The Roots
INTRODUCTION
• Annual plants complete their life cycle - from germination
through flowering and seed production to death - in a single
year or less.
Many wildflowers and important food crops, such as cereals
and legumes, are annuals.
• The life of a biennial plant spans two years.
Often, there is an intervening cold period between the
vegetative growth season and the flowering season.
• Plants that live many years, including trees, shrubs, and
some grasses, are perennials.
These often die not from old age, but from an infection
or some environmental trauma.
• A plant is capable of indeterminate growth because it
has perpetually embryonic tissues called meristems in
its regions of growth.
• Meristem cells divide to generate additional cells, some of
which remain in the meristematic region while others
become specialized and incorporated into the tissues and
organs of the growing plant.
• The cells that remain meristematic are called initials.
• Those that are displaced from the meristem are called
derivatives. They continue to divide for some time until the
cells they produce begin to specialize within developing
tissues.
• The pattern of plant growth depends on the location of
meristems.
Apical meristems, located at the tips
of roots and in the buds of shoots,
supply cells for the plant to grow in
length.
This elongation, primary growth,
enables roots to ramify through
the soil and shoots to extend their
exposure to light and carbon
dioxide.
ROOTS
• ROOTS “the hidden half”
• Functions of roots:
• Anchorage
• Absorption of water & dissolved
minerals
• Storage (surplus sugars, starch)
• Conduction water/nutrients
Root systems
Tap Root
Calibri
The Root Internal Structure
Epidermis
Cortex
Pericycle
Endodermis
Phloem
Xylem
The Root Internal Structure
Root Epidermis
• Outermost, single layer of
cells that:
• Protects (from diseases)
• Absorbs water and nutrients
• ROOT HAIRS: tubular
extensions of epidermal
cells.
• Increase surface area of
root, for better
water/nutrient
absorption
Root Hairs: water and mineral
absorption
Root hairs
increase surface
area for better
absorption
Root Cortex
• In roots, the cortex ( storage parenchyma) provides support,
and often stores sugars and starch (for example: yams,
sweet potato, etc.)
Epidermis
Cortex
Pericycle
Endodermis
Phloem
Xylem
Cortex
Endodermis
• Endodermis: the innermost layer of the
cortex
Casparian strip
• The Casparian strip is a water-impermeable strip of
waxy material found in the endodermis (innermost
layer of the cortex).
• The Casparian strip helps to control the uptake of
minerals into the xylem: they have to go through the
cytoplasm of the cell!
Carrot (Daucus carotta) Manioc (Manihot
esculenta)
Sugar Beet (Beta vulgaris)
Some
Modified
Roots
Sweet Potato (Ipomoea batatas)
The Stem
STEMS
• Above-ground organs (usually)
• Support leaves and fruits
• Conduct water and sugars
throughout plant (xylem and phloem)
The Stem Internal Structure
Dicots
Epidermis
PITH
Monocots
Epidermis
Cortex
Vascular Bundles
Xylem
Phloem
Cortex
Vascular Cambium
Vascular Bundle
Xylem
Phloem
Typical Dicot Stem
Typical Dicot
Stem
Stem Vascular tissue
• Vascular bundles – composed of both xylem and
phloem
• Xylem
• Conducts water
• Support
• Phloem
• Conducts food
• Support
Vascular
cambium
Vascular cambium
• Occurs in woody stems
• Vascular cambium located in the middle of the
vascular bundle, between xylem and phloem
Typical Monocot Stem
Typical Monocot Stem
Details Vascular Bundles
Modified Stems
Tubers
Bulbs
Rhizomes
Primary and
Secondary Growth
Giant Baobab (Adansonia digitata)
Namibia (internet source)
Types of stems
• Herbaceous
vs.
Woody stems
Primary growth is the growth in length performed by
addition of cells by the apical meristems.
Secondary growth is the growth in width or thickness.
This is the product of the cell division of the lateral
meristems. The lateral meristems are cylinders of
dividing cells extending along the length of roots and
shoots and it results in the progressive thickening of roots
and shoots. The vascular cambium is an example of a
lateral meristem.
Woody plants show secondary growth. In addition, the
epidermis is replaced with bark.
