine consists of a tenuous matrix component identified tentatively as cellulose, traversed by radially directed rods, the probacula. Initially these possess a lamellated or granular appearance, and are not resistant to acetolysis; their chemical identity at this early developmental stage is unknown. The distribution of the probacula and the interconnections they form above and below the loose matrix material establish the basis of the final exine pattern. As the tetrad ages, the patterned component formed by the probacula becomes resistant to acetolysis. On release from the tetrad, the spores expand, and there is a rapid accretion of new materials in the patterned wall component. For a period, free aldehyde groups appear in the wall, but this characteristic is lost in the mature exine. The cellulose matrix of the primexine is dispersed and probably partly degraded after release of spores from the tetrad, but a residue persists in the exine cavities.