Macrosporogenesis and Embryology of Melilotus

Abstract

M. alba, M. oflicinalis, and the variety Redfield Yellow (possibly a derivative of a hybrid between the 2 spp.) were studied. A single primary sporogenous cell functions as the macrospore mother cell in M. officinalis and in the variety; in M. alba 1-3 are usually formed, only 1 undergoing further development. Of the row of 4 macrospores, the chalazal cell develops into a 7-celled embryo sac of the usual type. The micropylar end of the nucellus breaks down and the embryo sac elongates greatly, the basal portion becoming deeply imbedded in the nucellus, the apical portion coming to lie in direct contact with the inner integument. The pollen tube enters the sac between the synergids and the egg. Fertilization occurs 18-24 hrs. after pollination. By transverse divisions the zygote forms a filament of 3 or 4 cells. The terminal cell develops into the embryo and the basal cells form a multicellular suspensor. The endosperm is at first multinucleate; by the time the embryo reaches the 16-celled stage, cell division begins in the micropylar region. The endosperm is entirely absorbed by the time the embryo is mature. n=8; M. alba and the variety each have 1 pair of satellite chromosomes.