CHANGES IN MEGAGAMETOPHYTE STRUCTURE IN PAPAVER NUDICAULE L. (PAPAVERACEAE) FOLLOWING IN VITRO PLACENTAL POLLINATION

Papaver nudicaule placentae with attached ovules were dissected out of unpollinated gynoecia 1–3 days after anthesis, dusted with pollen, and cultured on modified Nitsch's growth medium at 23 C. Ovules were removed from expiants at 15, 24, 31, and 48 hr postpollination, fixed in GA-OsO₄, embedded in Spurr's resin and sectioned (1.0 μm) for light microscopy. Placentae, 15 hr after pollination, were fixed and processed for scanning electron microscopy. Pollen germinates within 1 hr. Although most pollen tube growth appears random, there is directional growth toward the micropyle. The crassinucellate ovule contains an embryo sac consisting of three antipodals, two polar nuclei, and an egg apparatus composed of two synergids and a polarized egg having a large chalazal vacuole and micropylar nucleus. Pollen tube access into the megagametophyte is through a degenerate synergid, with fertilization occurring between 24 and 31 hr after pollination. Zygote establishment is accompanied by polarity reversal in which the nucleus assumes the chalazal position subtended by a large micropylar vacuole. Fertilized ovules normally develop into germinable seeds.