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Bio1020, Diversity of Life: March 5.docx

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BIOL 1020
Robert Laird

March 5, 2014 – Biology Gymnosperm evolution • Seed plants: o Extant clades of seed plants  Gymnosperms (naked seed: the seeds are exposed on sporophylls that form cones (strobili)) • Numerical dominance, biomass, geographical expanse= dominate.  Angiosperms (seeds are found in fruit, which are mature ovaries) sometimes separated into male and female plants. • Began to replace gymnosperms near the end of the Mesozoic (251 MYA) • Angiosperms now dominate more terrestrial ecosystems • Today, cone-bearing gymnosperms called conifers dominated in the northern latitudes o Gymnosperms appear ~308 millions years ago and dominated Mesozoic (251-65 millions years ago) terrestrial ecosystems o Better suited than non-vascular plants to drier conditions o Seed plants originated in Late Devonian period (380 MYA) o Progymnosperms acquire some adaptations that characterize seed plants. (Archaeopteris: woody plant, heterosporous, no seeds) Secondary growth • Secondary growth occurs in stems and roots of woody plants but rarely in leaves • The secondary plant body consists of the tissues produced by the vascular cambium and cork cambium • Secondary growth is characteristic of gymnosperms and many eudicots (most angiosperms), but not monocots • Vascular cambium between the primary xylem and primary phloem (this is what grows) Fig. 35.19 o Anything beyond phloem is considered as Bark.  Bark has the cork cambium • Dendrochronology: Growth rings, Higher indices represent bigger rings, and greater temperature. Closer the rings the less growth. Types of wood • As a tree of woody shrub ages, the older layers of secondary xylem, the heartwood, no longer transport water and minerals (supports the tree) • The outer layers, known as sapwood, still transport materials through the xylem • Older secondary phloem sloughs off and does not accumulate. (ruptures, shed off the tree with bits of bark) • Secondary xylem: heartwood and sapwood • Bark: secondary phloem and layers of periderm • Figure 35 Diversity of gymnosperms: • Four extant phyla o Cycadophyta (cyads)
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