Stems and Roots
Chps 9 and 10
Chp 9 Vocabulary
• Secondary xylem
• Vessel elements
• Secondary phloem
• Primary xylem • Cork
• Primary phloem • Periderm
• Vascular bundles • Suberin
• Sieve elements • Pressure (mass) flow
• Companion cells • Vascular cambium
• Symplastic • Cork cambium
Stems are Fundamental Plant Organs
• Vascular plants are those plants that have a conducting system
composed of vascular tissue (xylem and phloem).
• Stems are indispensable organs for most plants. All other organs
(leaves, buds, roots) are attached to stems. Stems enable plants to
increase their height or length, mass, and surface by the activity of
• Plant stems are usually branched, which allows increase in mass
and the amount of surface available for attachment of leaves and
reproductive structures. The more leaves on a stem, the greater
the amount of sunlight they can harvest in photosynthesis.
• Stems transport water and minerals collects by roots from the soil
to the leaves where these materials are needed for photosynthesis
(xylem), and conduct sugars produced in the leaves to roots and
any other places where sugar fuel is needed (phloem)
• Xylem is defined by the presence of the tough, waterproofing
compound lignin on walls of specialized cells – tracheids and vessel
elements. Lignin also provides support to vascular tissue and thus
Structure and Function of Stems
• In herbaceous (nonwoody) stems and the
young stems of woody plants, xylem and
phloem tissues differentiate from precursor
tissue (procambium) formed by the apical
• Mature conducting tissues formed in this way
are known as primary xylem and primary
phloem. These primary conducting tissues are
located near each other within elongate
• The vascular tissue in a plant is interconnected
– extending from the roots, through thte stem,
into branches and leaves and other organs like
the water pipes in your house.
• Phloem tissues include pipeline
components known as sieve elements,
which may consist of sieve cells or sieve
tube members end to end.
• Sieve elements possess pore-containing
end walls known as sieve plates. Their
perforations develop by expansion of
plasmodesmata. Pores in the end walls of
sieve elements allow phloem sap – a
watery solution of sugars and other organic
molecules – to move freely from one cell to
• Phloem sieve elements are alive at
maturity, but the nucleus and some other
cell components are degraded during sieve
element development and are thus absent
from mature cells. In order to function,
sieve elements require the help of the
adjacent companion cells, which have
nuclei and provide materials to the sieve
cells via plasmodesmata.
• Ex. When a plant is cut or wounded, P protein
(phloem protein) masses along the sieve plate of
sieve elements, forming a “slime plug”. Such plugs
function to reduce the loss of phloem sap, much as
clots reduce blood loss from the vascular system of
• Another wound response is deposition of the
carbohydrate callose, which also helps to plug
phloem sap leaks.
Phloem Conducts Sugars
• Phloem provides plants with a long-distance
transport system. The direction of transport in
phloem is from the source of organic molecules
to sites, known as “sinks”, where molecules are
utilized like roots, flowers, etc.
• Direction is “source to sink”
• In some plants, sugars are loaded from the cells
producing them either directly into sieve
elements or indirectly, via companion cells
through plasmodesmata. This process is known
as symplastic loading.
• Other plants have apoplastic loading of
phloem, which occurs from intercelular spaces.
This means that it will require active transport
through the cell membrane.
• The force that moves organic compounds
within the phloem is known as pressure flow or
mass flow and is based on osmosis and cell
Water Moves bc of Transpiration
• With a few exceptions, plants obtain their water and minerals
from the soil and move these materials via the root’s xylem
into the stem. The stem’s xylem’s main function is to
transport water and minerals to other organs.
• Water moves in the xylem as the result of transpiration, the
evaporation of water from plant surfaces like the stomata. A
stream of water rises through the plant as each water
molecule lost from a cell at the surface is replaced by another
from inside the cell, which in turn exerts an attractive force on
nearby water molecules, causing the water to rise.
• Xylem will help new leaves and flower buds grow by placing
sugar in a watery solution to flow up the tree (days are warm,
nights are cold in spring). Maple trees are good sources of
such xylem sap and are cultivated in large plantations called
sugar bushes. With care, 150 L can be tapped per year
without harming the tree.
