Pollen wall and tapetum development in Anaxagorea brevipes (Annonaceae): sporoderm substructure, cytoskeleton, sporopollenin precursor particles, and the endexine problem

Abstract

Two oppositely directed processes — uptake and secretion — occur probably simultaneously at the cell surface of a young tetrad microspore in Anaxagorea brevipes. Spiral elements, being the structural units of the microspore glycocalyx, are also seen in the tapetal cell glycocalyx, associated with the pro-orbicules. The glycocalyx spiral elements are connected with the cytoskeleton during the tetrad period. The sporopollenin precursor accumulations become evident at the middle tetrad stage as dark roundish particles. They are associated with the tops of the spiral elements and are the beginning of the protectum. Later sporopollenin precursor particles fuse with each other, resulting in a continuous, unbroken tectum. The granular infratectum is initiated at the late tetrad stage, the granules are also connected with the glycocalyx spiral elements but do not fuse with each other during the further development. The uneven foot layer of the ectexine forms at the early post-tetrad stage on the surface of the primordial lamella. The lamellated endexine forms at the middle post-tetrad stage. The distal aperture region lacks the infratectum and foot layer. The intine consists of two layers, the inner one is channelled. At the early tetrad stage, the secretory tapetum attempts to invade between the tetrads, but fails. The tapetum secretes orbicules and dark globular concretions. Several points are discussed: (1) the connection between the substructural exine units and the cytoskeleton; (2) the native form of sporopollenin precursor particles and its significance for phylogeny and ontogenetic structure-formative processes; (3) the existence of the endexine in primitive angiosperms, the problem of homologies in palynology; (5) the behaviour of the tapetum; and (6) the hypothetical possibility of the relationships between form and function.