• In woody plants, primary growth is restricted to the
youngest parts of the plant - the tips of the roots and
shoots.
• The lateral meristems develop in slightly older regions
of the roots and shoots. Secondary growth adds girth
to the organs.
• Each growing season, primary growth produces young
extensions of roots and shoots, while secondary
growth thickens and strengthens the older part of the
plant.
Apical Meristem Revisited Origin and
Development of Stems
• Apical meristem at stem tip
• Contributes to increase in
stem length
• Dormant before growing
season begins
• Protected by bud scales
and by leaf primordia
• Leaf primordial: Tiny
embryonic leaves that
develop into mature
leaves
Longitudinal section
through stem tip
Origin and Development of Stems
• Apical meristem cells form 3
primary meristems.
• Protoderm - Gives rise to
epidermis
• Procambium - Produces
primary xylem and phloem
• Ground Meristem Produces pith and cortex,
both composed of
parenchyma cells
Longitudinal section
through stem tip
Vascular Bundles of Stems Revisited
• Leaf primordia and bud
primordia develop into
mature leaves and buds.
• Traces branch off
from cylinder of
xylem and phloem,
and enter leaf or
bud.
• Trace = strand of
xylem and phloem
• Each trace leaves a
gap filled with
parenchyma in the
cylinder of vascular
tissue, forming leaf
gap or bud gap
Tissue Patterns in Stems Revisited
Herbaceous Dicotyledonous Stems
• Herbaceous dicots have discrete vascular bundles
arranged in a cylinder.
• Vascular cambium arises between primary xylem and
primary phloem.
• Adds secondary xylem and secondary phloem
Tissue Patterns in Stems
Woody Dicotyledonous Stems
• Wood = Secondary xylem
• Differences in wood:
• Vascular cambium and cork cambium active all year:
• Ungrained, uniform wood produced
• Some tropical trees
• If wood produced seasonally:
• In spring: Relatively large vessel elements of secondary xylem
produced - Spring wood.
• After spring wood: Fewer, smaller vessel elements in
proportion to tracheids and fibers - Summer wood.
• In conifers, vessels and fibers absent
• Tracheids in spring larger than later in season
Development of early wood in dicots
Epidermis
Cortex
Cortex
Vascular Cambium
Vascular Bundle
Xylem
Phloem
Vascular Cambium
Primary phloem
Primary xylem
Secondary phloem
(phloem ring)
Secondaryxylem
(xylem ring)
Cortex
Secondary xylem
(second ring)
Secondary xylem
(first ring)
Primary xylem
Primary phloem
Secondary phloem
(first ring)
Secondary phloem
(second ring)
Vascular Cambium
Origin and Development of Stems
• Narrow band of cells between the primary xylem and
primary phloem may become vascular cambium.
• Cells produced by the vascular cambium become
components of secondary xylem toward center and
secondary phloem toward surface.
Vascular tissue forms rings in trees
• Annual rings: xylem formed by the vascular cambium during
one growing season. One ring = one year
Tissue Patterns in Stems
Woody Dicotyledonous Stems
• One year’s growth of xylem = annual ring
• Vascular cambium produces more secondary xylem than
phloem.
• Bulk of tree trunk consists of annual rings of wood.
• Indicates age of a tree
• Indicates climate during tree’s lifetime
• Vascular Rays consist of parenchyma cells that function
in lateral conduction of nutrients and water.
• Xylem Ray - Part of ray within xylem
• Phloem Ray - Part of ray through phloem
Tissue Patterns in Stems
Woody Dicotyledonous Stems
3-D view of
dicot wood
The Bark
• In many plants cork cambium (phellogen) produces cork
cells with suberin and phelloderm cells.
• Function to reduce water loss and to protect stem against
injury
• Lenticels - Parenchyma cells in cork for exchange of gases
Tissue Patterns in Stems
Woody Dicotyledonous Stems
Cross section of
young stem with
secondary
growth
Tissue Patterns in Stems
Woody Dicotyledonous Stems
• Tyloses - Protrusions of adjacent parenchyma cells into
conducting cells of xylem
• Prevent conduction of
water
• Resins, gums, and
tannins accumulate, and
darken wood, forming
heartwood.
