Plant Reproduction Chapter 22 Gymnosperm Reproduction Typically, the male pine cones are quite small and develop near the tips of lower branches. Each scale of the male cone has two or more microsporangia on the underside. Inside the microsporangia are microspore cells that undergo meiosis and develop into mature pollen grain, which is a spermbearing male gamete. The female pine cones are larger and located near the top of the tree. Each scale of the female cone has two ovules that lie on the upper surface. Within the ovule, a megaspore cell undergoes meiosis and develops into mature female gamete (egg), each containing a single, large egg lying near the ovule. After pollination, the pollen tube discharges two non-flagellated sperms. Only one of the sperms fertilizes an egg in the ovule 15 months after pollination. The other sperm fertilizes the endosperm to make a food source. After fertilization, the ovule matures and becomes the seed composed of the embryo, its stored food, and a seed coat. Finally, in the third season, the female cone opens to release its seeds, whose wings are formed from a thin, membranous layer of the cone scale. When a seed germinates, the sporophyte embryo develops into a new pine tree, and the cycle is complete Angiosperm Reproduction In angiosperms, the reproductive structures are located in the flower. The flower attracts insects and birds that aid in pollination, and it produces seeds enclosed by fruit. There are many different types of fruits, some of which are fleshy (apple, tomato, peach) and some of which are dry (pea in a pod, nut, grain). Fruits provide protection for the seeds. Within a flower, there is a diploid megaspore mother cell in each ovule of the ovary. The mother cell undergoes meiosis, producing one functional megaspore, whose nucleus divides mitotically until there are eight haploid nuclei. Male gametophytes are produced in the anther. An anther contains four pollen sacs with many microspore mother cells, each of which undergoes meiosis to four microspores. Pollination is simply the transfer of pollen from the anther to the stigma. It is brought about by wind or with the assistance of a particular pollinator. The plant uses the pollinator to ensure cross-pollination. The pollinator uses the plant as a source of food in the form of nectar. When a pollen grain lands on a stigma of the same species, it germinates, forming a pollen tube. The pollen tube grows as it passes between the cells of the stigma and the style to reach the female gametophyte. The outer layers (integuments) of the ovule harden and become the seed coat. A seed is a structure formed by the maturation of the ovule; it contains an embryo plus stored food. The ovary and sometimes other floral parts develop into the fruit. A fruit is a mature ovary that usually contains seeds. Therefore, angiosperms are said to have covered seeds. Flowers and Pollinators Most flowering plants coevolved with pollinators Insects, birds, and other animals that withdraw pollen from a flower and transfer it to the female reproductive parts are pollinators.