Fig. 2.

Ovule and female gametophyte development in cowpea. a Initial stages of ovule primordium growth; several proliferative cells differentiate in the L2 and L3 layers (arrows); the gynoecium is approximately 0.5 mm in length. b Ovule primordium prior to archespore differentiation (gynoecium 1 mm). c Developing ovules harboring a single archespore (gynoecium 1.5 mm). d Developing ovule showing an MMC at the onset of megasporogenesis (gynoecium 1.75 mm). e Ovule with an MMC undergoing meiosis I; the nucleus of the MMC is at zygotene stage (gynoecium 2.5 mm). f Developing ovule with a fully differentiated MMC (gynoecium 3 mm); the outer integument is longer than the inner integument that covers approximately half of the nucellus. g Developing ovule at the end of megasporogenesis. The inner integument has resumed its growth; the functional megaspore (FM) is the meiotically derived cell located at the chalazal pole, adjacent to three degenerating megaspores (gynoecium length 3.5 mm). h Developing ovule at the onset of megagametogenesis; a fully differentiated megaspore and degenerated megaspores are within the same cellular plane (gynoecium length 3.75 mm). i Developing ovule at the four-nuclear stage of megagametogenesis. j Differentiating female gametophyte showing three antipodal cells at the chalazal end (inset). k Fully differentiated ovule containing a cellularized female gametophyte; the egg cell and polar nucleus are visible. Arc archespore, CC central cell, EC egg cell, DMg degenerating megaspore, F funiculus, FG female gametophyte, FM functional megaspore, II inner integument, L1 L1 layer, MMC megaspore mother cell, OI outer integument, PN polar nucleus. Scale bars a = 18 μm; b = 21 μm; c = 28 μm; d = 23 μm; e = 13 μm; f = 31 μm; g = 12 μm; h = 27 μm; i = 37 μm; j = 12 μm; k = 56 μm