–
Heartwood - Older, darker wood in
center
–
Sapwood - Lighter, still-functioning
xylem closest to cambium
Tissue Patterns in Stems
Woody Dicotyledonous Stems
• Softwood - Wood of conifers
• No fibers or vessel elements
• Hardwood - Wood of dicot trees
• Resin canals - Tubelike canals scattered throughout
xylem and other tissues
• Lined with specialized
cells that secrete resin
• Common in conifers
• In some tropical
flowering plants
–
Frankincense and myrrh
Resin
canals
in pine
Tissue Patterns in Stems
Woody Dicotyledonous Stems
• Bark - Tissues outside the vascular cambium, including
secondary phloem
• Mature bark may consist of alternating layers of crushed phloem
and cork.
Cross
section of
young stem
with
secondary
growth

Laticifers - Ducts found mostly in phloem that
have latex-secreting cells
•
Rubber, chicle (chewing gum), morphine
History of the tree: annual rings
Dendrochronology : tree time-keeping
1492: Columbus lands in
the Americas
1917 & 1945: Tree
Survives two World
Wars
1776: Declaration
1969: Man
of US independence
lands on Moon
1620: Pilgrims land
in Plymouth, Mass.
1489: Tree is planted
by Native American
1861: Start of
Civil War
1971: Birth Year
of the person
who cut down
this tree!
Tissue Patterns in Stems
Monocotyledonous Stems
• Monocots stems have neither a vascular cambium nor a
cork cambium.
• Produce no secondary vascular tissues or cork
• Primary xylem and phloem in discrete vascular bundles
scattered throughout the stem
–
Vascular bundles oriented
with xylem closer to center
of stem and phloem closer
to surface.
–
Parenchyma (ground tissue)
surrounds vascular bundles.
Cross
section
of
monocot
stem
Tissue Patterns in Stems
Monocotyledonous Stems
• In a typical monocot vascular bundle:
• Two large vessels with several small vessels
• First formed xylem cells
stretch and collapse.
–
Leave irregularly shaped air
space
• Phloem consists of
sieve tubes and
companion cells.
• Vascular bundle
surrounded by sheath
of sclerenchyma cells.
Monocot vascular bundle
Wood and Its Uses
Types
of Sawing
• Radially
cut (quartersawed)
boards show annual rings in
side view.
• Tangentially cut (plain-sawed
or slab cut) boards are cut
perpendicular to rays.
• Show annual rings as irregular
bands of light and dark streaks
Wood and Its Uses
• Knots - Bases of lost branches covered by new annual
rings produced by the cambium
• Wood Products
• About half of U.S. and Canadian wood production is
used as lumber, primarily for construction.
• Sawdust and waste used for particle board and pulp.
• Veneer - Thin sheet of desirable wood glued to cheaper
lumber
• Pulp is second most widespread use of wood.
• Paper, synthetic fibers, plastics, linoleum
• In developing countries, about half of cut timber is used
for fuel.
• Less than 10% in US and Canada.
Specialized Stems
• Rhizomes - Horizontal stems that
grow below-ground and have long
to short internodes
• Irises, some grasses, ferns

Runners - Horizontal stems
that grow above ground
and have long internodes.
•

Strawberry
Stolons - Produced beneath the
surface of the ground and tend
to grow in different directions.
•
Potato
Specialized Stems
• Tubers - Swollen, fleshy, underground
stem
• Store food
• Potatoes - Eyes of potato are nodes
• Bulbs - Large buds surrounded by
numerous fleshy leaves, with a small
stem at lower end
• Store food
• Onions, lilies, hyacinths, tulips
Specialized Stems
• Corms - Resemble bulbs, but
composed almost entirely of
stem tissue, with papery leaves
• Store food
• Crocus and gladiolus

Cladophylls Flattened, leaf-life
stems
• Greenbriars, some
orchids, prickly
pear cactus
Prickly pear cactus
Wood and Its Uses
• In a living tree, 50% of the wood weight comes from
water content.
• Dry part of wood composed of about 60-75% cellulose and
about 15-25% lignin.
• Density - Weight per unit volume
• Durability - Ability to withstand decay
• Tannins and oils repel decay organisms.