| Reproductive in Angiosperms - The Flowering Plants
As in the gymnosperms, the micro- and mega-gametophytes are retained and develop inside the microsporangium and megasporangium, respectively. Angiosperm sporangia develop within highly modified structures called flowers, rather than in the woody cones present in most gymnosperms. The parts of the flower represent modified sporophylls arranged around a central axis (= a strobilus), and are situated on a condensed stem called the receptacle. (A) Anatomy of the Flower Examine the flowers available for dissection and identify sepals, petals, stamens (with anther and filament), and pistil (with stigma, style, and ovary). Examine Rust, figs. 45 & 46, or C & V figs. 6.85, 6.118-6.131 for reference. Note that there is considerable variation between different species of flowers with respect to size, complexity, color, presence or absence in number of the various parts. You'll return to this subject in Lab 13. (B) From what did Flowers evolve? How can flower be compared to these other structures you have observed, such as sori on fern fronds, strobili in club mosses and gymnosperms? In flowers the calyx represents the lowermost series of sporophylls on the strobilus, and functions primarily as a protective organ in the bud stage. The corolla represents the second-lowest series of sporophylls, functioning primarily as an attractive organ to potential pollinating animals. The stamens correspond to microsporophylls, and produce pollen. Megasporophylls constitute the uppermost series of sporophylls on the strobilus, and have been modified into a gynoecium (pistil). The ovary is a chamber that contains the ovules, and is derived from an evolutionary inrolling of a laminar megasporophyll. Evidence supporting this model is found in fossils and in certain extant primitive angiosperms in which the gynoecium is not completely (almost) fused. (C) Gametogenesis in Angiosperms Gametogenesis in gymnosperms was described earlier in the lab. Angiosperm gametogenesis is similar to that in gymnosperms and may be regarded as a simplified variation on a theme of increased efficiency in the reproductive process. Review several prepared slides of the development of microgametophytes and megagametophytes that demonstrate evolutionary parallels with gymnosperms and non-seed plants.
Pollen begins as a single microspore covered by a resistant pollen wall. The original microspore nucleus divides mitotically to produce a) a pollen tube nucleus and b) a generative nucleus. The generative nucleus divides a second time to form two sperm nuclei. Examine the prepared slides of early (immature) anthers. These represent the microsporangia prior to meiosis. Are there many or few microspore mother cells (pollen mother cells)? Examine the slides labeled pollen tetrads.
Megagametogenesis in angiosperms is distinct from that in gymnosperms in its relative simplicity. Up to a point, the process is similar, but the functional megaspore does not produce a many-celled megagametophyte; rather, it gives rise to a 7-celled, 8-nucleate embryo sac. No archegonia are produced. This reduced megagametophyte represents the ultimate in reduction (an ever-present theme in evolution). An egg, surrounded by a pair of synergids (that aid in pollen reaching the egg), three antipodal cells, and two polar nuclei, which fuse with one of the sperm nuclei to form a highly nutritive tissue, the endosperm. As with the gymnosperm ovule, there is an integument, but unlike gymnosperms, angiosperms often have a double integument. An angiosperm ovule has greater protection via a double integument, plus a specialized food reserve for the developing embryo, while at the same time showing reduction (= increased efficiency?) in a) loss of archegonia, and b) reduction in number of cells constituting the megagametophyte. Examine the slides labeled megasporocyte and the one called mature embryo sac. The former contains the megaspore mother cell.
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