EEB340 Lecture 18.docx

36 views2 pages
16 Apr 2012
Lecture 18: Alternation of Generations
Alternation of Generations (Gymnosperms)
- in Gymnosperms, the arrival of pollen on the ovule initiates the process of cell
division single epidermal cell undergoes one periclinal division
o leads to primary neck cell (which gives rise to 2 neck cells) and a central cell
o the central cell enlarges and divides into the egg nucleus and the ventral
canal nucleus (which degrades)
o reduction in number of divisions
- arrival of pollen on ovule initiates this whole process of division
Ginkgo (and true for all gymnosperms)
- male gametophyte at time of arrival at ovule is 3 or 4 celled development until
now endosporic
- once pollen lands on ovule, shifts to exosporic development
- pollen tube grows via tip growth
o exosporic male gametophyte development
o prothalial cell responsible
o everything with gametophyte is mitosis
- generative cell gives rise to sterile cell and spermatogenous cell (gives rise to
sperm) only 2 sperm, reduced
o occurs as pollen tube is growing
- pollen tube grows in cell wall space of megasporangium and dependent on this
tissue to support its development
- cavity with liquid that sperm swim through before reach 2 neck cells (separated so
sperm may enter)
- in Ginkgo and Cycad, these sperm are flagellated
o Cycad sperm have up to 500,000 flagella in hulla skirt fashion
- Single fertilization event zygote (first cell of new sporophytic generation)
- In Pine, the pollen tube grows directly to the central cell to deliver unflagellated
sperm derived water not needed for fertilization
- in seed plants, the sporophyte grows down into the female gametophyte uses the
gametophyte’s nutrient stores and energy
Cycadophyta: Zamia
Unlock document

This preview shows half of the first page of the document.
Unlock all 2 pages and 3 million more documents.

Already have an account? Log in

Get OneClass Notes+

Unlimited access to class notes and textbook notes.

YearlyBest Value
75% OFF
$8 USD/m
$30 USD/m
You will be charged $96 USD upfront and auto renewed at the end of each cycle. You may cancel anytime under Payment Settings. For more information, see our Terms and Privacy.
Payments are encrypted using 256-bit SSL. Powered by Stripe.