Wood and Bark
• Many plants produce no wood or bark, but
woody plants produce wood tissue and bark
by the action of two meristems (vascular
cambium and cork cambium).
• The girth increase of a tree trunk is known as
secondary growth. The cambiums are
• Mature vascular cambium takes the form of a
cylinder. It produces lignin-rich secondary xylem
tissue on the inside (wood) and secondary phloem on
the outside (inner bark).
• Addition of a thick cylinder of wood requires that the
circumference of the vascular cambium must increase,
necessitating the addition of new vascular cambium
cells by cell division.
• Ray initials produce ray parenchyma cells and ray
tracheids (together form vascular rays). Rays store
things and transport food laterally across the stem.
• During each growing season, the
vascular cambium produces new
cylinders of secondary xylem,
adding new wood and growth
rings. Rings from previous years
may still transport water, but
really old rings (toward the
center) can become clogged by
tyloses from neighboring
• Innermost wood is heartwood:
full of decay-resistant chemicals,
good for furniture
• Phloem exists towards the
outside, in the inner bark layer.
They are susceptible to bark
damage because it would cut off
its food supply (girdling or
ringing a tree will kill it).
Cork Cambiumbegin to enlarge, the
• As young woody stems
delicate epidermis eventually ruptures and its
protective role is replaced by cork.
• Cork is produced to the outside of a secondary
meristem called the cork cambium.
• Together, the cork, cork cambium and
parenchyma cells make up the periderm.
When periderm become worn out, they will
be replaced by a new periderm on the inside.
Eventually the old periderm will create outer
bark. The outer bark is dead whereas inner
bark is still living.
• Cork cell walls have layers of lignin and
suberin. Suberin helps prevent microbial
attack and also waterproofs the stem’s
• Lenticels are slightly raised patches of various
shape that interrupt the bark’s cork layer to
allow gas exchange for the inner stem tissues.
Human Uses for Stems
• Paper: Paper started as papyrus, which was made from the stems of
the papyrus plant. Papyrus also made rafts, sails, cloth, and cord. To
make paper, the Egyptians peeled the outer layers of papyrus stemss
off, exposing the pith. The pith was sliced into thin strips and laid
across one another. Workers then pounded the layers making starch
release from the cells and thus gluing the strips together and then
dried in the sun. Today, most paper is made from wood pulp. Genetic
engineers are working on making trees with more cellulose and less
lignin for paper. Lignin byproducts of the pulp are toxic and can
threaten water supply.
• Cork: The cork oak’s cork layer is several inches thick. It can be
stripped without hurting the tree. Cork is able to float and is used as
insulation, floor covering, shoe soles and bottle stoppers.
• Bamboo: used for housing in some areas. UK is working on creating
earthquake-proof housing with bamboo.
• Wood: construction material, fuel, paper, furniture. Known for
strength and beauty and makes up more than 1% of the world’s total
economy. Species like redwood and white oak are desired for ship
building. Basswood, yellow birch, and black cherry are valued for
making musical instruments because their structure lends to a
Chapter 10 Vocabulary
• Embryonic root
• Storage root •Feeder root
• Prop root •Root hairs
• Aerial root •Gravitropism
• Buttress root
• Lenticel •Micorrhizal fungi
• Epiphytic plants •nitrogen-fixing bacteria
• Taproot system
Roots Play a Variety of Roles
• When seeds germinate, the first plant organ to emerge is the
embryonic root, the radicle,– and a primary root is soon present on
• Shoot development depends on enlargement of cells by water uptake.
And photosynthesis requires water to serve as the necessary electron
donor. Both of these processes are highly dependent on an early
water and mineral supply.
• Bryophytes do not have roots. Moss and bladderworts are examples.
Because their leaves are so thin, they can directly absorb water and
minerals from their very wet environment.
• Some roots store carbohydrates during the first year of growth of
biennials. Carrots, sugar beets, parsnips, and rutabagas are biennials
grown for their food-roots.
Roots are Hormone and Secondary
• Roots produce the plant hormones cytokinins and
gibberellins, which are transported in the xylem to
the shoot, where they influence growth and
• Roots are also a site for producing protective
secondary compounds. Ex. Nicotine is made in the
roots of tobacco plants and moved to the leaves to
act as a poison that helps prevent herbivore attack.
