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BIOL 1902-A-Plant notes.doc

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BIOL 1902
Michael Runtz

Week 3 – TERMS AND EXAMPLES PLANT DEFENCES For 400 million years plants have been under attack from animals. Like an evolutionary arms race or ecological chess game, animals on offensive, plants on defensive 1) PHYSICAL DEFENCES Also called mechanical protection 1) body armour = tough epidermis ("outer skin") = bark and woody stems; trees, shrubs, strengthened with lignin; Vulnerable embryos are often housed inside seed coats fortified with lignin 2) other armour: structures that pierce - Physical or mechanical protection is often enhanced with nasty piercing structures Prickles are epidermal outgrowths: rose stems have prickles, raspberries and blackberries too Thorns are modified branches; Honey Locust trunks and Hawthorn branches have thorns Spines are modified leaves on a thistle; most spines are dead with highly lignified cells (sclereids); thistles have spines Sometimes these are aposematically coloured; Automimicry might explain why rose buds look like prickles 3) Trichomes: small hooked or clubbed hairs = dense tangles impede small animals such as mites and small insects. Mullein leaves have very dense trichomes Some trichomes release sticky glandular secretions = glandular hairs or glandular trichomes One type - separately stored phenols and enzymes, when animal brushes them they break open, contents mix like epoxy, create glue-like ooze that hardens. Some release chemicals that repel; Stinging nettles produce painful trichomes that inject chemicals Trichomes can be inducible: Water Smartweed grows in the water and has floating leaves that lack trichomes. If the habitat driesout and smartweed leaves develop glandular trichomes on the leaves and flower stems. 2) DIGESTIBILITY REDUCERS When eaten, these make it hard for the animal to digest the plant; most have dual purpose = structural and defensive A) Digestibility Reducers that are Structural Elements : Plant tissues have cells with stiff walls that can be thought of as reinforced concrete - cement with steel rods stuck in for support 1) cellulose + pectin and hemicellulose = gluey cement with “rods.” Together these elements give rigidity to cell walls Benefit: hard to digest even for insects; omnivores and carnivores digest very little of the material, if at all. 2) Lignin = another structural agent, also impregnates and stiffen cell walls; often stored in cells called sclereids. Provides woodiness in stems and toughens leaves 3) silica = cell walls strengthened by silica, derived from silicon - one of the most common elements in the earth’s crust; silica is found in grasses, horsetails; stored in special cells called phytoliths B) Digestibility Reducers that are NOT Structural Elements: These have no other function but defence so these are called secondary metabolites 1) Calcium Oxalate Crystals = in leaves of Arum plants: Skunk Cabbage and Jack-in- the-Pulpit are Arums; Calcium Oxalate is caustic, has a corrosive taste (burns lips and tongues) 2) Tannins: common are tannins, which are especially abundant in oak leaves and woody plants. Tannins are not bound inside the cell walls as lignin is, so they play no role in structural support. They deter anima
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