Late Embryogeny and Histogenesis in Sesamum indicum L

Abstract

1) Late embryogeny and histogenesis in Sesamum indicum L. were described.
2) The protoderm of the embryo is established early by the periclinal division of the cells of the 8-celled embryo-globe: the cells of the surface layer produced by this division are divided only anticlinally thereafter, except for a few cells participating in the root-cap formation.
3) The tissue differentiation in the embryo starts with the appearance of cells with decreased stainability. The first such cells appear in the outer ground meristem, which makes the embryonic cortex, then follows the appearance of such cells in the inner ground meristem, which makes the pith in the hypocotyl. In consequence of the appearance of these less-staining regions, tissues whose cells are dark-staining and appear unaltered are blocked out as a procambial core and a presumptive region of cotyledons and plumule. The procambial core differentiates into the embryonic stele.
4) The presumptive region of cotyledons and plumule is regarded as an incipient, juvenile shoot apex. The shoot apex develops from the juvenile to the advanced structure through embryogeny. Even in the full-grown embryo, however, the apical meristem remains incompletely developed. No periclinal division occurs in the 2nd or 3rd cell layer at the leaf position prior to the formation of the first foliage leaves. This makes a contrast with the initiation of later leaves, which is preceded by periclinal divisions in the 2nd and 3rd layers. The apical meristem is connected with the procambium of the hypocotyl by two meristematic strands at opposite sides of the procambial cylinder in the intercotyledonary plane before the first leaf primordia are initiated.
5) The apical meristem of the radicle is organized at about the same stage with the cotyledons, earlier than the formation of the plumule. Initials of the root cortex and those of the epidermis and root-cap are distinguished from other cells, from the time of their very origin. The undermost cells of the procambial core, which are contiguous with the cortex initials, function as the initials of the root stele.
6) The embryonic stele is differentiated as a structure having the transitional pattern of vascular system between the cotyledons and the root: its fundamental tissue pattern is already determined when the first protoxylem and protophloem poles appear.