Morphogenesis of the sporoderm inPolystachia pubescens(Orchidaceae)

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Abstract
Pollen wall morphogenesis of Polystachia pubescens (Orchidaceae) was observed with light and transmission electron microscopy. Polystachia pubescens has pollinia which consist of isobilateral calymmate tetrads. Three sporoderm types are present within the pollinium: type 1 (forming the surface of the pollinium), with large homogeneous exine “flakes”, lacking columellae, footlayer and endexine, with a fibrous intermediate layer and a bilayered intine; type 2, which is found between tetrads within the pollinium, and whose exine is represented by small exine globules only, but otherwise is equivalent to sporoderm type 1; and type 3, found within the tetrads and solely consisting of the bi-layered intine. The whole pollinium is enveloped by a polysaccharidic film (“envelope”) which originates from tapetal and sporocyte cell walls and is formed after the tapetum has degenerated shortly before anthesis. Young microspores are separated by callose. The exine and the outer layer of the intine are formed simultaneously, while the inner layer of the intine is formed after the generative cell has become detached from the pollen wall. No primexinc could be found. The fibrous intermediate layer supporting the exine in sporoderm type 1 and type 2 derives from the “special mother cell wall”, whose callosic nature can be detected as far as the first mitosis.