Autumn.wmf (12088 bytes) Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Vegetative Structure & Function

I.  Life Span/General Terms

  • herb / shrub /  tree  / vine
  • herbaceous / woody
  • annual / biennial / perennial
  • 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.  The box is analogous to the cell wall.  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 (see below).

    If 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. These carbohydrates, which make up the outermost layer of the plant cell wall (called the middle lamella), bind adjacent cells together. Cooks use pectins extracted from the middle lamella to solidify jams and jellies.

    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. Trivia note: prized ginseng roots have a translucent appearance - apparently obtained by freezing).

    Putting a cell in a box presents one major 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 nor crossing any cell membranes. Cool!     

    The water balloon is analogous to the 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.

B.  Separating the balloon from the box
    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 pectinase, cellulase, and hemicellulase suspended in an inert osmotic agent (0.7 M mannitol or sorbitol) in a buffered solution. The enzymes degrade the components of the cell wall releasing the protoplasts. This will be the focus of at least one or our lab sessions.

C.  Wall Structure

  1. wall components - cellulose, lignin, suberin

  2. primary vs. secondary

  3. formation

D.  Plastids & Vacuoles


III.  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
  • Sclerenchyma - often dead, thick wall, primarily for support; fibers - long, thin, often associated with vascular tissue; sclerids - shorter, may be branched

IV. Plant Tissues

  • vascular (i.e., xylem, phloem)
  • dermal (i.e., epidermis)
  • ground (i.e., pith, cortex)

V. Major Vegetative Organs

  • roots – anchor, store nutrients, underground, positively gravitropic
  • leaves – photosynthesis, attached to stem, has a bud at its base
  • stems – supports leaves, flowers and fruits; usually negatively gravitropic, with buds

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.

VI. 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

VII. 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

VIII.  Stem Anatomy

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

B.  Woody Stems

C.  Hardwood vs. softwood
    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

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)

IX. 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

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.

X  Monocots vs. Eudicots: A Quick Comparison


Table 1: Comparison of Monocots and Dicots




Number of species 165,000 50,000
Growth form woody or herbaceous mostly herbaceous
Embryo cotyledons two cotyledon one (best character to distinguish)
Endosperm present or absent often present
Floral parts 4 or 5-merous 3-merous
Leaves net (reticulate) veined  parallel veined 
Petiole common, seldom sheaths stem not common, petiole often sheathing
Vascular system definite # bundles, ring (like broccoli) numerous, scattered (like asparagus)
Mature root system primary or adventitious, strong taproot often present wholly adventitious (primary root system of short duration, fibrous roots common, usually without strong taproot)

Good Web Sites:

| Top| SGS Home | CSB/SJU Home | Biology Dept | Biol116  Section Home Page | Concepts Home Page | Disclaimer |

Last updated: January 29, 2004        � Copyright by SG Saupe