BIOL 108 Topic 15

Reduced Gametophytes

• in seed plants, gametophytes develop within spore walls retained within parental sporophyte tissues

• gametophytes are protected from environmental stresses

• gametophytes doesn’t have vascular tissue, only the sporophyte does

• a trend in plant evolution is the progressive reduction in gametophyte size and independence

Heterospory

• different types of spores produced

Microsporangia

• produce microspores that develop into male gametophytes

Megasporangia

• produce megaspores that develop into female gametophytes

Ovules

• an ovule consists of a megasporangium (2n), megaspore (n) and one ore more protective integuments (2n)

• develop into seeds after fertilization

Female Gametophyte

• the megasporangium produces a megaspore via meiosis, enclosed in protective integuments

• the megaspore isn’t dispersed

Pollen

• microspores develop into male gametophytes within pollen grains

Male Gametophyte

• microsporangium produces microspores via meiosis

• microspores aren’t dispersed; they develop into pollen grains

• microspores undergo mitosis to produce tiny gametophytes

Seeds

• a seed develops from a fertilized ovule and consists of an embryo, food supply, and outer seed coat

• a fertilized ovule develops into a seed containing the next-generation sporophyte

• seed sizes vary greatly and is primarily determined by the amount of gametophyte-derived storage reserves

Pollination

• transports pollen to the ovule for fertilization

• pollen eliminates the need for water during fertilization

• the pollen tube discharges sperm into the female gametophyte within the ovule

Seed Characteristics

• the seed is the dispersal stage for seed plants (not the spores)

• seeds provide evolutionary advantages over spores

• seeds have external coats that protect the embryo

• seeds have an energy-dense supply of stored food that supports early seedling growth

Four Phyla of Gymnosperms

• Cycadophyta

• Gingkophyta

• Gnetophyta

• Coniferophyta

• they all form a monophyletic group

Phylum Cycadophyta

• diverse and dominant in the Mesozoic but are not limited to small populations in the tropics and subtropics

• Cycads grow very slowly, producing large palm-like leaves on short, unbranched stems

• have flagellated sperm

Phylum Ginkgophyla

• the only living species is Ginkgo biloba with distinctive bilobed leaves

• it has separate male and female sporophyte trees that are wind-pollinated

Phylum Gnetophyla

• comprises three genera:

  • Gnetum, Ephedra, Welwitschia

• phylum grouping based on molecular evidence

• gnetophytes are morphologically diverse and are adapted to various environments

Phylum Coniferophyta

• most speices rich phylum of gymnosperms

• most conifers are woody shrubs or trees

• conifers were widespread before the evolution of the angiosperms

• most conifers are evergreens and can carry out photosynthesis year round

• conifers are adapted to cold and dry habitats

Life cycle of Gymnosperms

• gymnosperms have dominant sporophyte generations

• gymnosperms develop seeds from fertilized ovules

• transfer to male gametophyte to ovules by pollen

Life cycle of a conifer

• pollination by wind

• seed dispersal by wind

• fertilization and seed development in conifers are slow