21. A pollen tube is produced, which grows into the nucellus to provide nutrients for the developing sperm cells. 22. Plants that are self-sterile often inhibit the pollen grains from their own flowers from growing pollen tubes . 23. The presence of multiple grains of pollen has been observed to stimulate quicker pollen tube growth in some plants. 24. Stigma have been shown to assist in the rehydration of pollen and in promoting germination of the pollen tube . 25. In this example, pollen tube incompatibility and slower generative mitosis have been implicated in the post-pollination isolation mechanism. 26. The pollen tube elongates and pierces and grows through the megasporangium wall and delivers the sperm cells to the female gametophyte inside. 27. In oaks, early fertilized ovules prevent the fertilization of other ovules by inhibiting the pollen tube entry into the embryo sac. 28. Thus, kin recognition at the level of the pollen tube apparently leads to post-pollination selection to avoid inbreeding depression. 29. Exo-PGs play a role in enabling pollen tube elongation since pectin rearrangement is necessary for the growth of pollen tubes. 30. Exo-PGs play a role in enabling pollen tube elongation since pectin rearrangement is necessary for the growth of pollen tubes .
