Microsporogenesis in Taxus baccata L.: The Formation of the Tetrad and Development of the Microspores

Abstract

Following meiosis II in Taxus microsporangia a small proportion of the tetrads regularly degenerated. Despite frequent inequality in the frequency of ribosomes between the spores of a tetrad, partial degeneration within a tetrad was never observed. The initial wall of the young spores was found to resemble the wall of the mother cell in containing a fibrillar layer, and the two walls may possess similar isolating properties. The symmetry of the tetrad was regularly iso-bilateral. The formation of the sporoderm began as the spores were released into the loculus by the rapid dissolution of the wall of the mother cell. Osmiophilic droplets emerged from the spore protoplast and entered the wall. The fibrillar layer ceased to be recognizable and the droplets coalesced to form an outer layer on which up to six sporopollenin lamellae, probably of tapetal origin, were deposited. The accretion of a single layer of sporopollenin droplets, in no recognizable pattern, gave rise to the outer verrucose part of the exine. Cytochemical tests showed that the tapetum was rich in acid phosphatases from the beginning of meiosis. Towards the end of its degeneration the tapetum intruded into the loculus and could therefore be regarded as partly invasive.