EMBRYO‐SAC DEVELOPMENT IN YUCCA RUPICOLA

Abstract

STAGES IN the development of the embryo-sac in species of Yucca have been noted by several investigators, most of whom have described isolated observations with little effort to record the sequence of developmental events. Koernicke (1901) discovered the formation of a linear tetrad of macrospores in Y. filamentosa, amending the statements of Vesque (1879), Guignard (1882), and Herail (1889) that in species of this genus the macrospore mother-cell gives rise to only three daughter-cells. Raciborski (1893) found that in Y. aloifolia the nuclei of the egg apparatus and the polar nuclei are red-staining, while the antipodal nuclei are blue-staining when the tissue is treated with a fuchsin-iodine green combination, which condition he reported also in the majority of a number of species of angiosperms selected at random. Reed (1903) has given a partial account of the development of the female gametophyte in Y. filamentosa. Folsom's (1916) study of the ovule and embryo-sac, fertilization, and embryogeny in Y. glauca is the most thorough available for any species of the genus. The present account confirms in general that of Folsom and includes certain additional intermediate developmental stages. The two species are very similar with respect to the manner of growth of the ovule, but certain differences occur as described below. METHOD.-Young ovaries were removed from flower buds of Yucca rupicola Scheele growing in the vicinity of Austin, Texas, cut in slices, and fixed in the Allen and Wilson modification of Bouin's solution. The material was imbedded in paraffin, and sections of the slices were cut 12 pV thick transversely to the original axis of the ovary. The sections were stained by the Heidenhain iron alum-haematoxylin method. A study of sections of developing ovaries of different sizes yielded a wide range of stages in embryo-sac development, from which the most significant have been reproduced. OBSERVATIONS.-The development of the embryosac in the species of Yucca thus far reported conforms in general with the normal type, which has been found to characterize the majority of flowering plants studied. In Y. rupicola the macrospore mother-cell undergoes the meiotic divisions and yields four macrospores, which are usually disposed in a linear series, the axis of orientation being the same as that of the erstwhile mother-cell. Frequently the two macrospores nearest the micropyle are separated by an oblique rather than an absolutely transverse wall, as seen in figure 2. The perfect linear tetrad, as in figure 1, was observed in approximately half the ovules examined in this stage. Folsom (1916) has noted these variations in macrospore arrangement in Y. glauca and has described also the occurrence of the