Autumn.wmf (12088 bytes)Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Vegetative Structure & Function

I.  General Terms

  • herb / shrub /  tree  / vine / forb vs. grass
  • herbaceous / woody
  • annual / biennial / perennial / winter annual / rosette
  • deciduous / evergreen

II.  Plant Cells  (remember the hierarchy of biological organization? cells → tissues → organs)

A. Plant Cells:  Water balloons in a box
    Consider a water balloon inside a cardboard box.  This is a good model for a plant cell. Thus, there are two major components to a plant cell: (1) the box which is analogous to the cell wall, and (2) the water balloon which is analogous to the cytoplasm or protoplast.  The protoplast is everything inside the cell wall. It is the "living" part of the cell and includes the cytoplasm, nucleus, vacuole, and other goodies. 
 Like a cardboard box, the cell wall is relatively rigid, it is non-living and highly structured. Its more obvious functions are to support and protect the cell. It is produced by the protoplast.

    Pores or air spaces (called intercellular spaces) exist between adjacent cells because of the difficulty of packing of cells with rigid walls. Try and squeeze a bunch of irregularly boxes into a room without leaving any space between them!  The intercellular spaces are important for gas exchange and water transport, some movements (i.e., sensitive plants - water moves into/out of theses spaces; nyctinastic movements - sleep movements) and freezing protection (i.e., water moves out of cells into the spaces to minimize cellular damage on freezing.

    Putting a cell in a box presents a problem - how do cells talk to one another since they are now effectively isolated in their own small compartment? Plants solved this problem by putting windows in the box! Or in other words, there are lots of specialized pores through the wall called plasmodesmata that provide a cytoplasmic connection ("cytoplasmic bridges") between adjacent plant cells.  The plasmodesmata are 40-50 nm in diameter.  The maximum sized object that can pass through as a molecular mass of 700-1000 daltons, which is equivalent to a molecule 1.5 - 2.0 nm.  Thus, the cytoplasm of a plant is essentially contiguous throughout the entire plant. Imagine hopping on board a tiny submarine like in the films Fantastic Voyage or Innerspace. You can essentially travel from cell-to-cell throughout the plant without ever leaving the cytoplasm or crossing any cell membranes. Cool!     

B.  Separating the balloon from the box (not on exam)
    Just as you can remove a balloon from a box, so too can a plant physiologist liberate the protoplast from the cell wall. This is accomplished by slicing up a leaf (or other tissue) and then floating it in a solution containing wall-digesting enzymes like cellulase suspended in an inert osmotic agent in a buffered solution. The enzymes degrade the components of the cell wall releasing the protoplasts.

C.  Wall Structure

  1. wall components - All walls contain cellulose, pectins, hemicelluloses, proteins.  Many walls are further strengthened by a 'hardening agent' called lignin.  Some walls are waterproofed with waxy materials such as suberin

  2. primary vs. secondary

  3. formation  - the wall is constructed by the protoplast.  For most of the wall materials, they are built somewhere within the cell (i.e., proteins via ribosomes; hemicellulose and pectins in Golgi) and then packaged in vesicles that fuse with the cell membrane to release their contents into the wall (=exocytosis).  Cellulose is too big to be packaged and transported so the enzymes necessary for making cellulose are moved to the cell membrane where the cellulose is made in situ.  Since the wall is produced by the protoplast, this means that the wall is made from the outside in.  Or in other words, the oldest region of the wall is the outmost area.  And, as the wall is made, the space available for the protoplast decreases.

D.  Cell Types

  • Parenchyma - living, thin-walled, storage and various metabolic functions
  • Collenchyma - living, irregular thickenings of the wall, especially in the corners, often beneath epidermis, support; celery is a good example.
  • Sclerenchyma - often dead, thick wall, primarily for support; fibers - long, thin, often associated with vascular tissue; sclerids - shorter, may be branched

