Pollen ontogeny and its wall formation in Cunninghamia lanceolata

Abstract

During meiosis, microsporocyte cytoplasm shows considerable changes, expressed in greatly increased organelles and in their equatorial distribution at telophase I and II. At the early tetrad stage, the formation of microspore wall is initiated by deposition of electron dense granules against the callose wall and the appearance of vesicles outside the plasmalemma except at the distal area. After a thin layer of granules, initial ectexine, and fibrillar materials are formed, the endexine starts with the apposition of tripartite lamellae. During this time, many osmiophilic granules are present in the cytoplasm. Both ectexine and endexine continue to develop in the free microspore period. The intine is a homogeneous layer of variable thickness. Intranuclear inclusions, appearing at the early tetrad stage, coincide with exine formation. The difference between sporopollenin synthesized by the microspore and by the tapetum is discussed.