Plant Morphology
I. PLANT CELLS AND TISSUES
A. How Plant Cells Grow:
1. The actual increase in root and stem size is due mainly to cell
elongation (growth).
2. Cell growth = Irreversible increase in cell size. Mostly due to
uptake of water by the vacuole.
3. The cell wall usually extends in only one direction which is
determined by microtubules of the cytoskelleton beneath the
plasma membrane. Also the orientation of cellulose fibrils
in the cell wall help determine the direction in which a cell
can grow.
4. When cells stop growing additional cellulose and other cell wall
related materials are added to the interior of the primary cell
wall which lends to additional rigidity.
5. Rapid elongation of shoots increases their exposure to light
while elongation of roots increases their exposure to soil
nutrients and water.
B. Types of Plant Cells
1. Parenchyma Cells:
Parenchyma cells are relatively unspecialized.
They have primary cell walls that are thin and flexible.
They lack secondary walls.
The protoplast usually has a large central vacuole.
Functions in synthesizing and storing organic products.
Some parenchyma cells have plastids.
Most mature parenchyma cells do not divide, but retain
the ability to divide and differentiate into other cell
types under special conditions.
2. Collenchyma Cells
Collenchyma cells have functional protoplasm and usually lack
secondary walls. However, their primary cell walls are
unevenly thickened in the corners of the cell.
Collenchyma cells are usually grouped in strands or cylinders
in young stems for support but do not restrain growth.
3. Sclerenchyma Cells
Sclerenchyma cells function in support. They have very rigid,
thick secondary walls strengthened by lignin.
Sclerechyma cells usually lack protoplasm at maturity and thus
are considered dead at maturity.
At maturity, sclerechyma cells cannot elongate and function
only in support.
There are two forms of sclerechyma cells
(1) fibers: long, slender, tapered cells occurring in bundles.
Commonly found in flax and other fibrous plant species.
(2) sclerids: shorter, irregularly-shaped cells. Commonly
found in hard seed coats, and as stone cells in fruits
like pears.
4. Water-Conducting Cells: XYLEM
Xylem consists of two cell types, both with secondary walls and
both dead at functional maturity.
(1) Tracheids are long, thing tapered cells having lignin-hardened
secondary walls with pits (thinner regions where only
primary walls are present).
Water flows from cell to cell through pits.
Also function in support.
(2) Vessel elements are wider, shorter, thinner-walled,
and less tapered. Their end walls are perforated for free
flow of water through long chains of vessel elements called
xylem vessels. Evolved from tracheids.
5. Metabolite-Conducting Cells: PHLOEM
Sieve-Tube Members: transport sucrose, other organic compounds,
and some minerals. Chains of cells are collectively called
phloem. Protoplasts lack a nucleus, ribosomes and a distinct
vacuole. Are alive at functional maturity. The porous end
walls between cells are called sieve plates. Some cells have
long strands of P-protein (function is unknown). Cells have
the polysaccharide callose which is involved in formation of
the sieve plates.
Companion Cell: connected to each sieve-tube member by many
plasmodesmata. Since the sieve-Tube Members do not have
nuclei at maturity, the companion's nucleus and ribosomes
may also serve the Sieve-Tube Members.
NOTE: As you learn these cells, it is useful to understand
that cell structure often reveals cell function, and
vice versa.
C. Plant Tissues:
Simple tissues consist of a single cell type with a common function.
Complex tissue consists of more than one cell type. For example:
Xylem has schlenchyma cells (fibers), parenchyma cells, tracheids,
vessel elements, and companion cells.
D. Tissue Systems: plants have three tissue systems, each one is
continuous throughout the plant body.
1. Dermal Tissue System
Dermal tissue system or epidermis = Single layer of tightly
packed cells covering the young plant body.
Functions in protection and has special characteristics consistent
with the function of the organ it covers.
For example, root hairs specialized for water and mineral
absorption are extensions of epidermal cells near root tips.
2. Vascular Tissue System
Vascular tissue system The xylem and phloem that functions
in transport and support.
3. Ground Tissue System
Ground tissue system Predominantly parenchyma that fills the
space between dermal and vascular tissue systems.
Has diverse functions including photosynthesis, storage
and support.
II. PRIMARY GROWTH
A. Indeterminate Growth
1. Indeterminate growth: plants grow as long as they live.
Most plants show indeterminate growth. Most animals cease
growing after reaching a certain size (determinate growth).
2. Some plant organs (flowers, seeds) show determinate growth.
3. Indeterminate growth is made possible by meristems (tissue that
remains embryonic). Meristematic cells are unspecialized
and divide to generate new cells near the growing point.
a. Apical meristems, located in root tips and shoot buds,
supply cells for plants to grow in length.
b. Primary growth is initiated by apical meristems and forms
primary tissues organized into the 3 tissue systems.
