The Bases of Angiosperm Phylogeny: Embryology

Abstract

Many of the embryological characteristics that may be useful, when employed judiciously and in conjunction with other characters, in arriving at taxonomic conclusions are listed. Several characteristics that show a non-random distribution between families of the Magnoliatae and those of the Liliatae are discussed. Features which are more predominant in the monocotyledons than dicotyledons are: monocotyledonous type of development of the anther wall; amoeboid tapetum; successive cytokinesis of the microspore mother cells to form isobilateral tetrads; helobial endosperm development; and a single cotyledon in the mature embryo. In contrast, those characteristics that are more prevalent among dicot families than monocot families include: basic and dicotyledonous patterns of anther wall formation; simultaneous cytokinesis of microspore mother cells with the formation of tetrahedral tetrads; hemitropous, amphitropous, or circinotropous ovules; ovules with a single integument; an endothelium; Oenothera, Penaea, Peperomia, Plumbago, or Plumbagella types of megagametophyte development; cellular endosperm; and two cotyledons in the mature embryo. In spite of these differences, those characteristics that are most common-occurring in at least 70%, and usually more, of all angiosperms -are evenly distributed between the two classes and afford a strong embryological unity to the angiosperms. These widespread characters include: four microsporangia per anther; differentiated endothecium; two-celled pollen grains; bitegmic anatropous ovules; Polygonum type of megagametophyte development; and nuclear endosperm. Within the two classes distribution of the various characters is not always proportional among the several subclasses and superorders; for example, a unitegmic, tenuinucellate ovule with an endothelium, cellular endosperm with haustoria and Solanad embryogeny predominates in the Ericanae and Asteridae; or helobial endosperm is a distinctive feature of the Alismidae. On the whole, embryological characteristics are remarkably constant at the family level. In those families where variation does occur, genera are usually constant, although a few notable examples of intrageneric, and even intraspecific, variation do exist, as for example, in patterns of megagametophyte development. In addition to the taxonomic usefulness of the grosser aspects of embryology-the major categories of structure or development-some evidence is presented suggesting that variation in details within a single category, such as size, shape, and cellular characteristics of the developing and mature megagametophyte, may be helpful in determining relationships within families, genera, or species. I Since the time of the nineteenth century landmarks in embryological knowledge and the men who made the observations (Amici's discovery of the pollen tube, Hofmeister's recognition of the general structure of the mature megagametophyte and meiosis in microspore mother cells, Strasburger's working out of stages in megagametophyte development and observation of fertilization, and finally the independent recognition that fertilization was double by Nawaschin and Guignard [see Maheshwari, 1950]), many descriptions of various aspects of sexual reproduction (embryology in the broad sense) in a wide variety of angiosperm plants have been published by a host of botanists. During the early part of this century some use was made of embryological characteristics for systematic purposes, for example, Samuelsson's (1913) treatment of the Ericaceae, but it was only with the publication of Schnarf's (1931) Vergleichende Embryologie der Angiospernen that impetus was given to this type of synthesis of the accumulating embryological information. Since that time a number of papers, review articles, or books have