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BIOL 120
Simon Cheung

Lecture 4 Challenge:  Dealing with gravity  Harvesting light energy  Strengthen cell wall  Strength through chemistry - lignins Lignins in seedless vascular plants:  strengthen cell walls by replacing water and coating or encrusting the cellulose (the principle building block of plant cells walls: chains of glucose molecules)  Bryophytes do not have lignins  Allow for increase in size and height Seedless vascular plants:  Paleobotanists are not certain how SVPs evolved because fossil record is incomplete  Have a vascular system of phloem and xylem  Do not have seeds or flowers  Speculate that vascular tissue, waxy epidermis and other characteristics evolved over millions of years in fresh water algae  SVPs dominated landscape around 350 mya and persisted as dominant plants for about 100 mya (BUT still need water for fertilization)  Today they represent only about 5% of living plant species  Three extinct ancestors without roots and leaves:  Rhyniophytes: stems with dichotomous branching (arms of equal length)  Zosterophylls: Short lateral branches  Ancestors to Lycophyta - club mosses • Arose from the Zosterophylls • All lycophytes have microphylls (leaves that have a single vascular trace or leaf vein (no leaf gap) • Sporangia on modified leaves (sporophylls) Lecture 4 • Sporangia axillary or cluster to form a terminal strobilus • Homosporous or heterosporous  Trimerophytes: Had more extensive branching and formed a central stem  Ancestors to 3 of 4 living phyla of SVPs: • Psilotophyta - whisk ferns o Do not have true roots (only rhizomes) o The stems have a)  small scale like nonvascularized flaps of green tissue called enations (Latin “enatus”, “to rise out of”) OR b) Single-veined leaves o Nutrients and water absorbed by rhizomes that have hair-like rhizoids (assoc
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