Structure and Development of the Caryopsis in Some Indian Millets. VI. Setaria italica

Abstract

1. Each spikelet in Setaria italica Beauv. is borne lateral to a bristle axis as are also the smaller bristles of a bristle shoot to a principal bristle axis. Therefore, the bristle shoot as a whole, and not the individual bristle, must be regarded as the morphological and homologous equivalent of a spikelet. 2. Microsporogenesis in this species is normal, and the divisions are of the successive type. The pollen grains are three-celled at shedding. 3. The ovule is anatropous, bitegmic, and crassinucellate. A narrow endostome, formed by the excessive growth of the inner integument, is hooded by part of the outer integument. A pair of nucellar cells abutting the micropyle becomes hypertrophied and richly protoplasmic. 4. A linear tetrad of megaspores is formed, and the innermost is functional. The embryo sac is organized in the usual fashion. Rarely the egg shows conspicuous lateral hooks otherwise found only in synergids. The synergids and the several, multinucleate antipodal cells commence to degenerate before fertilization. 5. The endosperm is of the Nuclear type. After it becomes cellular, the peripheral layer behaves very much like a "cambium." The aleurone layer is single. 6. Double fertilization occurs. The early divisions of the zygote occur in regular sequence but not the later ones. Organ differentiation is like that in other grasses. There is no epiblast. The insertion of the scutellum is at a short distance from the plumule sheath. 7. Only the outer and the inner epidermal layers of the pericarp form the outermost covering of the caryopsis. A nucellar epidermis of empty cells may occasionally persist, although in most cases the nucellar membrane alone survives. The outer integument is completely obliterated, and the inner collapses except toward the tip and in the insertion region near the funicle.