chapter 12 outline
chapter 12 outline BIOL 1110
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This 6 page Class Notes was uploaded by Caitrín Hall on Tuesday January 26, 2016. The Class Notes belongs to BIOL 1110 at University of Connecticut taught by Bernard Goffinet in Summer 2015. Since its upload, it has received 13 views. For similar materials see Introduction to Botany in Biology at University of Connecticut.
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Date Created: 01/26/16
Leaves (photosynthesis & transpiration) 12.1 As photosynthetic organs, leaves occur in a vast range of forms Leaves in most eudicots Blade – wide, flat portion o Variable shape o Simple – one blade; margins are smooth, toothed, or deeply lobed o Compound – blade consists of two or more leaflets Pinnately compound leaflets – arranged along central axis that extends from petiole Palmately compound leaflets – arise from common point at the end of the petiole **Axillary buds occur where leaves (NOT leaflets) join stems** Petiole – stalk portion Stipules – small, leaflike structures at base of petiole Axil – angle between petiole and stem o Axillary buds Node – point on stem where leaf is attached Internode – space between nodes Sessile leaves lack petiole Sheath – base of leaf that wraps around stem Most monocots – parallel veins Most eudicots – pinnately/palmately netted veins 12.2 The major tissues of leaves are epidermis, mesophyll, xylem, and phloem Epidermis – tough outer cell layer; helps protect inner tissues & supports structure Covers upper and lower surfaces Posses plastids (not green); translucence permits sunlight to pass through Secrete waxy cuticle to prevent evaporative water loss Trichromes – variety of functions that depend on plant/location o Reflect light, absorb water and minerals, or deter plant eaters Stomata – permit gas exchange o 2 guard cells surround each stoma o Turgid guard cells fill with water and bend to open pore o Bulliform cells of grass leaves act similarly when turgid Mesophyll – main photosynthetic tissue Between upper and lower epidermis Parenchyma cells packed with chloroplasts & surrounded by intercellular space (connected to stomata for gas exchange) Palisade layer (parenchyma) – columnar; surrounded by space; hosts most photosynthesis; upper Spongy layer (parenchyma) – irregular shape; open arrangement for gas exchange; lower Xylem and phloem – conducting tissues that form vascular bundles, or veins Xylem – conducts water/dissolved minerals to leaf; upper side of each vein Phloem – conducts sugars/photosynthates from leaf; lower side of vein Bundle sheath – parenchyma cells surrounding veins o Kranz anatomy – mesophyll cells form a ring around vascular bundles (associated with C4 photosynthesis) Environment determines leaf structure Hydrophytes – grow in wet environments o Wholly or partly submerged o Thickness of cuticle is related to degree of submersion—more submerged thinner o Stomata on upper epidermis (submerged leaves may lack stomata) Mesophytes – moderately moist environment o Thicker cuticles on upper leaf than on shaded lower surface o Stomata on lower epidermis to shield from direct sunlight Xerophytes – grow in dry habitats o Very thick cuticles + multiple layers of epidermal cells to prevent additional water loss o Many stomata on under surface, but shed leaves when very dry 12.3 Plants move large quantities of water by means of transpiration Transpiration – process by which water (more than 90%) passes through stomata and evaporates “Motor” that moves water through xylem Stomata open for gas exchange & lose water in process o CO2 is needed when sunlight is available; water is heated and evaporates faster o Guard cells gain dissolved substances (K+ ions) which increase concentration water moves to guard cells by osmosis o Water build-up creates turgor pressure – causes guard cells to bend and open Radially oriented cellulose allows cells to lengthen under turgor pressure; attachment to each other prevents guard cells from sliding past each other o Reverse process closes stomata at night Environmental factors affect stomatal movements o Plants with daytime stomata open: water stress causes production of hormone abscisic acid (ABA), which causes solutes to move out of cell, decrease in turgor pressure, pore closure o Very hot mid-day conditions: Increase in CO2—happens when transpiration > water absorption—causes stomata to close & use CO2 build-up for photosynthesis o Desert plants: CAM fixes CO2 as organic acid at night; close stomata in daytime; break organic acid to release CO2 for photosynthesis 12.4 The cohesion-tension theory explains movement of water through plants Root pressure = guttation o Soil moisture is high and transpiration is low o Water is forced into stems and out of leaves o Too weak to exist as only