Studies in Gentianaceae. Microsporangium and pollen

Abstract

The development and structure of the microsporangium and pollen in 22 taxa from the Gentianaceae have been studied and literature on the subject reviewed. Formation of the microsporangial wall follows the dicotyledonous sequence. In Exacum and Cotylanthera, the anthers dehisce by pores, and the endothecium is nonfibrous. The other genera have longitudinal anther dehiscence, and the endothecium is fibrous. Although a single middle layer is usual in the family, Exacum, Cotylanthera, Voyria and Swertia carolinensis show one to four middle layers. The tapetum is derived partly from the primary parietal layer and partly from the ground tissue on the connective side. A tapetum of sporogenous origin occurs in some Gentianinae and Erythraeinae where partitions of tapetal tissue (sterile trabeculae) subdivide the microsporangium. Ubisch granules are present in Exacum. Cytokinesis in the microsporocytes is simultaneous. Microspore tetrads are tetrahedral, isobilateral, or decussate. Pollen grains are two or three celled at the time of anther dehiscence. They occur as single grains or are held in tetrads or polyads. Single grains are tri-, tetra-, or penta-colporate or pericolporate or one to six porate with smooth, scabrous, striate, striato-reticulate or reticulate surface pattern. Variation among tribes and subtribes and lack of information on the microsporangium in Chironiinae, Tachiinae, Rusbyantheae, Helieae, Voyrieae, and Leiphaimeae are pointed out.