The Embryo Sac of Physostegia

Abstract

1. The archesporium of Physostegia consists of a single hypodermal cell, which, without formation of parietals, functions as the megaspore mother cell. 2. The megaspore mother cell by two successive divisions gives rise to a row of four megaspores; the chalazal one enlarges and gives rise to the embryo sac, while the other three disorganize. 3. The mature embryo sac contains an egg, two synergids, three antipodal cells which multiply to several, and two polar nuclei which fuse. 4. During the formation of the embryo sac a lobe develops from near its chalazal end, so that the sac consists of two distinct parts joined by a narrower portion. 5. Double fertilization of the usual type in all probability occurs. 6. The endosperm is cellular from the beginning, the wall accompanying the first division of the endosperm nucleus being longitudinal through the sac. The chalazal portion of the sac, or "endosperm lobe," becomes completely filled with endosperm tissue, which invades and destroys nearly all of the integument; while the micropylar portion of the sac never contains more than a very few endosperm cells, and later disorganizes, becoming completely obliterated by the encroaching endosperm. 7. The first division in the fertilized egg is transverse, and the chalazal cell, which becomes imbedded in the endosperm through the great elongation of the micropylar cell, develops very regularly into a typically dicotyledonous embryo, which displaces nearly all of the endosperm.