BSCI 124 Lecture Notes

Undergraduate Program in Plant Biology, University of Maryland


Levels of Organization to produce a functioning plant

  1. Plant cells- the basic building blocks.  
    1. each cell is approximately 1/10- 1/100th of a millimeter long
    2. cells can specialize in form and function to provide certain specialized functions to the whole plant
    3. Each cell can live on its own under certain conditions- however, by working together they provide a way to survive in more varied conditions
  2. Plant tissues- collections of similar cells that serve a specific purpose by functioning together
    1. Unlike animals, the major organs of plants (roots, stems, and leaves) are all composed of the same three tissues (epidermis, vascular tissues, and ground tissues).
    2. Each tissue carries out the same fundamental activities throughout the plant.
    3. Three types of tissues
      1. Epidermis - the exchange of matter between the plant and the environment.
        1. the epidermis on aboveground organs (leaves and stems) is involved with gas exchange
        2. the epidermis on belowground organs (roots) is involved with water and ion uptake
      2. Vascular tissues - the transport of water and dissolved substances inside the plant
        1. the xylem carries water and dissolved ions from the roots to stems and leaves
        2. the phloem carries dissolved sugars from the leaves to all other parts of the plant
      3. Ground tissues - metabolism, storage, and support activities
        1. the ground tissue of the leaf (called mesophyll) uses the energy in sunlight to synthesize sugars in a process known as photosynthesis
        2. the ground tissue of the stem (called pith and cortex) develops support cells to hold the young plant upright
        3. the ground tissue of the root (also called cortex) often stores energy- rich carbohydrates
  3. . Plant organs- tissues that act together to serve specific functions for the whole plant
    1. Roots
      1. Functions -
        1. Anchorage
        2. Absorption of water and dissolved minerals
        3. Storage (surplus sugars transported from leaves)
        4. Conduction
        5. Cross section of herbaceous dicot root
      2. See branch root of willow (Salix), x.s.
      3. Epidermis
        1. Single layer of cells for protection (from disease organisms) and absorption (water and dissolved minerals)
        2. Root hairs- tubular extensions of epidermal cells
          1. short lived
          2. greatly increase surface area of root, in contact with soil
          3. confined largely to the region of maturation of the root
      4. Cortex
        1. Store starch and other substances
        2. Contain numerous intercellular spaces - air spaces essential for aeration of the root cells (for cellular respiration)
        3. See monocot root of an orchid (Orchidaceae), x.s.
      5. Xylem
        1. Conducts water and dissolved minerals
        2. composed of
          1. See vessel elements of oak (Quercus), x.s.
            a) vessels: tube-like structures composed of hollow elongate cells (vessel members) placed end-to-end and connected by perforations
          2. See tracheids from pine (Pinus), x.s.
            a) tracheids: elongated conducting and supporting cells with tapering and pitted walls without perforations
            1. Upward movement caused by transpiration from the leaves aided by the properties of water: polarity of water molecules, cohesion of water molecules to each other, adhesion to xylem cell walls
            2. Very rapid- 2 feet/ minute
      6. Phloem
        1. Conducts food (dissolved sugar)
        2. Phloem composed of sieve elements (sieve tube members, companion cells)
          1. Sieve tube is a series of sieve tube members arranged end-to-end and interconnected by sieve plates
          2. Movement of sugars up or down through plasmodesmata of sieve elements
          3. One inch/ minute
    2. Stems
      1. Functions of stems - an important site with a thorough review and useful illustrations
        1. Support leaves and fruits
        2. Conduction of water and sugars throughout plant See cross-section of an herbaceous stemCross section of herbaceous dicot stem
      2. Tissues of stem
        1. Epidermis
          1. Protection
          2. Cuticle to conserve moisture
        2. Cortex
          1. Store food
          2. Photosynthesis (when stem is green)
          3. Some support cells
            - e. pith to store food
        3. Xylem
          1. Conduction of water and minerals
          2. Second function - has strong supporting cells (fibers)
        4. Phloem
          1. Conduction of food
          2. Second function - support

