jimtrue.com : school : BSC2011 : CH 35 & 36: Plant Structure & Growth/Transport in Plants

Posted by Jim True on September 28, 2004 6:24 PM. Last Updated October 22, 2006 9:23 PM

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CH 35 & 36: Plant Structure & Growth/Transport in Plants

CH. 35 - Flowering Plants

• The "green plants" are all members of the Kingdom Plantae.
• There are ~260,000 living species of plants, 905 of which belong to one group, the flowering plants.
• There are two groups of flowering plants, the Class Monocotyledones, usually called the "monocots" and the Class Dicotyledones, or "dicots".
• Most flowering plants that we are familiar with are dicots.
• Monocots (M) and dicots (D) can easily be told apart by serveral distinctive characteristics:
1. Leaf Shape - M, long and narrow; D, short and broad.
2. Leaf "veins" - M, parallel; D, "webbed" or "netted" appearance.
3. Flower parts - M, 3's or multiples; D, 4's or 5's or multiples.
4. Vascular bundles in stem - M, randomly scattered throughout stem; D, arranged in a ring along stem periphery.
5. Root system - M, fibrous root system, no roots distinctly larger than others; D, tap root system, possessing at least an enlarged central root.
• Monocots include all grasses, grains, lilies, orchid, palms (not really trees!).
• The dicots are the most diverse plants (>200,000 species) and include virtually all of the familiar flowering plants.

Ch.35 Plant Structure

• the plant body is divided into two main regions:
1. Root - part of the plant below ground.
2. Shoot - Part of the plant above ground.
• Roots usually arise at the base of the shoot, but some can arise from the stem or even leaves (plant "cuttings") Such roots are termed adventitious.
• Root hairs - Site of most water absorption through single celled extensions on roots surface. These increase available 'surface area' for maximum water absorption.
• The shoot system typically consists of stems and leaves.
• Stems consist of nodes (sites where leaves attach, internodes (spaces between nodes), axillary buds (locations where new stems or "branches" can arise) and a terminal bud at the growing tip, where the stem continues its growth.
• Leaves are simpler in construction, possessing a flattened blade, and a stalk for attachment to the stem, the petiole.
• Of course, blade shape and petiole length are enormously variable.
• There are a number of shoot modifications:
1. Stolon - stems aligned parallel to the ground but are above suface, e.g. spider plant, St. Augustine grass.
2. Rhizome - stems that are aligned parallel to the ground but are beneath the surface of the soil, e.g. some fern species, ginger root.
3. Tuber - expanded region of stem underground for food storage (e.g potato)
4. Bulb - underground structure mainly formed by specially modified non-photosynthetic leaves. Used for food storage e.g. onions, tulips, daffodils.
5. Tendrils - Leaves or stems form long winding strands that can attach to or around surfaces for support (peas (leaf), ivy (stem).
6. Spines - Leaves are very tough, with pointed ends, do not perform photosynthesis, used for protection, e.g. cacti.
7. "Fleshy" leaves - found in plants collectively called "succulents" Leaves are modified to hold water, eg. Kalanchoe, Aloe
8. "Attractor" Leaves - Brightly colored to draw attention to the flow, e.g. poinsettia.

