Macrosporogenesis and Embryology of Melilotus

Abstract

1. In Melilotus a single apical hypodermal cell is differentiated in the young ovule as an archesporial cell. This cell divides to form a primary parietal and a primary sporogenous cell. 2. The primary sporogenous cell functions as the macrospore mother cell in M. officinalis and in the Redfield Yellow variety of sweet clover. A number of sporogenous cells (1-3) are usually formed in M. alba, but only one of them undergoes further development. 3. In consequence of two divisions, the macrospore mother cell produces a row of four macrospores, the chalazal one of which develops into a 7-celled embryo sac of the usual type; the other three macrospores disintegrate. 4. The micropylar end of the nucellus breaks down and the embryo sac elongates greatly, the basal portion becoming deeply imbedded in the nucellus and the apical portion extending so as to lie in direct contact with the inner integument. 5. The polar nuclei fuse and the fusion nucleus comes to lie just basal to the egg apparatus. 6. The antipodal cells disintegrate just prior to fertilization. 7. The apices of the synergids elongate and a distinct filiform apparatus is produced. 8. The pollen tube enters the embryo sac between the synergids and the egg, and the synergids are rarely broken down in consequence of fertilization but persist for some time thereafter. 9. Fertilization occurs under greenhouse conditions between 18 and 21 hours after pollination. 10. The zygote, by transverse divisions, forms a filament of three or four cells. The terminal cell of this filament forms the embryo. The basal cells by further divisions form a multicellular suspensor. 11. The primary endosperm nucleus divides before the zygote, and there are usually four or more free nuclei in the embryo sac at the time of division of the zygote. 12. Cell division in the endosperm begins in the region of the suspensor at about the time the embryo has reached the 16-celled stage. 13. The endosperm is entirely absorbed by the time the embryo is mature. 14. Remnants of the suspensor are still present after the absorption of the endosperm. 15. The haploid number of chromosomes is 8. Melilotus alba and Redfield Yellow each have one pair of satellite chromosomes.