THE DEVELOPMENT OF TWIN EMBRYO SACS, EMBRYOS, AND ENDOSPERM IN POA ARCTICA R. BR.

The origin and development of twin embryo sacs especially in P. arctica R. Br. were studied microscopically in sections of whole spikelets representing consecutive stages in development from the time of emergence of the panicle until after pollination. Twin embryo sacs usually develop, one originating from the innermost of a row of four macrospores with the reduced chromosome number, the other from an aposporous cell that originates in the nucellus, near the chalaza and behind the normal archespore. The individual development of the normal and aposporous embryo sacs from their respective mother cells is traced and the competition between them discussed. Both aposporous and normal egg cells develop parthenogenetically. Pollen germination appears to activate the aposporous polar cell to form endosperm which in turn nourishes the aposporous (2n) and (or) the "sexual" (n) embryo. The greatest number of plants originate from the aposporous embryo. A type of apospory was found in P. alpina L. from Greenland. The work of other investigators, especially on P. pratensis L. is reinterpreted in the light of these findings.