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Cycad pollen grain structure
Has flagella; has three cells (generative, tube, prothallial); generative cell becomes body and stalk cell; body cell becomes the sperm, making it a mature pollen grain
Conifer pollen grain
Have wings (some, which are called pine nuts, don’t); has up to four cells; two prothallial cells, which get crushed up against the microspore wall
Gingko pollen grain?
Has flagellaSperm not as large as cycad, but same basic shape
Typical structure; no difference except for outer portion of the ovule; only two ovules produced; the integuments form a tube and the micropyle has a pollination droplet at the end of the micropylar tube
Outer and inner: very stonyMiddle: very fleshy
Outer seed coat smells
Same basic structure as cycadsSeeds can have wings, but some don’t (pine nuts)
Has integuments, pollen and archegonial chambers, archegonia, three layers, micropyle, nucellus
Development of the embryo
- Pollen grain usually develops first and splits into its sperm prior to meiosis even occurs in the female
- Once it gets into the ovule, it will begin to germinate and the tube cell will expand, producing the pollen tube that will be parasitic and feed its way through the tissue of the ovule until it reaches the archegonia
- Pollen grains are found in the pollen chamber prior to meiosis
Difference between conifers and cycads
- Whereas the cycad generative cell divides into the body cell (androgenous) and the sterile stalk cell, the conifer generative cell divides directly into two sperm, which raises debates as to whether these are actually sperm or sperm nuclei. Cycads have the largest sperm
- Whereas the male and female are produced on the same plant on conifers, the male and female are produced on different plants in cycads.
- Sperm have no flagella in conifers, whereas in cycads, they do. The pollen tube in conifers has to go further down
Difference between angioand gymno
- Gymnosperms (except for Gnetophyte) have tracheids and not vessel elements. Angiosperms have vessel elements.
- - Differences in ovule formation:
- o Development is the degree of protection.
- Ovules are protected and are inside
- Ovules on conifers, for example, lie outside the cone scale
- Ovules of each
- o Ovules have integuments, megasporangium, megasporocyte, which, by meiosis, produces four haploid spores, three of which disintegrate (same as in gymnosperms).What is different is the food source.
- In gymnosperms: the female gametophyte becomes the stored food
- In angiosperms: the second sperm will fuse with the polar nuclei in the center of the megagametophyte and form a triploid endosperm, which acts as the food source
What are the four characteristics that classify the Gnetophytes?
- - They have xylem vessels
- - Gnetum and Welwitschia have no archegonia
- - They have no motile sperm
- - they have compound strobili (they are classified as this)