Stages of ovule development in Iris tenax

Development begins early with megaspore production beginning when floral buds were just emerging from the soil. Development proceeded rapidly to the seven-celled stage prior to flower opening. Final maturation occurred during the male phase of flowering with degeneration of synergids. The center arrow indicates the cell (2n or diploid) that is dividing & will initiate 4 spores (each 1n or haploid). Arrow on left shows funiculus (stalk connecting ovule to ovary wall). Arrows on right indicates layers that surround ovule and form seed coat. Layers to the outside (integuments) grow up and surround (cylinder-like) ovule after development of spores begins. Arrows indicate four spores formed. Upper spore will form ovule while other three spores are beginning to degenerate. The upper spore has a dark staining nucleus and lighter staining vacuoles surrounding the nucleus. The lower two spores are stained darker and hard to distinguish because they have become crushed by the enlargement of the two upper spores. Notice how the integuments have grown up around the ovule. The spore that will develop into the ovule (at upper arrow) has enlarged, crushing the other three spores. The lower arrow indicates the ovary wall. Two cell divisions have taken place resulting in a 4-celled immature ovule with two cells at the upper and two cells at the lower pole of the developing ovule. Each of the four cells divided resulting in 7 cells and 8 nuclei (black arrows). The ovule is nearly mature at this stage. The synergids are already degenerating at this stage. Two antipodal cells have nuclei that are visible in this section while the third nucleus is out of the focal plane. The polar nuclei are contained within one cell wall so are two nuclei within one cell. With the fusion of the two polar nuclei and formation of a dark staining structure at the base of the degenerating synergids (filaform apparatus) the ovule is mature and ready for the sperm nuclei. The filaform apparatus is thought to help direct the sperm nucleus into one of the degenerated synergids and toward the egg cell. Notice that the nucleus of the polar nucleus cell is larger than other nuclei indicating its 2n condition. After fertilization the pollen tube can be seen within the ovule (left). The pollen tube grew out of the pollen that germinated on the stigma of the flower and two sperm nuclei moved within the tube toward the ovule. The polar nucleus is now 3n due to the fusion of one of the two sperm nuclei. The polar nucleus is dividing to form two endosperm cells (right). Endosperm cells will continue dividing and will form the nutritive tissue for the embryo within the seed. The second sperm nucleus fused with the egg cell to form the 2n zygote.