Seed Plants

Spermatophytes have a specialized form of the sporic cycle.  All seed plants are heterosporous, forming microspores in microsporangia and megaspores in megasporangia.  After their formation, the spores are retained within the sporangia where they develop into gametophytes, surrounded and nourished by sporophyte tissues.  Seed plant gametophytes are highly reduced, in some cases to just a few cells.  The microspores develop into sperm-forming male gametophytes (commonly called pollen grains or microgametophytes) and the megaspores develop into egg-forming female gametophytes (commonly called embryo sacs or megagametophytes).  Megasporangia and female gametophytes are formed within a protective sporophyte structure called the ovule.

Male gametophytes (pollen) are released from the sporophyte.  Pollination occurs when pollen is transferred to the vicinity of a receptive ovule.  Sperm are formed and released by the male gametophyte and ultimately fertilize the egg cell (within the female gametophyte inside of its ovule).  Following syngamy, the zygote develops into a sporophyte embryo, and the ovule develops into a seed.  A generalized spermatophyte life cycle is represented below:

Generalized Spermatophyte (Seed Plant) Life Cycle

Key Points in a Seed Plant Life Cycle:

• sporophyte is the dominant (long-lived and free-living) generation
• sporophytes are heterosporous
• gametophytes (male and female) are short-lived and highly reduced
• gametophytes are retained and nourished within the sporophytes
• female gametophytes develop within a protective sporophyte structure called the ovule
• zygotes are formed and develop into embryos within ovules
• each fertilized ovule (containing a new embryonic sporophyte and a food supply) develops into a seed

A key to understanding seed plant reproduction is understanding ovules.  Ovules are ovoid structures formed by the sporophyte generation.  They consist of the megasporangium surrounded by one or two layers of sterile tissue (called integuments).  An opening through the integuments called the micropyle allows the passage of sperm or pollen into the interior of the ovule.  Each megasporangium forms one functional megaspore, resulting in the formation of one female gametophyte per ovule.  Following fertilization and development of an embryonic sporophyte, the integuments of the ovule develop into a protective seed coat and the entire structure develops into a seed.

The five phyla of seed plants are:

• Pinophyta (conifers)
• Cycadophyta (cycads)
• Ginkgophyta (ginkgoes)
• Gnetophyta (gnetophytes)
• Magnoliophyta (flowering plants)

The first four of these phyla are relatively small clades with a combined number of species of around 900. The members of these four phyla are commonly called gymnosperms (naked seeds) because seed are formed on the surface of scales or other structures.  The flowering plants, on the other hand, are the largest clade in the plant kingdom with about 250,000 species.  These organisms are commonly called angiosperms (vessel seeds), because the seeds develop within an enclosing structure (the carpel) which ultimately develops into a fruit.  Gymnosperms share commonalities in their life cycles, and will be described first.  Angiosperms have a fairly specialized life cycle and will be described second.