DEVELOPMENT OF THE OVULE AND FEMALE GAMETOPHYTE IN EUSTACHYS PETRAEA AND E. GLAUCA (POACEAE)

The unilocular pistil in Eustachys contains a single ovule with lateral placentation. In E. petraea and E. glauca, the mature ovule is bitegmic, tenuinucellate, and amphitropous with the endostomic micropyle oriented toward the base of the locule. A single hypodermal archesporial cell enlarges to form the megasporocyte. The chalazal dyad member is larger than the micropylar one, and meiosis II is nonsynchronized. Two-thirds of the tetrads are linear and one-third T-shaped. The chalazal megaspore is functional. Initially ovoid, the two-nucleate female gametophyte becomes curved as it enlarges. The four-nucleate stage becomes wider at its extremities and constricted in the center. Synchronous mitotic divisions establish the eight-nucleate stage with four nuclei at each pole separated by a large central vacuole. In E. petraea, the maturation sequence begins with antipodal differentiation, followed by differentiation of the egg apparatus, migration of the polar nuclei to the center, and division of the antipodals to produce twelve cells. The sequence in E. glauca begins with migration of the polar nuclei followed by differentiation of the antipodals, egg apparatus, and antipodal replication to six cells. The polar nuclei fuse to form a secondary nucleus appressed to the egg cell in E. glauca and separated from it by a vacuole in E. petraea. T-tests for length measurements for various stages of development indicate that the functional megaspore and two-nucleate female gametophyte are significantly larger in E. glauca than in E. petraea. There is no significant difference in gametophyte length at the four-nucleate stage, and at the eight-nucleate stage, length in E. petraea surpasses that in E. glauca. This gap widens significantly at the mature stage. Nuclear volumes are significantly greater in E. glauca than in E. petraea in the functional megaspore and two-nucleate stage, but the volumes are similar at the four-nucleate stage. Consideration of the differences in structural complexity between the sporophyte and gametophyte generations leads to the conclusion that the female gametophytes of these species are more distinctive than are the sporophytes.