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Structure of the microgametophyte versus the megagametophyte
micro: three cells
How does the angiosperm differ from gymnosperms?
Differs from gymnosperms because the microgametophyte has three to four cells and the megagametophyte develops completely differently. It doesn’t have much other than the archegonia, which angiosperms do not have, the stored food, which develop differently (from the female gametophye in gymnosperms and from the fusion of the sperm with polar nuclei in angiosperms), chambers, micropyle (which they both have).
Angiosperm development of egg
involves meiosis and results in the formation of megaspores within the ovule in the nucellus
developemtn of megaspore into embryo sac, which is the megagametophyte (ovule)
Microgametophyte formation (summary)
- Within the anthers, microsporocytes develop meiosis microspores each microspore divides once to form a tube cell and generative cell, which is the IMMATURE MICROGAMETOPHYE (pollen grain) germination generative cell divides, forming two sperm conveyed to egg as a mature male gametophyte
Megagametophyte formation (summary)
- Within the ovule, a single megasporocyte develops meiosisfour megaspores, three of which disintegrate fourth develops into megagametophyte
Method of fertilization for angiosperms
- - Pollen grain germinates on the stigma, producing a pollen tube, which grows down the style into the ovary, entering the ovary through the micropyle- One sperm fuses with egg zygote
- - Another sperm fuses with two polar nuclei (2n) of the embryo sac, producing a triploid endosperm nucleus
- - The embryo develops within the embryo sac, and the integuments become the seed coat
- - Eventually, the seed is shed from the fruit
Detailed megagametophyte development
Diploid megasporocyte meiosis four haploid megaspores three disintegrate and one (farthest from the micropyle survives) enlarges with expansion of the nucellus nucleus of the megaspore divides by mitosis four nuclei mitoses (three of them) eight nuclei arranged in two groups of four at opposite ends of the megagameophyte one from each ends migrates to the center to become the polar nuclei cell awll formation around the other six cells to become the egg cell, synergids, and antipodals 8-nucleate, seven celled structure called the megagametophyte (embryo sac)
Detailed Microgametophyte Development
Sporogenous cells of the anther become microsporocytes meiosis haploid microspores (pollen grains) during meiosis, each division is followed by cell wall formation to form the major features of the pollen grains microspores divide mitotically, forming the tube cell and generative cell, which moves to the interior of the pollen grain (2-cell structure is the immature microgametophyte) generative cell divides mitotically to produce the sperm 3-celled mature microgametophyte
When is pollen mature?
when the sperm develop
Why are pines considered gymnosperms?
- Because the seeds aresitting on the cone scale without any protective covering
True or False:
Angiosperms are the only plants with cotyledons.
o FALSE: In the cycads, there are two. In the conifers, there are about eight to ten.
In conifers, explain the cotyleons.
o There is a part that becomes a shoot. There is a root hypocotyls axis. Hard to tell when the hypocotyls and root starts. The hypocotyls is the region between the cotyledon and where the root starts. Hypocotyl literally means underneath the cotyledon
What is a large sequioa tree?
General Sherman tree
True or False:
Yew is a good plant to grow.
- False: The plant is toxic. The female plant hasseeds and leaves that can kill kids due to alkaloids. But, the outer fleshy red covering is not. Plant male plants only so the attraction is not there. This plant is toxic.
How are gnetophytes similar to angiosperms?
- They have xylem vessels. No other gymnosperms but these havethem.
- Gnetum and Welwitschia do not have archegonia. Ephedra has two or three.
- Double fertilization occurs in Gnetum and Ephedra. However, the second fertilized egg simply aborts. It does not become the food like in angiosperms.
- Their strobili resemble inflorescent flowers
What is interesting about Ginkgo fertilization?
- It does not usually happen until the ovules have been shed from the parent tree.
Difference in dispersal of gymnosperms and angiosperms?
gymnosperms: usually wind