Patterns of embryo-sac organization, synergid degeneration and cotyledon orientation in Linum usitatissimum L.

Abstract

The embryo sac of Linum usitatissimum consists, as in most angiosperms, of the egg, two synergids, central cell and antipodals. In Linum the embryo sac is strongly polarized in the longitudinal axis, but rotationally random relative to the vascular bundles of the ovule and ovary. Synergids designated right or left based on their orientation relative to the egg can be distinguished unambiguously if viewed from one pole of the embryo sac — in this case, the chalaza was used as the reference point. The synergid most likely to degenerate is the left (60%), proximal (52.3%) or septal-facing (52.7%) synergid. The volume and the surface area of the filiform apparatus of the left synergid is significantly smaller than that of the right synergid. Synergid-degeneration patterns varied between individual plants, indicating genetic control; however, the preference for the proximal and septal-facing synergid, although weak, indicates the possibility of some physiological influence. The cotyledons appear to assume bilateral symmetry with respect to the ovule only once endosperm digestion has begun. As the cotyledons grow, the embryo rotates to occupy the widest part of the embryo sac, thus imposing bilateral symmetry between the embryo and seed; prior to that time, the early and heart-shaped embryos are randomly oriented.