force that moves water Cohesion-tension theory o Hydrogen bonds in water cohesion o Water from mesophyll cells replaces water evaporated at surface Concentration of solutes in mesophyll cells rises More water is drawn into cells by osmosis, creating tension Tension draws water to veins where tension transfers to xylem Water, a continuous piece, is pulled from roots to xylem in leaves **Living cells in vascular tissue play no role** (as far as we know) 12.6 Leaves perform many functions in addition to photosynthesis Leaves undergo modifications to perform other functions: o Acting as support structures o Attracting pollinators o Traps for animal prey o Providing winter cold protection o Food/water storage o Defense against animals Tendrils – threadlike structures that help plants climb over things for light/space Bud scales protect buds of woody plants in temperate climates over winter o Wax-impregnated – prevent desiccation & insulate bud o Growth inhibitors prevent bud growth until spring Bracts – modified to attract pollinators; surround small, inconspicuous flowers Food/water storage o In arid environments, succulents have thick & fleshy leaves to store water and carry out photosynthesis o In tropical environments, epiphytes grow on branches of other plants & usually have leaves modified to store rainwater o Garden plants (cabbage, lettuce, kale, spinach, etc.) store nutrients o Bulbs – short, conical, underground stems surrounded by fleshy food-storage leaves Self defense o Desert plant leaves form sharp, nonphotosynthetic spines to deter animals **Spine = modified leaf; thorn = modified stem that arises from leaf axil; prickle = sharp outgrowth from epidermis/cortex of stem** Some leaves capture animal prey o Carnivorous plants grow in temperate/tropical regions where nitrogen in soil is deficient o Passive traps – attractive leaves modified into pitchers that trap rainwater & secrete digestive enzymes; insects drown because slippery trichromes prevent escape o Active traps Venus flytrap – touch-sensitive hair causes trap to snap; digests insect within a week Sundews –maculated hairs attract insects; hairs bend and trap insects; mucilage digests insects while hairs absorb nutrients Bladderworts – submerged/floating in nutrient-deficient water; tiny bladders contain water and are lined with specialized glands; water leaves & creates negative charge; insects touches hair and is pulled in Chapter Wrap-up Examine and Discuss Self Test 1. A pinnately compound leaf has: a. leaflets arising from a common point at the end of the petiole b. leaflets arising along a central axis that is an extension of the petiole c. leaflets arising in whorls along a stem d. sessile leaflets arranged along a central stem e. sheathed leaflets wrapped around an extension of the petiole 2. Which of the following statements about the distribution of stomata on plants is true? a. Some xerophytes may have stomata only in depressions on the lower surface of leaves. b. In hydrophytes with floating leaves, the stomata occur only on the lower epidermis. c. Mesophytes have stomata on the upper and lower epidermal layers of their leaves. d. Hydrophytes with submerged leaves may lack stomata entirely. e. Both a. and d. are true. 3. In which of the following plants and environments would you expect to find closed stomata? a. mesophytic plants with ample water in the daytime b. xerophytic CAM plants in the night c. mesophytic plants at high midday temperatures with excess CO inside the leaves d. xerophytic CAM plants in 2 the daytime e. both c and d 4. In the experiment shown in Figure 12.18 of your text, the roots of a jewelweed (Impatiens) were cut off, and the plant inserted into a test tube containing water and blue dye. Within minutes, the blue dye begins moving up the stem. In what major plant tissue is the dye most likely to be moving? a. epidermis b. phloem c. parenchyma d. xylem e. mesophyll tissue 5. Which of the following are not formed from modified leaves? a. traps on the Venus fly trap b. tendrils of the grapevine c. bud scales surrounding the buds of temperate woody plants d. bracts of the poinsettia e. spines on cacti Applying Concepts 1. Imagine you are walking through the produce section of a well-stocked grocery store. Can you name at least five different vegetables that consist primarily of leaves? Celery, onions, lettuce, cabbage, spinach 2. On a trip to the grocery store with a friend, you casually mention that celery is basically a bunch of leaves from the celery plant. Your friend disagrees, arguing that celery stalks are stems. Who is correct, and how could you settle the question? Celery is a bunch of leaves. The stalk is a petiole with axillary buds between it and the stem. Axillary buds exist where leaves meet stems.
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