The formation of integuments, megasporogenesis and megagametogenesis in Dendrobium catenatum, with special discussions on embryo sac types and section techniques

The formation of integuments, megasporogenesis and megagametogenesis in Dendrobium catenatum, an economically important orchid, are observed. After pollination, mitotic cell divisions of the placental epidermis result in the formation of a branching system of outgrowths. The tip of each branch consists of an archesporial cell derived from the differentiation of the terminal subepidermal nucellar cell. It differentiates directly into a megasporocyte. The first division of the meiosis of the megasporocyte produces a dyad approximately equal in size, in which the micropylar cell promptly degenerates. The second meiotic division of the remaining dyad cell results in the formation of two megaspores of unequal size. The larger chalazal cell becomes functional and eventually develops into a mature megagametophyte. The development of the megagametophyte conforms to the Monosporic Polygonum type. The final arrangement of the mature embryo sac conforms to a sevencelled/ eight-nucleate structure. The mature ovule is bitegmic, tenuinucellate and has an anatropous orientation. In the present study, we also discuss the differences between three main types of embryo sac development and the improvement of section techniques.

Cytological and histological studies on female gametophyte of Leucojum aestivum (Amaryllidaceae)

In this study, gynoeceum, development of megasporangium, megasporogenesis, megagametogenesis and female gametophyte of Leucojum aestivum were examined cytologically and histologically. Ovules of L. aestivum are of anatropous, bitegmic and crassinucellate type. Inner integument forms the micropyle. Archesporial cell develops directly into a megasporocyte. Embryo sac development is of bisporic Allium type. Filiform apparatus is observed in synergids. Polar nuclei fuse before fertilization to form secondary nucleus near the antipodals.

Embryology of Epidendrum ibaguense. I. Ovule development

The orchids are unique among angiosperms in that ovule development is initiated after successful pollination. The monandrous orchid Epidendrum ibaguense has three placental ridges at anthesis. After pollination, mitotic activities result in the formation of a dichotomously branching system of outgrowths. The tip of each branch consists of five to six nucellar cells covered by the epidermis. A subterminal nucellar cell differentiates into the archesporial cell approximately 12 days after pollination. By day 18, it differentiates directly into a megasporocyte. The first meiotic cell division produces a dyad in which the micropylar cell begins prompt degeneration. The second meiotic cell division results in the formation of two megaspores of unequal size. The larger cell at the chalazal end will become the functional megaspore. Callose is present in the walls of the megasporocyte, the micropylar dyad cell, and the megaspore destined to degenerate. The development of the megagametophyte conforms to the Polygonum type. One of the chalazal nuclei delays its final mitotic division until fertilization, making it appear that only two antipodals are present. The mature ovules are bitegmic and have an anatropous orientation.

Cytoembryology of Amaryllis hybrids

Karyotype analysis, sporogenesis, and the development of gametophytes are described for four hybrids of garden Amaryllis. The basic chromosome number is x = 11, and all four hybrids are tetraploid. The basikaryotype, which consists of two median, five submedian, and four subterminal chromosomes, is traceable in four hybrids. The anther wall is five to six layered. The tapetum is of the secretory type, and its cells become two nucleate at the onset of meiosis in the microsporocytes. Bivalents, trivalents, and quadrivalents are formed during meiosis. The endothecial cells lack fibrillar thickenings. Pollen grains are shed at the two-celled stage. About 60–70% of the pollen grains are fertile. Ovules are anatropous and bitegmic. The archesporial cell may or may not undergo periclinal division. Development of the megagametophyte is monosporic in 90% of the ovules and bisporic in the remaining 10%. All four hybrids show degeneration of embryo sacs at various stages of development which accounts for the low percentage or lack of seed set.

