BIOL 005B Lecture Notes - Lecture 21: Pollen Tube, Megaspore, Microspore
BIOL 005B Lecture 21: Seed Plants
●Overview: Transforming the World
○The evolution of the true seed changed the course of plant evolution, enabling
their bearers to become the dominant producers in most terrestrial ecosystems
○Seeds and pollen grains are key adaptations that allow plants to inhabit terrestrial
ecosystems
○A seed consists of an embryo (an entire individual) and nutrients surrounded by a
protective coat
○Common characteristics among the seed plants
■Heterospory
●The ancestors of seed plants were likely homosporous, while seed
plants are heterosporous
●Seed plants produce two different spore sizes: microspores → male
gametophytes, megaspores → female gametophytes
●Megasporangia (2n) produce megaspores (n) and microsporangia
(2n) produce microspores (n)
■Reduced gametophytes (virtually microscopic)
●Provides protection as the gametophytes of seed plants develop
within the walls of spores that are retained within tissues of the
parent sporophyte
■Ovules (integument + megasporangium + megaspore or egg)
●Gymnosperm megasporangia have one integument
●Angiosperm megasporangia usually have two integuments
■Pollen
●Microspores develop within pollen grains, which contain the male
gametophytes
●Pollination is the transfer of pollen to the part of a seed plant
containing the ovules
●Pollen eliminates the need for a film of water and can be disperses
great distances by air or animals
●If a pollen grain germinates, it gives rise to a pollen tube that
discharges two sperm into the female gametophyte within the
ovule
○The Evolutionary Advantage of Seeds
■A seed develops from the whole ovule
■A seed is a sporophyte embryo, along with its food supply, packaged in a
protective coat
■Seeds provide more evolutionary advantages over spores:
●The may remain dormant for days to years, until conditions are
favorable for germination
●They may be transported long distances by wind or animals
○Through time, plants tend to greater complexity
■Embryonic attachment to the mother plant
■Cells joined in tubes for water and nutrient transport vascular tissues
■Seed stage- embryo packaged with a food supply in a protective coat
●Gymnosperms bear “naked” seeds, typically on cones
○The gymnosperms have “naked” seeds not enclosed by ovaries and consist of four
phyla:
■Cycadophyta (cycads)
■Ginkgophyta (one living species: Ginkgo biloda
)
■Gnetophyta (3 genera: Gnetum, Ephedra, Welwitschia)
■Coniferophyta (conifers, such as pine, fir, and redwood)
○Gymnosperm diversity
■Phylum Ginkgophyta (one species, the ginkgo or maidenhair tree, Ginkgo
biloba
)
●Ginkgos have a high tolerance to air pollution and is a popular
ornamental tree
Document Summary
The evolution of the true seed changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems. Seeds and pollen grains are key adaptations that allow plants to inhabit terrestrial ecosystems. A seed consists of an embryo (an entire individual) and nutrients surrounded by a protective coat. The ancestors of seed plants were likely homosporous , while seed plants are heterosporous. Seed plants produce two different spore sizes: microspores male gametophytes, megaspores female gametophytes. Megasporangia (2n) produce megaspores (n) and microsporangia (2n) produce microspores (n) Provides protection as the gametophytes of seed plants develop within the walls of spores that are retained within tissues of the parent sporophyte. Ovules (integument + megasporangium + megaspore or egg) Microspores develop within pollen grains , which contain the male gametophytes. Pollination is the transfer of pollen to the part of a seed plant containing the ovules.
