what fertilization in angiosperms is siphonogamous ?
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Answer:
The pollen grains of angiosperms cannot land directly on the ovules because the ovules are enclosed in a floral structure called an ovary (the ovary is indicated in the figure at the top of the page). Thus, the pollen grains land on a specialized surface (the stigma), where they germinate. Angiosperms are siphonogamous (Greek, siphon + gamia = tube union), meaning that their sperm are not motile (i.e., lack flagella and cannot swim) and are delivered to the egg by means of a pollen tube. The pollen tube must grow from the pollen grain on the stigma, down the style, and into the ovary to make contact with an ovule.
Answer:
Double fertilization occurs in siphonogamous seed plants (i.e., seed plants with nonmotile sperm) when both sperm in a pollen tube unite with structures in a megagametophyte. In angiosperms, one sperm unites with the egg to form a diploid zygote, the first cell of a new sporophyte. The other sperm unites with the nucleus or nuclei in the large central cell of the embryo sac to form a primary endosperm nucleus. This nucleus is the first nucleus of the endosperm (Greek, endon + sperma = within seed), a type of food tissue unique to the seeds of angiosperms.
After fertilization, the ovule becomes a seed. The seed is a structure containing a young, diploid sporophyte embryo and, typically, stored food for the embryo. In angiosperms, the food in the seed may be stored in the form of endosperm, it may be stored in the cotyledon(s) (seed leaves) of the embryo, or it may be stored in both structures. The seed is protected by a seed coat, which develops from the integuments of the ovule.
2-Panel figure showing longitudinal sections of seed containing embryos. Panel 1. Bean embryo with two cotyledons. Panel 2. Corn seed with copious endosperm.
Longitudinal sections of seeds with sporophyte embryos. Left: Bean (Phaseolus) split lengthwise to show the parts of the embryo, including the two food-storing cotyledons, the hypocotyl-root axis (sporophyte embryo axis below the cotyledons), and the first foliage leaves. No endosperm is apparent. Right: Corn (Zea mays, a monocot) embryo with one cotyledon and conspicuous endosperm. Credits: Phaseolus seed (Bruce Krichoff, via flickr, CC BY 2.0); Zea kernel (Jon Houseman and Matthew Ford, Wikimedia Commons, CC BY-SA 4.0). Images modified from originals.
Diagram showing the steps in the angiosperm life cycle
Generalized angiosperm life cycle. Summary diagram for the life cycle of an angiosperm. The diploid (2n), multicellular sporophyte bears flowers. Embryo sacs (megagametophytes) develop in the ovules, which are found in the ovary. Pollen grains (microgametophytes) develop in the pollen sacs (microsporangia) of the anther. One megaspore mother cell occurs in each ovule; it undergoes meiosis, typically giving rise to one functional megaspore. Many microspore mother cells occur in the pollen sacs; each undergoes meiosis to produce four microspores. Megaspores develop into embryo sacs, microspores into pollen grains. The pollen grains are released from the anther to land on a stigma during pollination. The pollen grain germinates, forming a pollen tube that delivers the sperm to the egg in the ovule during fertilization. The sporophyte embryo develops in the seed. Upon germination of the seed, the sporophyte resumes growth. Credit: Diagram by E.J. Hermsen (DEAL).
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