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1 LABORATORY EXERCISE 4 GYMNOSPERMS The first seed-bearing plants appeared about 360 million years ago. The surviving lineages are two monophyletic sister clades, the gymnosperms and angiosperms. In this 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 gymnosperms. 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 . The gametophytes are very reduced, and are protected by the sporophyte as they develop. All seed plants are heterosporous, producing mega-and microspores that develop into mega- and microgametophytes. The male gametophyte is the pollen grain . 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 seeds , which develop from ovules , have a protective seed coat and a food supply . Most gymnosperms (“ naked-seeded” ) form seeds on the surface of modified leaves called megasporophylls . By contrast, in angiosperms the megasporophylls (carpels) enclose the seeds. In gymnosperms the sporophylls are often aggregated into strobili commonly known as cones. All gymnosperms are woody. We will focus on the Phylum-level clade Coniferophyta to explore the above terrestrial adaptatations, and briefly examine Ginkgo , a representative of another gymnosperm clade. I. Phylum-level clade Coniferophyta This clade contains a diverse group of species, many of which are evergreen and bear needle-like or scale-like leaves . Species of pine, juniper, fir, spruce, bald cypress, redwood , and yew 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). The
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This note was uploaded on 02/04/2012 for the course BIO 1225 taught by Professor Caddell during the Fall '11 term at University of Central Oklahoma.

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