LABORATORY EXERCISE 4
The first seed-bearing plants appeared about 360 million years ago. The surviving
lineages are two monophyletic sister clades, the gymnosperms and angiosperms.
exercise, we will focus on the gymnosperms, and explore the terrestrial adaptations that
helped the seed plants become the dominant plants on land.
The first gymnosperms
evolved about 305 million years ago.
At that time, the seedless vascular plants were the
dominant terrestrial vegetation, but as the climate became drier, it favored the spread of
By 251 million years ago, gymnosperms were the dominant land plants,
and they remained dominant for almost 200 million years.
Gymnosperms still dominate
many ecosystems, including forests at high altitudes and high latitudes.
In both gymnosperms and angiosperms, the
sporophyte is the dominant generation
gametophytes are very reduced, and are protected by the sporophyte
develop. All seed plants are
producing mega-and microspores that
develop into mega- and microgametophytes.
The male gametophyte is the
Pollen grains do not require water for dispersal, and with a few exceptions in the
gymnosperms, the sperm carried inside the pollen grains have no flagella. Consequently,
water is no longer required for fertilization
in these plants. The
, which develop
, have a
protective seed coat and a food supply
) form seeds on the surface of modified leaves called
By contrast, in angiosperms the megasporophylls (carpels) enclose the seeds.
gymnosperms the sporophylls are often aggregated into
commonly known as
cones. All gymnosperms are woody. We will focus on the
to explore the above terrestrial adaptatations, and briefly examine
a representative of another gymnosperm clade.
Phylum-level clade Coniferophyta
This clade contains a diverse group of species, many of which are evergreen and
needle-like or scale-like leaves
pine, juniper, fir, spruce, bald
belong to this clade.
Some bristlecone pines are over 4,600
years old, and are the oldest known organisms on earth. The Coniferophyta also includes
the tallest (coast redwood) tree on earth as well as the largest in volume (giant sequoia).