Gymnosperms

Gymnosperms

• Are Vascular plants
• 4 divisions of gymnosperms the 3 largest are as follows

1. Conifers - Cone bearing needled trees (eg.pines, firs)
2. Cycads - cone bearing palmlike plants
3. Ginkgos

• Exhibit alternation of generations

• Sporophyte (2N) generation is dominant.
• Produce true seeds
• Seed = embryonic sporophyte, stored food & protective coat
• Seeds of gymnospermsare "naked"
• Sporophyte generation produces heterospores - spores are not identical
• Microspore (male) will develop into a pollen grain which later develops into a sperm producing male gametophyte
• Megaspore (female) will develop into an egg bearing female gametophyte.

Life Cycle of the Pine

• Pines produce 2 cones - male & female

• Male cone is smaller and often located in the upper portion of the tree
• Male cone scales contain pollen sacs where microspore parent cells undergo meiosis to make microspores.
• Microspores develop into pollen grains.
• .Female cone is larger (located lower on tree)
• Contain 2 ovules where megaspore parent cells undergo meiosis to form megaspores.
• Only one of 4 megaspores develops into mature gametophyte.

 

• Pollenation.
• Pollen grains are carried by wind to open scales of female cone.
• Pollen grain grows a pollen tube to the mature egg. Where it is fertilized by 2 non motile sperm (only 1 fertilizes egg, other dies). Embryo will develop into a seed.
• Fertilization requires ~15 months.
• After 3 years seed is released and will germinate to form an adult sporophyte.

Pine Life Cycle