• The roots of an African tree has long been known to
produce a yellow substance used by healers to treat
syphilis and leprosy (both caused by bacteria).
Recent studies showed that the yellow compound
(identified as a terpene) does in fact kill bacteria and
• If you look closely at the
base of corn plants, you
may notice prop roots,
growing from the stem
into the soil. These
specialized roots help
the tall corn plants stay
upright even though
they lack woody tissue.
• Some tropical trees grow
in thin soil and use
buttress roots to help
keep from falling over on
a windy day.
• Aerial roots form from a
stem and form massive
columns to support the
• Pneumatophores (“breath bearers”) are specialized roots of some types of
mangrove trees. They grow upward into the air, absorb oxygen rich air via
surface openings – lenticels.
• When the tide is up, the lenticels are protected by waterproofing
substances. When the tide goes back down, air is sucked into the lenticels.
• Some herbaceous plants like dandelions have contractile roots which
shorted by collapsing their cells. This allows the root to pull deeper into
the ground where it’s warm to survive changing early spring weather.
• Parasitic plants like the dodder obtain water, minerals, etc from host plants
by producing rootlike organs that penetrate the host’s stem and tap into
the host’s vascular system.
• Epiphytic plants grow non-parasitically on other plants and have
specialized roots Their roots are aerial and are photosynthetic. Ants often
form an association with such plants and provide nitrogen to the plant with
Types of Underground Root Systems
• Plant roots differ in their external form. These differences
result from variations in the fate of a seedling’s primary root.
For example, in gymnosperms and eudicot angiosperms, the
primary root generates a taproot system – single main root
from which many branches emerge.
• In grasses and other monocots, the primary root lives for a
short time and is replaced by a system of roots that develop
from the bottom of the plant’s stem. Roots from a stem are
called adventitious roots. Many adventitious make up a root
• If no single root is most prominent, then we
say it’s a fibrous system (many branched
roots). These are usually shallower in the
ground than a taproot.
• Feeder roots, produced by both taproot and
fibrous root systems are fine (<2mm in
diameter) peripheral root that are most active
in absorbing water and minerals from the soil.
Feeder roots have limited lifespans and are
• Knowledge of feeder roots is helpful in
landscaping. When transplanting a plant, make
sure you know where the roots end so you do
not risk cutting them when digging the plant
up, otherwise the plant may not be able to
obtain nutrients from the injured roots and not
Root Structure and Function
• Feeder roots are young branch roots. Soil texture influences
root branching. Plants that must grown through hard, dry soil
have fewer branch roots than those growing in moist, loose
• Branch roots and the main root axis are covered by an
epidermis, which is sometimes covered by a cuticle.
• A region closer to the root tip is fuzzy with countless root
hairs – fingerlike extensions from some epidermal cells. For
most roots, these hairs are the main location of water and
mineral absorption, and root hairs are a major site of uptake
selectivity, the ability of plant roots to discriminate between
useful and harmful soil minerals.
• At the cone-shaped tip, there is a root apical meristem
(RAM). This region of meristematic cells, which divide rapidly,
increasing the number of cells in the main portion of the root.
• Protecting the RAM is a root cap, whose cells are also generated by
the apical meristem. Cells in the center of the root cap contain
starch-rich plastids, amyloplasts. Some experts think that
amyloplasts operate as gravity sensors since they are heavy enough
to fall as the root grows, thus signaling the downward growth path
normal to most root cells.
• Other experts believe there are different mechanisms for
gravitropism, a root’s growth response to gravity. Plant biologists
do not fully understand why plants know which way is down.
• Root cap cells slough off the root tip a few days after being created,
so they must continually be replaced. These dispersal cells, known
as root border cells, do not then just die, but apparently help
modify the external root environment in ways that prevent attack
by microbes and tiny soil worms.
