STRUCTURE OF THE MALE GAMETOPHYTE IN GYMNOSPERMS

Summary Analysis of the morphological nature of the gymnospermous male gametophyte has been based on evidence from fossil gymnosperms, on a comparative survey of living vascular plants, and on ontogeny within the various families of the gymnosperms. Although fossil coniferous and taxad pollen is unknown, the fossil pollen of other gymnosperms is rather uniform in the presence of a layer of parietal cells (presumably an antheridial jacket) which surrounds a central, probably spermatogenous region. The same organization occurs in endosporal gametophytes of lower Tracheophyta of the present day. Reduction in the number of vegetative prothallial cells appears to have occurred early in gymnosperm evolution, so that the male gametophyte may be regarded essentially as a reduced antheridium. Between the fossil gymnosperms and the present forms there has occurred a reduction of the antheridial jacket and the corresponding production of a pollen tube. For various reasons, the balance of probability is in favour of a homology between the antheridial jacket of Palaeozoic gymnosperms and the tube cell of modern gymnosperms. In the lower tracheophytes and in Cordaimthus and the Recent gymnosperms, a polar organization of the endosporal male gametophyte is characteristic. On several grounds, it is shown that in this structure the presence of an antheridial stalk is precluded, and that therefore the use of the term ‘stalk cell’ is greatly in error. The cell which is often called ‘stalk cell’ is potentially–sometimes actually–spermatogenous and is sister to the virile spermatogenous cell. It is frequently sterile, however, and is therefore recognized here as the ‘sterile cell’. In the evolution of the Coniferales and the Taxales (as well as in the lower Tracheophyta) disappearance of the prothallial cells from the male gametophyte is a characteristic trend. Prothallial cells are lacking in the Taxodiaceae, Cupressaceae, Cephalotaxaceae and Taxaceae. (However, in two families, Araucariaceae and Podocarpaceae, there has developed a secondary proliferation of the prothallial cells). Recognition of the same trend in the Chlamydospermophyta and consideration of details of ontogeny indicate that prothallial cells must be absent from the gametophytes of Welevitschia and Gnetum. In these, the embryonal cell functions as an antheridial initial and produces a tube and a generative cell. The generative cell then divides into spermatogenous and sterile cells. Later, two male gametes result from division of the spermatogenous cell. Based on the morphological analyses indicated above, a terminology has been proposed which attempts to recognize morphological identities but does accept non-committal terms of common usage, as long as they do not violate the morphological concepts. A representative synonymy is given, and some examples of the use of the suggested terminology are shown. Details of the ontogeny of the male gametophyte in the conifer and taxad families have been reviewed. Although many genera are still completely unknown, it appears correct to state that on the whole the development of the male gametophyte within a family is quite uniform. The Pinaceae are characterized by the production of two senescent primary prothallial cells from the embryonal cell of the spore. The latter then serves an antheridial initial, forming a peripheral tube cell and an inner generative cell. All the preceding divisions are periclinal, and the generative cell also divides periclinally into an inner sterile cell and an outer spermatogenous cell. Eventually the latter divides to form two male gametes. In the Taxodiaceae, Cupressaceae, Cephalotaxaceae and Taxaceae no prothallial cells are formed, but the embryonal cell functions directly as an antheridial initial. Differences among these four families occur mainly in the relative sizes (and functional state) of the male gametes, the time of the occurrence of the various gametophytic divisions, and in the number of male gametes formed (which may exceed two in the Cupressaceae). In the Araucariaceae the two primary prothallial cells are not senescent but undergo an active period of cell division. Also the division of the generative cell is anticlinal rather than periclinal. Otherwise the sequence of ontogeny is similar to that of the Pinaceae. The Podocarpaceae resemble the Araucariaceae in the anticlinal division of the generative cell and usually the secondary proliferation of the primary prothallial cells. However, this proliferation is limited, and one or both cells may not divide. In one genus of the Podocarpaceae, no primary prothallial cells are produced. This article owes its inspiration to the period of the author's association with the late John T. Buchholz, Professor of Botany at the University of Illinois. It is a privilege to dedicate such a study to his memory. Professor O. H. Selling of the Swedish Museum of Natural History graciously supplied the print of PI. 1, fig. A, and Dr Walter Tulecke of the Boyce Thompson Institute kindly allowed his negative photograph to be reproduced as Pl. 1, fig. B. Dr Rudolf Florin has given both his personal permission and that of the Swedish Botanical Society to copy Text-fig. 1. Permission to copy figures from C. J. Chamberlain's Gymnosperms: structure and evolution and from the Botanical Gazette was given by the publisher, the University of Chicago Press. Many thanks are due to these individuals and organizations and to G. Fischer Verlag, Jena, J. Heslop-Harrison, editor, Annals of Botany, and P. Maheshwari, editor, Phytomorphobgy, who have allowed figures to be copied for the illustrations here.