Biology Semester II

Sections:

Introduction |  Section 1 | Section 2 | Section 3

Section Two:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9

Biology: Flowering Plant Reproduction: Part Six The Gynoecium The gynoecium is the collective term for all of the female reproductive parts of a flower. It consists of the stigma, the style, and an ovary containing one or more ovules. Together, these three structures are called the pistil or carpel. In many plants, the pistils fuse for all or part of their length. Dissected gynoecium of a lily. Angiosperms are the “covered seed plants.” In these plants, the ovary wall develops into a fruit around the seed. Like the male stamen, the female carpel is thought to be a modified leaf. The supposed ancestors of flowering plants had sporophylls (leaves that produce spores), but they lacked sporangia that were completely surrounded by what would become the ovary. During the 1950s, Harvard University botanist I.W. Bailey and his students proposed an evolutionary path for carpels. They began with the leaf-like carpels of the primitive angiosperms and progressed to the "normal" carpels, like those of the modern Lilium. Fossil evidence seems to support this proposed sequence. The evidence also supports the stamen-as-modified-leaf hypothesis mentioned above. Hypothesized evolutionary sequence producing a closed carpel by folding of a sporophyll. The Stigma and Style The stigma functions as a receptive surface for pollen. Once the pollen lands, it germinates its pollen tube. In most plants, there is a clear boundary between the stigma and the underlying style. Corn silk, however, is part stigma and part style, with no clear delineation between these two parts. The style serves to place the stigma some distance from the ovary. This distance between stigma and ovary is species specific. The pollen tube that grows from a pollen grain has a genetically determined length. Thus, if the style is too long, the pollen’s sperm cells cannot reach the egg tucked away inside the ovary. Since pollen from many species may land on any given stigmatic surface, the length of the style acts as a needed reproductive isolation mechanism. The Ovary The ovary contains one or more ovules, and each ovule in turn contains one female gametophyte. In angiosperms, the female gametophyte is also called the embryo sac. Some plants, such as the cherry tree, have only a single ovary that produces two ovules. Only one of the two ovules will develop into a seed (the cherry pit). Cross-section of an ovary of Lilium. Note the ovules in the center of the ovary. The Gametophytes Plants have a haploid phase, the gametophyte. This phase often is a separate, free-living plant in its own right. Such is the case in ferns and the non-seed plants, known as the fern allies. In seed plants, the female gametophyte develops inside the ovule, or megaspore, of the plant. The male gametophyte develops inside the pollen grain. In flowering plants (like all seed plants), the gametophyte phases are reduced to a few cells that depend on the sporophyte phase for their nutrition. This is the reverse of the pattern seen in the nonvascular plant groups, such as liverworts, mosses, and hornworts (the Bryophyta). The angiosperm male gametophyte has two haploid cells (the germ cell and tube cell). These are contained inside the exine of the pollen grain (the microspore). The germ cell divides to produce two sperm cells. This division occurs either prior to release of the pollen or before the germ cell begins its journey down the pollen tube. Female gametophytes of flowering plants develop within the ovule (megaspore). The ovule is contained within an ovary at the base of the pistil of the flower. The megaspore mother cell divides to produce a single haploid cell. This cell then divides by mitosis to produce the other cells of the female gametophyte. There are usually eight haploid cells in the female gametophyte: a) one egg cell, with two synergid cells on either side of the egg cell; b) two polar nuclei in the center of the embryo sac; and c) three antipodal cells (at the opposite end of the embryo sac from the egg cell). Stages of the female gametophyte in lily.  a) megaspore mother cell; b) eight-celled female gametophyte. Now go on to the next part.

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