Megasporogenesis and megagametogenesis in Pelargonium × hortorum

The ovary of Pelargonium × hortorum contains five pairs of superposed ovules in five locules. These ovules are bitegmic and crassinucellar and the upper ovule of each pair is campylotropous while the lower one is anatropous. A single archesporial cell functions directly as the megaspore mother cell. Meiotic division of the megaspore mother cell results in the formation of a linear tetrad of megaspores of which the chalazal megaspore is functional. Embryo sac development is of the polygonum type. Rapid degeneration of the three antipodals occurs followed by the fusion of the two polar nuclei. Therefore, the mature embryo sac contains the egg, the two synergids, and the fused polar nucleus. Double fertilization takes place. Ninety-two percent of the fertilized ovules of P. × hortorum cv. ‘Purple Heart’ are found in the upper position.The two integuments are initiated before the differentiation of the archesporial cell. Cells of the outer layer of the outer integument and the inner layer of the inner integument deposit tannins. The nucellus develops through divisions of the parietal cells of the nucellar epidermal cells.

Embryological features of taxonomic significance in the genus Nyssa

The embryological investigation of Nyssa aquatica L., N. biflora Walt., and N. sylvatica Marsh. reported here clearly delimits similarities and variations among the three species. Such evidence supports the retention of Nyssaceae as a family apart from Cornaceae. The major distinctions between the two families occur in the differentiation of the megasporocyte, development of the nucellus, degeneration of defunct megaspores, and growth pattern of the megagametophyte. Major points of similarity between the two families are the occurrence of unitegmic, pendulous, anatropous ovules, a single hypodermal archesporial cell, unequal dyad cells, hooked synergids, polar nuclei fusing before syngamy, and cellular endosperm. These same features serve also to show that Nyssaceae belongs in the order Cornales rather than Myrtales as suggested by several workers.Strong evidence is provided for the recognition of N. biflora as a distinct species rather than a variety of N. sylvatica as previously proposed by several workers. Points of distinction between the two taxa are found in the shape of the ovule, growth pattern of the nucellar apex and integument, position of the nuclei in the four-nucleate megagametophyte, and the growth pattern of the megagametophyte.Embryological features further indicate a closer relationship between N. aquatica and N. biflora than between either one and N. sylvatica. The features of greatest similarity between N. aquatica and N. biflora are the shape of the ovule and megasporocyte, growth pattern of the megagametophyte, presence of hooked synergids, and the occasional occurrence of bisporic megagametogenesis.

Embryological studies in the compositae, I. Sporogenesis, Gametogenesis, and Embryogeny in Cotula australis (Less.) Hook. F

Cotula australis has a discoid heterogamous capitulum in which the outermost three whorls of florets are female and naked. The bisexual disk florets are fully fertile and have a four-lobed corolla with four shortly epipetalous stamens. The anthers contain only two microsporangia. Wall formation and microsporogenesis are described and the pollen grains are shed at the three-celled condition. The ovule is teguinucellate and the hypodermal archesporial cell develops directly as the megaspore mother cell. Megasporogenesis is normal and the monosporio embryo sac develops from the chalazal megaspore. Breakdown of the nucellar epidermis takes place when the embryo sac is binucleate and its subsequent development follows the Polygonum type. The synergids extend deeply into the micropyle and one persists until late in embryogeny as a haustorium. The development of the embryo is of the Asterad type, and the endosperm is cellular. C. coronopifolia agrees with C. australis in the presence of only two microsporangia in each anther and the development of a synergid haustorium.

Embryological studies in the compositae. II. Sporogenesis, Gametogenesis, and Embryogeny in Ammobium alatum R. Br.

Ammobium alatum is a perennial herb whose discoid-homogarnous capitula are surrounded by several rows of involucral bracts with white radiating laminae. Four microsporangia are present in each anther and their development and dehiscence are described. The ovule is anatropous, unitegmic, and tenuinucellate. The archesporial cell is hypodermal in origin and, following considerable increase in size, it functions directly as the megaspore mother cell. Cytokinesis and wall formation are postponed until after Meiosis 11 and a dyad is formed in which each cell is binucleate. The embryo sac is bisporic and its development is a variation of the Allium type. After fertilization, the surviving synergid may increase greatly in size but it does not extend into the micropyle and it collapses when the embryo reaches the seventh cell generation. Embryogeny is of the Asterad type and the sequence of events leading up to maturation of the embryo and of the fruit is described.