• The tips of roots are embedded in a blanket of mucigel, a gluey
substance secreted from the Golgi apparatus of root tip epidermal
cells. Mucigel lubricates the root and helps in water and mineral
absorption and creates a favorable environment for beneficial
Root Mineral Absorption
• Root xylem obtains minerals and water in one of two ways
– Water and minerals are selectively taken up by root hairs and
transmitted via plasmodesmata
– Water and minerals that penetrate root tissues within intercellular
spaces and cell walls are selectively absorbed at the cell membrane
of nonsuberized surfaces of endodermal cells and released on the
• Mineral passage from root hairs through the cortex and endodermis via
plasmodemata is known as symplastic transport. This allows beneficial
minerals to be absorbed and harmful ones to be excluded.
• When minerals dissolved in water diffuse from the root’s environment into
epidermal cell walls, then through walls of cortical cells to the
endodermis, such movement is known as apoplastic transport. In
apoplastic transport, harmful minerals are unable to be excluded which
could cause the plant to be injured.
Root Hairs Have Selective Absorption
• Epidermal root hairs and cells of the endodermis
filter out mineral content of water.
• Many metal ions (iron, copper, manganese, and
magnesium) are needed by plants for the proper
functioning of enzymes and other complex molecules
in plant cells. Magnesium is used in chlorophyll and
iron is an electron carrier in photosynthesis and
• Aluminum is abundant in soil but toxic to plants. It
can bind to things like proteins and nucleotides
causing disruption in membrane function. The first
symptom of aluminum toxicity in plants is that roots
stop elongating within 5 minutes of exposure.
• Aluminum toxicity is a major
limitation in growing crops. Its
more prominent in acidic soils in
high industry areas.
• Acid rain can turn soil acidic.
When soil gets below a pH of 5,
positively charged metals stick to
the soil releasing aluminum ions
that are then dissolved in the soil
water and available for
absorption. Plants can bind the
aluminum by releasing organic
acid, but the best way to avoid it,
• Phosphate is needed to construct phospholipid
membranes, ATP, and DNA/RNA. Phosphate is one of
the most important components for plants.
• Phosphate forms strong chemical bonds with iron
and aluminum oxide minerals in solid, reducing its
availability. The organic acids mentioned earlier can
help dissolve the aluminum, freeing phosphate.
• Plant root cell membranes contain transporter
proteins whose shapes enable them to bind even
small amounts of soil phosphate and move it into the
cell. When soil phosphate is low, root cells increase
the number of phosphate transporter proteins
Roots Need Food and Oxygen
• Plant root cells are efficient at mineral absorption,
but they use a lot of ATP. ATP is also required for cell
division at the root tip.
• The most efficient mode of ATP production is aerobic
respiration (uses O2). Roots cannot photosynthesis,
so the phloem must bring them sugar sap.
• Roots of a plant are called heterotrophic because
they (like animals) must consume their food (don’t
• Root produced carbon dioxide dissolves in soil water
to produce carbonic acid which will cause weathering
of the soil. This is another way plants help reduce
• Beneficial microorganisms can form symbiotic relationships
with plant roots.
• These include mycorrhizal fungi and nitrogen-fixing bacteria,
which live within roots of legumes and some other plants.
• These microbes help plants obtain the large amouts of
minerals needed for growth (this growth would otherwise be
limited). This allows for a better competing plant and a higher
• Mycorrhizal fungi are important in providing phosphate.
Almost all vascular plants have mycorrhizal fungi.
• Nitrogen-fixing bacteria supply the nitrogen compounds
required by plants to produce amino acids and proteins.
Legumes have much closer associations with such bacteria,
producing special root nodules whose tissues harbor
nitrogen-fixing bacterial partners.
• Legume-bacterial relationships begin with a
• Legume roots secrete flavonoids into the soil
• Legume-root flavonoids signal to soil N-fixing
bacteria, which respond to the flavonoid signal
by secreting small organic molecules into the
• Legume-root epidermal cell membranes contain
receptor molecules that recognize and bind
molecules excreted from specific bacteria.
These bacterial compounds cause the root hairs
to curl within special root cells.
• Root nodules contain both infected and
uninfected cells and mature nodules possess
vascular tissue that connects with the root
vascular system that distributes compounds
containing nitrogen throughout the plant.
• The bacteria are plant specific due to the
secondary compounds released.