Alternation of Generations: Life Cycle Stages

All Embryophytes and some algae undergo a specialized life cycle known as Alternation of Generations. This term reflects the fact that the generations change ploidy, with a diploid generation (the sporophyte) giving "birth" to a haploid generation (gametophte), and that haploid generation giving "birth" to the next diploid (sporophyte) generation, and so on.

The life cycle is the same in all plants, though the life cycle stages may look completely different.

First, recall the generalized cycle of the Alternation of Generations:

And now consider this list of terms related to the Alternation of Generations:

I. TERMS RELATED TO THE (HAPLOID) GAMETOPHYTE GENERATION:

• gametophyte - a haploid plant which produces gametes
• gamete - haploid ovum (female) or sperm (male), produced via mitosis in the sex organs of the haploid gametophyte which are either the...
  • archegonium - the multicellular sex organ of the female gametophyte, analogous to an ovary, or the...
  • antheridium - the multicellular sex organ of the male gametophyte, analogous to a testis.

• zygote - single, diploid cell produced by the union of sperm and egg inside the archegonium. This will grow and develop into the sporophyte.

II. TERMS RELATED TO THE (DIPLOID) SPOROPHYTE GENERATION:

• sporophyte - a diploid plant which produces spores.
• spore - haploid cell produced via meiosis in the sporangium of the sporophyte. It will grow and develop into the haploid gametophyte.
  • megaspore - a spore that develops into a female gametophyte
  • microspore - a spore that develops into a male gametophyte
• sporangium - diploid sporophyte structure, essentially a compartment inside which diploid sporophyte cells undergo meiosis to become spores.
  • megasporangium - a sporangium that produces (female) megaspores
  • microsporangium - a sporangium that produces (male) microspores
• sporophyll - a specialized leaf bearing sporangia on its leaf blade.
  • megasporophyll - a sporophyll bearing megaspores
  • microsporophyll - a sporophyll bearing microspores

III. GENERAL TERMS:

• dioecious - each individual is either male or female (separate sexes)
• monoecious - each individual has both male and female reproductive structures (bisexual)

To fully understand the cycle, though, it's sometimes helpful to create an analogy.

What does a GAMETOPHYTE look like?

Bryophyte Gametophytes
A liverwort gametophyte, not reproducing at the moment:

Moss gametophytes:

Tracheophyte Gametophytes
In vascular plants, the gametophyte becomes smaller and smaller as we move away from the ancestral condition.

Fern gametophytes (last one with a sporophyte sprouting out of it):

These are bisexual: each gametophyte has both antheridia and archegonia.

Pine gametophytes:
The male gametophyte

• is a pollen grain (left).
• It has no antheridia, and produces only two sperm.
  
• The male pine cones grow on the ends of pine tree branches in the spring (center)
  
• The pollen develop from microspores in microsporangia on male pine cones (longitudinal section, right)
  

The female gametophyte

• remains inside the female pine cone megasporangium (left).
• Each gametophyte produces 1-6 archegonia (depending on species), but only one produces a viable ovum.
  
• The female pine cones grow on the ends of pine tree branches in the spring (center)
  
• The female gametophytes develop from megaspores in megasporangia on female pine cones (longitudinal section, right)
  

Flowering Plant gametophytes:
The male gametophyte

• is a pollen grain (right).
• It has no antheridia, and produces only two sperm.
  
• The pollen develop from microspores in microsporangia known as anthers (left)
  
• The anthers are located just inside the petals in a complete flower (i.e., one that has sepals, petals, stamens, and pistils).

The female gametophyte

• remains inside the female megasporangium inside the pistil (right).
• It has no archegonia, and produces eight identical nuclei.
  
• The female gametophyte (also known as the 8-nucleate embryo sac develops from a megaspore inside a megasporangium surrounded by a coat of mother sporophyte tissue, the ovule (center)
  
• The ovules are located inside the pistils (megasporophylls) in a complete flower (i.e., one that has sepals, petals, stamens, and pistils), and the pistils are located in the very center of the flower. (left)

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What does the ARCHEGONIUM look like?

The archegonium is the sex organ (analogous to an ovary) of the female gametophyte. It looks remarkably similar across taxa, though its location becomes less obvious as the gametophyte becomes smaller in the more derived plants.

Where is the archegonium found?

• liverworts: on the underside of the little female gametophye "palm tree" archegoniophore
• mosses: on the tip of a female gametophyte's shoot
• ferns: close to the little notch of the heart-shaped, bisexual gametophyte
• gynnosperms: inside the female gametophyte, locked in her ovule (immature seed)
• angiosperms: lost! Angiosperms have no archegonia.

Pictures of the above, in order:

Female liverwort:

Female moss:

Bisexual Fern:

Female Pine (cone):

Angiosperm: (the female gametophyte is a mass of cytoplasm with eight nuclei, but no archegonia)
: