16 Apr 2012
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Lecture 18: Alternation of Generations
Alternation of Generations (Gymnosperms)
Ginkgophyta
Cycadophyta
Coniferophyta
- 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