Pollen tube penetration and fertilization in spinach

The nucellus of immature crassinucellate ovules of Spinacia cannot be penetrated by pollen tubes, which cluster together in and around the micropyle. After maturation of the ovule, pollination activates the embryo sac, i.e. the synergids, to excrete substances which dissolve the middle lamellae of the nucellus tissue in front of the synergids. Eventually the nucellar cuticle near the micropyle becomes interrupted. At that developmental stage pollen tubes can penetrate into the nucellus, initially growing intercellularly, and subsequently following various pathways. More than one tube can reach the embryo sac, but only one pollen tube penetrates the degenerated synergid via the filiform apparatus. Sperm cells, vegetative nucleus and tube cytoplasm with many amyloplasts are discharged into the degenerated synergid. The vegetative nucleus and the sperm cells are pressed to the chalazal part of the synergid, of which the plasma membrane is degenerated previously. The discharge and the fusion processes of the sperm cells with the egg cell and central cell (7-9 hrs after pollination) are described and discussed. Apparently no or very few organelles of the sperm cells enter the female gametes.