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Plant Diversity II: The Evolution of Seed Plants (Seeds and pollen grains…
Plant Diversity II: The Evolution of Seed Plants
Seeds and pollen grains are key adaptations for life on land
These adaptations include the seed, the reduction of the gametophyte generation, heterospory, ovules, and pollen.
Bryophyte life cycles are dominated by the gametophyte generation, while seedless vascular plants have sporophyte-dominated life cycles.
Nearly all seedless plants are homosporous, producing a single kind of spore that forms a hermaphroditic gametophyte.
In contrast to the few species of heterosporous seedless vascular plants, seed plants are unique in retaining their megaspores within the parent sporophyte.
The sperm of some gymnosperm species retain the ancestral flagellated condition, providing evidence of this evolutionary transition.
Gymnosperms bear "naked" seeds, typically on cones
The most familiar gymnosperms are the conifers, cone-bearing trees such as pine, fir, and redwood.
The four phyla of extant gymnosperms are Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta.
The gymnosperms probably descended from progymnosperms, a group of Devonian plants that were heterosporous but lacked seeds.
The life cycle of a pine demonstrates the key reproductive adaptations of seed plants.
Ovulate cones produce megaspore mother cells that undergo meiosis to produce four haploid cells, one of which will develop into a megaspore.
The reproductive adaptations of angiosperms include flowers and fruits
The flower is the defining reproductive adaptation of angiosperms.
Angiosperms, commonly known as flowering plants, are vascular seed plants that produce flowers and fruits.
A flower is a specialized shoot with up to four circles of modified leaves: sepals, petals, stamens, and carpals.
The life cycle of an angiosperm is a highly refined version of the alternation of generations common to all plants.
Angiosperms have diversified into more than 250,000 species that dominate most terrestrial ecosystems.
Human welfare depends on seed plants
The absolute dependence of humans on Earth’s flora is a specific and highly refined case of the more general connection between animals and plants.
Gymnosperms and angiosperms are sources of wood, which is absent in all living seedless plants and consists of an accumulation of tough-walled xylem cells.
Plant diversity is a nonrenewable resource.
As the forests disappear, thousand of plant species and the animals that depend on these plants also go extinct.
Researchers have investigated fewer than 5,000 plant species as potential sources of medicines.