CH.35 Plant Structure

• Plant tissues may be simple (composed of one cell type) or complex (composed of several cell types).
• There are three tissue systems in plants which are typically found throughout the entire plant:
  1. Ground - These tissues form the main support framework, and also are involved in storage and production of food and plant growth.
  • Include 3 simple cell types, which combined make up most (the "background") of plant tissues:
  1. Parenchyma - Most common tissues
  • Shapeless nondescript cells
  • Have only primary cell walls
  • Cells are alive when mature
  • Functions: Storage, secretion, photosynthesis, general support.
  2. Collenchyma - rigid but flexible support tissues
  • Cells have unevenly thickened primary cell wall, so cells look "squared off" under microsope
  • Cells are alive at maturity
  • Usually found near outside tissue layers
  • Cells can form long strands (e.g. leaf "veins", celery "strings".
  • Functions: Strength and support framework.
  3. Sclerenchyma - Hard, rigid, inflexible tissues.
  • Have thick secondary walls.
  • Cells are dead when mature.
  • Cells have two different shapes:
  1. Fibers - Cells are long and tapered, forming long strands, e.g. flax ("linen")
  2. Sclereids (AKA "stone cells") Irregular cube or pebble shaped cells.
  • Tiny individual sclereids form the "grit" in pears.
  • Fused sclereids form peach and cherry "pits".
  2. Vascular Tissues - Two complex tissues.
  • Not all plants possess these tissues. Those that do are called vascular plants.
  1. Xylem (xylo - "wood") Two tubular cell types plus two simple tissues.
  • Carry water and minerals (essential nutrients) UPWARD and OUTWARD through plant.
  • Tracheids and vessel elements are tubular cells that are dead at maturity.
  • Tracheids are found in ferns and evergreens (pines and their relatives).
  • Tracheids cells are tapered and occur as overlapping clusters or bundles.
  • Water passes between cells and to other tissues via pits, tiny spots where secondary cell wall has not formed.
  • Vessell elements are found in flowering vascular plants.
  • These are connected end to end to form long tubes (like a water pipe).
  • Vessel elements pass water to other tissues via pits, and to each other via holes in the ends of the cell.
  • Also assocated with tracheids or vessel elements are special xylem paraenchyma cells for storage and sclerenchyma fibers for structural support.
  2. Phloem (phlo - "bark of a tree" ) Also has two special cell types and two special tissues.
  • Function to transport dissolved food molecules produced by Photosynthesis, usually INWARD and DOWNWARD throughout plant.
  • Cell types are called sieve tube members and companion cells.
  • Sieve tube members connect end to end at sieve tube plates, which are perforated to allow cytoplasm to interconnect.
  • Sieve Tube cells are alive at maturity but have lost most of their organelles, including the nucleus.
  • These actually transport the food.
  • Companion cells are also alive at maturity, possess their nuclei and are joined to sieve tube cells by plasmodesmata.
  • They help move food to the sieve cells.
  • Like xylem, phloem also has phloem parenchyma and sclerenchyma fibers
  • Xylem and phloem frequently lie side by side throughout the body of the plant and are referred to as vascular bundles, e.g "veins" in a leaf.
  • However, they provide unidirectional flow and DO NOT interconnect!
  3. Dermal Tissue - Complex tissue, in which there are three cell types:
  1. Epidermis - A single layer of parenchyma which forms the epidermis. ("over" + "skin") of leaves and the stems of herbaceous plants.
     • Epidermal cells may secrete a waxy material to form a cuticle, for waterproofing.
     • Epidermal cells lack chloroplasts.
     • Cuticle not only prevents H₂O loss, it prevents gas exchange.
     • Embedded in epidermis are guard cells, kidney bean shaped, and possessing chloroplasts.
     • Two guard cells surround openings called stomata (stoma - "mouth") through which gas exchange takes place.
     • Found mainly on leaves and green stems.
  2. Trichomes (tricho - 'hair") - Hairlike modifications of epidermal cells on leaves, stems and roots.
  • Root hairs - trichomes that increase surface area for H₂O absorption.
  • On stems and leaves, may be used to prevent overheating and H₂O loss.
  • May also be used for protection by secreting irritant, e.g. nettles.
  • In woody plants, the epidermis of the stem is replaced as the plant ages.
  3. Periderm (peri - "around")
     • Formed by cork cells, dead at maturity with very thick waterproofed cell walls (the "bark" of trees) and cork parenchyma for storage.