4. Some plants have finite life spans. Some are genetically
determined. Some are environmentally determined.
a. Annuals complete their life cycles in one year or
growing season.
b. Biennials life spans typically two years.
c. Perennials, such as trees and some grasses, live many years.
B. Primary Growth of Roots:
1. Root growth is concentrated near its tip. The root tip is covered
by a root cap, which protects the meristem and secretes a
polysaccharidc slime that lubricates the soil ahead of the
growing root.
2. The root tip contains 3 zones of cells:
a. The zone of cell division is located near the tip
of the root. Here, apical meristem produces cells for
primary growth and replaces the abraded root cap cells.
Mitosis is concentrated here.
b. The zone of cell elongation is where cells grow and elongate
to push the root tip, including the meristem ahead.
c. The zone of cell differcntiation, furthest from is where cells become
functionally mature and root hairs extend from epidermal cells.
3. The apical meristem produces three primary meristems, which give
rise to the three primary tissues:
a. Protoderm: outermost primary meristem. Gives rise to the epidermis.
b. Procambium: forms the stele (central cylinder)where xylem and
phloem develop. In dicots, xylem radiates from the center
of the stele in 2 or more spokes. The phloem located
in between the spokes. In monocots, the vascular tissue
forms a ring around the pith (central core of parenchyma cells).
c. Ground Meristem: is between the protodcrm and procambium, and
gives rise to the ground tissue system. The ground tissue
is mostly parenchyma. Important in food storage. The
cell membranes are active in mineral uptake. Fills the
cortex (root area between the stele and epidermis). The
ground meristem contains the endodermis, the single-cell
thick innermost layer of the cortex that forms the boundary
between the cortex and the stele. [It selectively regulates
passage of substances from soil to the vascular tissue of
the stele.]
4. Lateral roots may sprout from the outermost layer of the stele of
a root. The pericycle, just inside the endodermis, is a layer of
cells that may become meristcmatic and divide to form the
lateral root. The lateral root maintains its vascular connection
to the stele of the main root.
C. Primary Growth of Shoots:
1. Shoot apical meristem is a dome-shaped mass of dividing cells at
the tip of the terminal bud. Forms the primary meristems that
differentiate into the 3 tissue systems.
On the flanks of the apical dome are leaf primordia
which form leaves.
The at the base of the leaf primordia are meristematic cells
that develop into axillary buds.
In dicots growth is concentrated near the shoot tip. In monocots
growth occurs at each node along the stem.
2. The vascular tissue of the stem is organized into strands of
vascular bundles. The vascular systems of the roots and shoots
converge at the transition zone (shoot-root).
3. In dicots, vacular bundles are arranged in a ring with pith
inside and cortex outside. Xylem faces the pith, phloem faces
the cortex. Pith and cortex are connected by pith rays.
4. In monocots, vascular bundles are scattered throughout the ground
tissue.
5. Ground tissue of stems is mostly parenchyma, strengthened in many
plants by collencyma located beneath the epidermis.
6. Leaves are covered by the epidermis. The epidermis protects
against physical damage and pathogens. The waxy cuticle prevents
water loss but cuts down of gas exchange. Stomata are pores
flanked by guard cells which regulate gas exchange. Stomata
also allow transpiration (water loss from plant by evaporation).
7. The ground tissue of a leaf is mesophyll. Consists mainly of
parenchyma with chloroplasts for photosynthesis. Dicots usually
have two distinct layers in their leaves:
a. Palisade mesophyll: columnar cells of the upper half of leaf.
Specialized for photosynthesis.
b. Spongy mesophyll irregularly-shaped cells surrounded by air
spaces through which oxygen and CO2 circulate. In lower
half of leaf.
8. The leaf vascular tissue is continuous with that of the stem by a
leaf trace. Continues in the petiole as a vein, which branches
repeatedly throughout the mcsophyll of the leaf blade.
Functions also as a skeleton to support the shape of the leaf.
III. SECONDARY GROWTH
A. Secondary Growth of Stems
1. Secondary growth = Increase in girth produced by two
lateral mcristems: vascular cambium and cork cambium.
2. Lateral meristems = Cylinders of dividing cells running the
length of the plant except where primary growth is occurring.
3. Tissues formed by secondary growth arc called secondary tissues.
Most monocots produce only primary tissues.
4. Vascular cambium is f ormcd when meristematic parenchyma between
the xylem and phloem of each vascular bundle extends laterally
into the ground tissue separating the bundles. During secondary
growth, vascular cambium forms new xylem internal to itself and
new phloem external to itself. The accumulated layers of
secondary xylem become wood.
5. Wood = Thick, lignified, dead tracheid, vessel element and fiber
cells. Has rays of parenchyma cells radiating through secondary
xylem to function in storage and lateral transport of water and
solutes. Forms annual growth rings due to yearly activity:
cambium dormancy, spring wood production and summer wood production.
