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Comparative developmental studies of the floret and embryo sac in five species of Oryzopsis (Gramineae) Kam, Yew Kiew

Abstract

Development of the floret and embryo sac of Oryzopsis virescens and O. hymenoides was studied. Evidence from this study and from other studies on grass floret and embryo sac development has brought the following interpretations. Histogenesis of the lemma, palea, posterior lodicule and the gynoecial wall is similar, and indicates their foliar nature. They are determinate organs, have a shallow site of initiation, and exhibit marginal growth. The anterior lodicules differ from them in having a deeper initiation site. The interpretation of the anterior and posterior lodicules as reduced perianth structures of one whorl rather than as structures 'de novo' is preferred. The callus is formed by the downward projection of the base of the lemma. Developmentally, the stamens are stem-like. The gynoecium consists of a unit ascidiform gynoecial wall surrounding a terminal ovule. There are two styles, each of which develops from the lateral portions of the gynoecial wall. The floret apex is not used up in the formation of the gynoecial wall. The residual floret apex develops into the ovule. The grass gynoecium may be considered acarpellate. The ovule is hemianatropous, bitegmic and pseudocrassinucellate. The micropyle is delimited by the inner integument. Embryo sac development is of the monosporic, 8-nucleate type. The antipodals are proliferated. The development of the floret and embryo sac of three other species of Oryzopsis was also studied. They are, namely, O. micrantha, O. kingii, and O. asperifolia. Developmental features of all five species of Oryzopsis were compared with developmental features of Oryzopsis miliacea, and of four species of Stipa, a closely related genus. These are, namely, S. lemmoni, S. hendersoni, S. tortilis and S. richardsoni. Cytotaxonomic studies by Johnson (1945. Bot. Gaz. 107: 1-31) in the genus Oryzopsis indicate that O. virescens (n = 12) and O. miliacea (n = 12) are members of the Old World section Piptatherum; O. micrantha (n = 11), O. kingii (n = 11), and O. asperifolia (n = 23), belong to the New World section Oryzopsis; O. hymenoides (n = 24) belongs to the New World section Eriocoma. Intergradation of the genera Oryzopsis and Stipa occurs in North America in the sections Oryzopsis and Eriocoma. Qryzopsis micrantha resembles O. miliacea in certain morphological features, while O. kingii is a 'borderline' Oryzopsis-Stipa species. Oryzopsis hymenoides is known to hybridise with eleven species of Stipa. Thirty-one characters were abstracted from the developmental data and were analyzed statistically. The results indicate that O. virescens is set apart from the five other species of Oryzopsis and the four species of Stipa. The affinity of O. hymenoides on the basis of development is with Stipa. This further supports data from morphology, distribution and hybridization studies and suggests that Oryzopsis hymenoides belongs to the genus Stipa. There does not appear to be a discontinuous variation in development between O. miliacea, O. micrantha, O. asperifolia, O. kingii, S. richardsoni, O. hymenoides (Stipa) , O. hendersoni, S. lemmoni and S. tortilis. It would seem that more comprehensive studies of the genus Oryzopsis will either lead to its mergence with Stipa or at least to a redefinition of the sections of Oryzopsis.

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