E.  Consequences of having a cell wall

  1. How can walls be strong and provide support, yet allow for cell growth/elongation?  Actually, very few cells have the ability to elongate and grow because their thick cell wall prevents growth.  However, some cells have thinner walls that are capable of growth.  But, even these walls won't allow for elongation unless the wall is first temporarily loosened.  Wall loosening occurs when a transmembrane protein in the cell membrane pumps protons from the cytosol into the wall.  The increased protons acidifies the wall which: (1) temporarily loosens weak bonds, such as hydrogen bonds, between wall components: and (2) activates enzymes (extensin) that cleave covalent bonds between wall components.  The end result is that the wall can now elongate and then locks back into place.  The pump is driven by ATP.
  2. Growth is restricted to meristems - these are the only cells with walls capable of being loosened.
  3. Morphogenesis in plants occurs by cell addition rather than by cell migration as in animals
  4. Plants respond to their environment by development rather than behavior
  5. Growth in plants occurs by the  accumulation of similar units.

III. Plant Tissues

A.  Why do plant cells stick together?
    Returning to our balloon in box model, i
f you stack up a bunch of boxes you generally can’t make a very large tower before it comes crashing down. Since a plant is essentially constructed of numerous small boxes, why don’t they fall apart, too? The answer is glue! Plants glue their cells together with pectic polysaccharides (pectins). These carbohydrates, which make up the outermost layer of the plant cell wall, which is called the middle lamella, bind adjacent cells together. Cooks use pectins extracted from the middle lamella to solidify jams and jellies.

B.  Types

  • vascular (i.e., xylem, phloem) - plumbing
  • dermal (i.e., epidermis) - covering
  • ground (i.e., pith, cortex) - stuffing

IV. Major Vegetative Organs

There is often no clear anatomical distinction between leaf and stem. The crown is the junction of the root and stem. A shoot is a stem with leaves.

V. Roots

A.  Structure

  • tap – one main root, carrot
  • fibrous – many roots, no one root is dominant, like grasses
  • adventitious – a root that develops from a part of the plant other than another root, like the prop roots of maize, or the tendrils of ivy
  • root hairs � tiny outgrowths of the epidermis designed to absorb water and minerals

B.  Functions

  • support
  • anchoring plant
  • absorption of water and nutrients

C.  Roots as food

  • Some select root crops (cassava or manihot, parsnip, carrot, rutabaga, turnip)
  • The importance of biennials � spend first year accumulating nutrients in roots to prepare for flowering the following season
  • Roots or stems? � A few crops, such as radish and beet, grow underground and appear to be a taproot.  These crops actually develop from the hypocotyl, part of the embryonic stem.  Thus, even though they look and act like taproots, they are probably best considered stems

D.  Mycorrhizae - fungal interactions with plant roots

VI. Stems
     Botanically, a stem is the structure to which the leaves and roots are attached.  The stem supports the plant and is characterized by having buds and leaves.

A. Structures

  • node – region to which a leaf is attached
  • internode – region between nodes
  • bud – found at base of leaf, immature shoot system
  • axillary (or lateral) bud – bud at base of leaf, along stem
  • terminal bud – bud at end of stem
  • bud scales – protective covering over bud, modified leaves
  • bud scale scar – scar left on stem where the terminal bud scales fell off
  • leaf scar – scar left on stem where leaf detached
  • lenticel – areas on stem for gas exchange
  • vascular bundle scar – in leaf scar, where vascular bundle went into leaf

B. Specialized Types

  • rhizome (ginger)
  • stolon (strawberry)
  • bulb (onion)
  • corm (gladiolus)
  • tuber (potato)

C.  Stems As Food
      We typically think of stems as growing above the ground, but as we discussed earlier, some specialized types of stems (i.e., tubers, rhizome, corm, bulb) grow underground.

  • Aerial stems - asparagus, bamboo, kohlrabi
  • Tubers � white potato, yams
  • Rhizomes � Jerusalem artichoke (sunflower), arrowroot (starch), ginger
  • Corm � water chestnuts, taro
  • Bulbs � onion, garlic, leek

VII.  Stem Anatomy

A.  Herbaceous Stems - pith, vascular bundles, xylem, phloem, vascular cambium

B.  Woody Stems

C.  Hardwood vs. softwood (not on exam)
    These terms refer to the species of tree from which the wood is obtained. Hardwoods are angiosperm (flowering) trees like the oaks, birches, maples, and basswood. Softwoods refer to conifers (gymnosperms) like pines, firs, and spruces. These terms are also roughly equivalent to the degree of "hardness" of the wood. In general, the hardwoods have harder wood than softwoods. However, there are hardwoods with "soft" wood such as basswood and the poplars, and similarly there are softwoods with relatively "hard" wood like southern yellow pine. The following table compares the two:

Comparison of hardwood and softwood
Feature Softwood Hardwood
species Conifers (gymnosperms) Flowering trees (angiosperms)
cell types Tracheids only Vessels & tracheids
texture Homogenous Heterogeneous
hardness "soft" easily split "hard"
uses Building, paper Furniture, fuels

D. Wood Appearance (not on exam)

1. Cuts � the "Jelly Roll" or Onion Model

  • Transverse (Cross) Section � across the stem, perpendicular to the long axis; annual growth rings appear as concentric circles
  • Radial Section � parallel to the long axis, through the center; grain pattern a series of parallel lines
  • Tangential Section � parallel to the long axis, anywhere but through the center; grain pattern wavy and variable, not all parallel

2. Boards

  • Quarter-sawn � radial cuts � note grain pattern (linear with perpendicular rays)
  • Plain-sawn � tangential cuts � not grain pattern ("wavy")

3.  Grain

  • due to annual rings and cell structure
  • runs in the direction of the tree
  • coarse grained wood usually with conspicuous annual rings, large pores

E. Lessons from Wood  (not on exam)

1. Forensics
    Wood structure has helped to solve several crimes including the conviction of Bruno Hauptmann for the abduction and murder of Charles Lindbergh�s infant son. By carefully studying the anatomical structure of the wood in the ladder left at the scene of the crime, technologist Arthur Koehler, was able to determine that:

  • The ladder was constructed from scraps of ponderosa pine, Douglas fir and birch
  • Part of the ladder had been constructed from wood removed from a joist in Hauptmann�s attic
  • The edge of one of the rails had been smoothed by a plane found in Hauptmann�s toolbox.

2. Past History � Dendrochronology
    Trees rings provide a window on the past. Tree rings can be used to:

  • Date archaeological artifacts by comparing tree rings in wood samples taken from the site with rings of known age.
  • Monitor climate trends can be monitored since the ring width in many species is sensitive to growth conditions (i.e., temperature, moisture, carbon dioxide concentration).
  • Determine past geological events like volcanic activity, sunspots, fire
  • Determine past forest types (i.e., alder wood wheels on ancient carts were much larger than those of trees today, canoe from bog 50 foot log � none that big today)

VIII. Leaves

A . Parts

  • blade – main photosynthetic part
  • petiole – fancy term for the leaf stalk
  • stipules – appendage are base of petiole in some leaves. Stipules can be glandular, leafy, spiny, or scale-like. In many cases, the stipules fall off shortly after the leaf expands. Many plants completely lack stipules.

B. Leaf structure:  The leaf blade may be all one section or broken up into smaller sections (called leaflets)

C.  Function:  Photosynthesis (= food production).  Leaves are well-adapted for photosynthesis:

  • broad - light & carbon dioxide absorption
  • flat (thin) - light penetration, carbon dioxide diffusion
  • pores (stomata) - carbon dioxide uptake, minimize water loss
  • cuticle - prevent water loss
  • veins (xylem/phloem) - water/nutrient transport, help support thin leaves

D. Specialized Leaves – leaves may be modified in a variety of different ways. For example, the leaves of carnivorous plants are modified into traps. Tendrils are common in vines and used for support. Tendrils can actually be derived from modified leaves or stems

ELeaves as food  - Crops can be derived from the leaf blade ("greens") or petiole

  • Greens (leaf blade) - spinach, lettuce, parsley, endive, kale, beet greens, swiss chard, dandelion, amaranth, etc.

  • Leaf Stalk (petiole) - celery, rhubarb

  • Onion group - onion, garlic, leek. The tubular leaves of chives and other onions are often eaten.  Also, a bulb, which we classified as a stem, is actually made up of the fleshy bases of the onion leaves attached to a small stem.  Thus, even though onions are described as modified stems, the leaf base is the major part that is eaten.

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Last updated: January 28, 2009        � Copyright by SG Saupe