6. The secondary phloem does not accumulate extensively: it develops
into bark which eventually sloughs off the trce trunk.
7. Bark = All tissues external to the vascular cambium. Lenticcls in
bark allow for gas exchange.
8. Cork cambium = Cylinder of meristematic tissue that forms
protective layers of the secondary plant body. Forms in the
outer cortex first. As continued secondary growth splits this,
it is replaced by new cork cambium formed deeper in the cortex.
When no cortex is left, it develops from parenchyma cells in the
secondary phloem (only the youngest secondary phloem, internal
to cork cambium, functions in sugar transport).
9. Periderm = The protective coat that replaces epidermis. It is
made of cork cambium, cork and phelloderm. Cork is produced by
cork cambium (to its outside). The cells are impregnated with
suberin, a waxy material, and are dead at functional maturity.
Phelloderm is a parenchyma tissue produced by cork cambium
(to its inside).
10. Wood has two zones:
1. Heartwood, the older layers of secondary xylem blocked with
resins to become nonfunctional for water transport.
2. Sapwood, the younger secondary xylem, vascular cambium,
young secondary phloem, and cork cambium. Conducts water and
nutrients.
B. Secondary Growth of Roots
1. Vascular cambium and cork cambium also function in secondary
growth of roots.
2. The vascular cambium produces secondary xylem to its inside and
secondary phloem to its outside. First located between xylem
and phloem of the stele. As stele grows, cortex and epidermis
split.
3. The cork cambium forms from the pericycle of the stele and
produces the periderm, which becomes secondary dermal tissue.
4. Roots also become woody.
IV. THE PARTS OF A FLOWERING PLANT
During its adaptive evolution on land, the plant body became
differentiated into a subterranean root system and an aerial shoot
system, consisting of stems, leaves and flowers.
Roots depend on shoots for organic nutrients.
Shoots depend on roots for minerals, water and for support.
Xylem conveys water from roots to shoots.
Phloem conveys food f rom shoots to roots, and from storage roots to
actively growing shoots.
A. The Root System
Roots anchor plants, absorb and conduct water and mincrals and
store food. There are two major types of root systems:
1. Tap root System
One large, deep, vertical root (the taproot) produces many
smaller secondary roots. Mostly seen in young dicots.
Provides firm anchorage. Some are modified to store a large
amount of food (for example, carrots and turnips).
2. Fibrous Root System
Mat of thread-like roots spread out below the soil surface.
Provides expansive exposure to soil water and minerals.
3. Roots are concentrated in the upper few centimeters of soil,
preventing soil erosion. Mostly seen in monocots.
a. Absorption of water is greatly increased by root
hairs, which increase the surface area of the root.
b. Adventitious roots = Roots arising from stems or leaves.
Some, such as prop roots of corn, help support the plant stem.
B. Shoot System
1. Shoot system = Stems and leaves (flowers consist of modified leaves).
2. Nodes = The points at which leaves are attached to stems.
3. Internodes = Stem segments between the nodes.
4. Apical dominance = The inhibition of axillary bud growth by the
terminal bud.
5. Some can develop into flower-bearing shoots or vegetative branches.
Sometimes possible to stimulate growth by removing terminal bud.
Terminal bud = The bud on a shoot tip, usually responsible for
most growth in length.
6. Stolons = Horizontal stems growing along the ground (strawberry).
7. Rhizomes. = Horizontal stems growing underground (irises).
Some end in enlarged tubers where food is stored (potatoes).
8. Bulbs = Vertical, underground shoots with leaves modified for food
storage.
9. Leaves = Main photosynthetic organs of a plant. Petioles join the
leaf to the node of a stem. [Most monocots lack petioles;
instead, the leaf base forms a sheath enclosing the stem.]
a. Monocot leaves have parallel major veins running the length
of the blade.
b. Dicot leaves have a multi-branched network of major veins.
Can be palmate or pinnate.
c. All leaves have numerous minor cross-vcins.
10. Classification of leaf arrangement on stem:
a. Opposite = 2 leaves at each node 180o apart.
b. Alternate = Each node has one leaf and leaves at adjacent
nodes point in opposite directions.
c. Whorled = A node has 3 or more leaves attached.
d. Simple leaf = One undivided blade.
e. Compound leaf = Divided into several leaflets.
11. Classification by leaf shape:
lanceolate, oval, cordate (heart-shaped) or triangular.
12. Classification by leaf margin:
entire (smooth), undulate, serrate or lobed.
13. Modifications of leaves:
a. Tendrils are modified leaflets that cling to supports.
b. Spines of cacti function in protection.
c. Many succulents have leaves modified for storing water.
d. Some plants have brightly colored leaves that help attract
pollinators to the flower.