The Physiology of Flowering Plants - KHAM PHA MOI

294 REPRODUCTIVE DEVELOPMENTMEGASPOROGENESISMEGAGAMETOGENESISmegasporocytemeiosis Imeiosis IIfunctionalmegasporemitosismitosismitosismaturegametophytenucellusmegasporocyteouterintegumentinnerintegumentfunctionalmegasporeembryosacchalazaantipodalspolar nucleisynergiddegeneratemegasporesintegumentsegg cellfuniculusmicropyleFig: 11:16 Polygonum-typeembryogenesis. Redrawn fromReiser & Fischer (1993).protective integuments grow to surround the nucellus, leaving asmall gap, the micropyle; the funiculus provides a vascular supplyfor the ovule. The megasporocyte undergoes two successive meioticdivisions to form four haploid cells. The one closest to the chalazalend enlarges to become the functional megaspore, the other threedegenerate and are crushed. A series of three successive mitoticdivisions of the megaspore then occurs, forming the female gametophytecomposed of seven cells containing a total of eight nuclei (Fig.11.16) with the egg cell at the basal end. Already a polarity has beenestablished, which is important in the future development of theembryo.Over 15 other types of gametophytic development have beendescribed. In Arabidopsis the polar nuclei fuse to form a diploid secondaryendosperm nucleus whilst the antipodal cells degenerate. Incontrast, in maize the polar nuclei only partially fuse and the antipodalcells proliferate to about 100. The Polygonum type is termedmonosporic since only one megaspore nucleus produces the embryosac; in the Allium type (bisporic) two nuclei contribute, and in theAdoxa type (tetrasporic) all four nuclei from the meiotic divisionsurvive to form the embryo sac.Although the ovules are normally developed in unpollinatedflowers, further development may occur after pollination. In some