CH.35 - Plant Growth

• Flowering plants are either herbaceous, the stems always remain green, or they are woody, the stems produce the hardened material we call "wood".
• Plants live varying amounts of time:
1. Annuals - Entire life is one year. All annuals are herbaceous, e.g. petunias, geraniums, corn.
2. Biennials - also herbaceous, but with a 2 year life span. In the first year, these grow and store food, during the second, reproduction and death, e.g. carrots, beets.
3. Perennial - The life span is > 2 years, in some cases 100's to 1000's of years.
   • All woody plants are perennials, e.g., almost all trees.
   • Some herbaceous plants are also perennials, but they die back in cold months and maintain minimal metabolic levels until warmer weather, e.g. onions, lilies, tulips, daffodils).
• Indeterminate Growth - Plant stems and roots typically grow throughout life.
• Determinate Growth - Leaves, flowers and fruits grow to some genetically predetermined size and then stop.
• Plant growth consists of 3 stages:
  1. Mitosis and cell division.
  2. Elongation - enlargement of tissues.
  3. Differentiation - tissues & organs form.
• Meristem - Undifferentiated cells that undergo continuous mitosis to produce plant tissues.
  • Analogous to animal germ layers.
  • Growth is produced along two different axes by meristem:
  1. Primary Growth - Vertical growth provided by apical meristem - cells in tips of roots and stems.
     • Apical meristem is found in both herbaceous and woody plants.
     • Produces the lenghtening of stems and roots and the formation of branches and leaves.
     • In leaves, primary meristem undergoes mitosis and cell division until leaf reaches full size, then division stops.
     • Three types of primary meristem:
       1. Protoderm - ("first" skin) - Differentiates to epidermis.
       2. Procambium ("first" + "exchange") - Differentiates to primary vascular tissues, primary xylem and phloem. ONLY TYPE OF VASCULAR TISSUE PRESENT IN HERBACEOUS PLANTS!
       3. Ground meristem - Differentiates to ground tissues.
  2. Secondary growth - Horizontal (lateral) growth which increases plant diameter, which ONLY OCCURS IN WOODY PLANTS!
  • Secondary meristems are found along the entire length of the roots and stems, except for the tips.
  • Two different secondary meristems:
    1. Vascular cambium - a thin layer within stems and roots.
       • Occurs between wood and bark in a woody plant.
       • Cell division of the vascular cambium produces secondary xylem to the inside, secondary phloem to the outside.
       • Secondary xylem is produced annually, forming a substance called wood.
       • The newest secondary xylem is fully functional, but as it grows older, the flow decreases. In old wood, there is no flow.
       • There may be an accumulation of organic substances in the older xylem.
       • Only the current years secondary phloem is present and functional. each preceding years phloem is crushed between the bark and the vascular cambium.
       • The secondary phloem forms the inner bark of the plant. If this layer is damaged all the way around the plant ("girdling), the plant will die.
    2. Cork cambium - layer of meristematic cells outside the secondary phloem.
       • Produces the periderm.
       • The functional phloem PLUS the periderm is all considered the bark of the tree.

CH.36 Transport

• Unlike animals, plants do NOT have a circulatory system, although fluids may flow through tubular channels (not cross-connected, and no pumps pushing fluids the plant).
• Transport in plants includes cellular level (uptake of water and nutrients), short distance (cell to cell) and long distance (through vascular tissues).
• We will focus on long distance transport only.
1. Water transport in xylem - water and dissolved minerals move inward through the roots to the tracheids or vessel elements of the xylem (throgh diffusion and osmotic pressure).
• The fluid is then pulled upwards. along its length, water and minerals "leak" out into tissues at pits.
• The movement of water in this manner is explained by the tension-cohesion model (also known as the transpiration-cohesion model).
• Water is pulled in an unbroken column throughout the plant due to the strong cohesion of water molecules.
• Adhesion of water allows it to "cliing" to the xylem walls, also helping the flow.
• Flow is created by transpiration, the evaporation of water at the leaves.
• This creates a lower air pressure at the leaves.
• Water rises through the xylem in response to lower pressure (like sucking liquid through a straw). Moderate wind and sunlight assist with transpiration.
• Occurs rapidly when there is a lot of water and transpiration level is high (hot, windy days), more slowly when it is cool, still or there is little H₂O in soil.
• To a lesser extent, water is also moved upward through the stems by root pressure, caused by the osmosis of H₂O into the roots from the soil.
• Osmosis into the xylem from the soil is due to active uptake of minerals into the roots, causing an osmotic shift in xylem.
• Root pressure is most significant in small plants and moist soil.
2. Transport in Phloem - Although we can generalize that phloem moves food inward and downward, in actuality, phloem moves food in any direction from where it is concentrated (source) to where it is not (sink).
• Food moves as the disaccharide sucrose.
• Pressure-flow hypothesis - how sugars are thought to be transported.
• Phloem is "loaded" with sugar at the source by active transport of sugar into the sieve tube cell.
• Powered by energy from the proton pump.
• This causes water to diffuse into the sieve tube cell which increases the pressure in those cells, thus forcing the liquid/sugar to move from higher to lower pressure.
• At the sink, sugars are "unloaded" by both diffusion and active transport, H₂O diffuses out of sieve tube cell and fluid pressure drops.

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