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
-
wall components -
cellulose, lignin, suberin
-
primary vs. secondary
-
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
- Vascular cambium � meristematic tissue that produces the xylem and phloem.
Occurs between the bark and wood, lateral meristem (vs. apical meristem),
results in growth in width of trees
- Wood � xylem
- Inner bark � phloem
- Outer bark (or periderm) � comprised of three layers: cork (phellum), a
cork cambium (phellogen) and phelloderm (inner layer)
- Sapwood � lighter color, functional xylem
- Heartwood � darker colored wood near center of stem, non-functional xylem,
clogged with tannins, resins and other materials
- Rays � for lateral (inside/outside) movement of water and other materials
- Spring (or early) wood � larger xylem cells resulting from better growing
conditions early in the growing season when moisture is generally plentiful
and temperatures are ideal
- Summer (or late) wood � smaller xylem cells produced latter in the growing
season
- Annual rings � caused by the transition from the summer wood of one
growing season to the spring wood of the next.
- Ring-porous - pores in early wood larger than in late wood and appear in a
ring. Common in oak, elm and ash
- Diffuse-porous - spring wood pores the same size as those produced laters.
Common in buckeye, basswood, maple, cherry
- tylose - outgrowths that plug up the pores of white oak and other species
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
E.
Leaves 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 |
|
Feature |
Eudicot |
Monocot |
| 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) |
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Last updated: January 29, 2004
� Copyright by SG Saupe