  4. Leaves- organs of photosynthesis See leaf of privet (Ligustrum), x.s.
    1. Relate anatomy of leaf to its primary function of photosynthesis
      1. carbon dioxide + water -------> sugar + oxygen
        2. Cross section of mature leaf
    2. Major tissues of the leaf
      1. Epidermis
        1. Transparent- light goes right through
          (a) Main function - protects against drying out (cuticle)
          (b) Stomata with guard cells
        2. Function- gas exchange, especially common on lower epidermis
      2. Mesophyll
        1. Site of photosynthesis
        2. Air spaces between cells for gas exchange to each cell
      3. Veins
        1. Xylem- water conduction
        2. Phloem- food conduction
        3. Bundle sheath- one or more layers of fiber cells surrounding a vein; strengthens vein to support leaf
        4. Branching extensive in veins- no mesophyll cell is far from a vein
    3. Transpiration- loss of water vapor
      Abscission- leaf fall

Plant physiology - how all the tissues and organs work together

  1. Water and ion transport pathway (water is needed in leaves but available only in soil)
    1. Water and ion uptake occurs at the root hairs and the rest of the root epidermis.
    2. Water and ions move in the cells and the intercellular spaces of the root cortex.
    3. The Casparian bands in the endodermis (the innermost layer of the cortex) function as an impermeable barrier, which allows the endodermis to selectively absorb desirable ions (e.g., K, Ca, PO4 , NO3, Cl) and block undesirable ions (Na, Al).
    4. The water and absorbed ions diffuse into the hollow water-conducting cells (tracheids and/or vessels) in the root xylem.
    5. The water and ions move up in the water-conducting cells in the xylem which form many microscopic channels like straws connected end- to-end that reach into all organs in the plant.
    6. The water and ions move from the xylem into the mesophyll of the leaf.
    7. The water not needed for metabolism or growth evaporates from the surface of small pores called stomates in the leaf epidermis via a process called transpiration.
    8. How water moves up the plant
      1. In essence, water moves via the same mechanism that we use to suck soft drinks up a straw.
      2. In very thin channels like the water-conducting cells, the water molecules are said to have great cohesive force, meaning that they cling very tightly to each other.
      3. The evaporation of water molecules at the surface of the stomates in the leaves generates the sucking force that pulls the adjacent water molecules up to the leaf surface.
      4. Like a lengthy chain extending all the way back to the roots, each water molecule pulls up the molecule below it, and so the whole water column moves up the plant.
      5. What is truly amazing about this process is that it does not involve the input of biological energy. Water moves up the tallest tree simply using the energy from sunlight necessary to evaporate the water molecules at the stomatal surface.
      6. The rate of water movement up the tree must therefore depend on the rate of water evaporation (transpiration ) at the stomates. The plant regulates transpiration by opening and closing its stomates.
  2. Dissolved sugar transport (sugars are made in leaves through photosynthesis, but must be moved to other parts of the plant to power growth and life functions)
    1. Sugar diffuses from the mesophyll in the leaf to the phloem cells in the vascular bundles.
    2. Specialized cells called companion cells load the dissolved sugar into the sugar-conducting cells (called sieve elements) of the phloem by using cellular ATP as an energy source.
    3. Since the high concentration of dissolved sugar dilutes the water in the conducting cells, more water molecules diffuse via osmosis from the intercellular spaces (with high water concentration) around the vascular bundles into the sugar conducting cells (with low water concentration).
    4. This osmotic water flow generates a high hydraulic pressure that moves the dissolved sugar solution through the phloem conducting cells from the leaves to the rest of the plant where the sugar is unloaded by other companion cells.

Links to other sites

Major review of plant anatomy
Excellent Review
Primary Growth: A review with access to good illustrations
Secondary Growth: A review with access to good illustrations
Stem anatomy of a tomato: Lots of detailed illustrations
Leaf anatomy of a tomato: Lots of detailed illustrations

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Last revised: August 11, 1998 - Straney