Plant%20Anatomy

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Plant Anatomy Unlike mammals, plants have only a
few basic organs inflorescences leaves stems
roots Each has its own structure, but there are
features (tissues or cell types) in common among
a number of these organs.
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Each organ has an epidermis, the outermost cells.
Generally, this is a single layer of flattened
cells. Leaves and stems secrete a protective
layer of cutin (a wax, forming the cuticle) on
the outside surface of the epidermis.
(cork)
(related to cork cambium, but longer lived)
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Some plants have trichomes (hair-like
projections) from the epidermis. Trichomes may
aid in protection from insect herbivory, and/or
may be glandular. Tomato leaves (shown below) are
an example.
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In woody plants, as they become woody, the
epidermis is replaced by a surface tissue called
periderm. The periderm consists of the phelloderm
(a continuous source for cork cambium), the cork
cambium (which produces the cork, cells that are
dead and empty at maturity), and the cork layer.
That is the surface structure depicted on the
earlier slide. There are 3 tissues in common
among plant organs parenchyma, collenchyma,
and sclerenchyma Where do we find those tissue
types and what are their functions?
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Tissue type Characteristics Location Function
Parenchyma living cells, primary cortex
and basic cell walls only, pith of
metabolism usually spherical roots,
stems or only somewhat xylem, phloem
elongated leaf mesophyll Collenchyma living
cells, leaf petioles, support elongated
young stems, uneven cell
walls petals Sclerenchyma non-living
at fibers support maturity, xylem,
phloem, elongate or various cortex
sclereids thickened secondary cortex,
pith, cell walls mesophyll
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Parenchyma Collenchyma Sclerenchyma
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Throughout the plant body there must be a means
to move water, minerals, and photosynthate. The
vascular system achieves that. It is comprised of
two systems xylem conducts water and minerals
from the roots upward phloem transports
organic materials synthesized by the plant
xylem
Vascular bundles in a dicot stem
phloem
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There is important structure within the phloem
and xylem bundles, and theyre different. Xylem
is comprised of tracheids, vessel elements,
fibers, and some parenchyma.
Vessel elements indicate that this is an
Angiosperm
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Tracheids function in support. They have both a
primary and a secondary cell wall. At maturity,
these cells are dead. They function in support,
but they are also the only conducting cell in
vascular plants other than Angiosperms. They have
numerous pits along their lateral cell walls, so
that water and minerals can move between
cells. Vessel elements are shorter, wider, and
have either many perforations in their end cell
walls, or those end walls have virtually
disappeared. They become what their name suggests
pipelike tubes. Remember, they are only present
in the xylem of Angiosperms.
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Phloem is basically composed of sieve tube
members. There are also companion cells, fibers,
and parenchyma.
Sieve tube members are (strange) living cells.
They have only primary cell walls, and, when
mature, the nucleus and most organelles have
degenerated. Their end walls connect neighboring
cells with sieve plates.
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Sieve tube members are closely associated with
companion cells (both developmentally and
physiologically). There are numerous
plasmodesmata between the cells that permits the
companion cell to control transfer of organic
material into the sieve tube. The companion cells
also have large nuclei, and metabolically
regulate sieve tube members. Youve already seen
one picture of the xylem and phloem in a stem. Is
there more structure? Of course! Each vascular
bundle is (generally) surrounded by a bundle
sheath, and how they are placed within a stem
differs in differing types of plants. In
monocots, the vascular bundles are scattered
through the stem in dicots they form a ring.
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In a monocot stem
xylem
bundle sheath
phloem
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In a dicot stem
xylem
parenchyma
(parenchyma and fibers)
phloem
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Annual rings in the cross section of a woody
plant represent the annual growth of xylem from
the vascular cambium. The rings are visible
because the cells of spring growth (called
springwood) are larger (due to the wetter
conditions) and apparently lighter in colour than
those produced during summer (summerwood). The
size (thickness) of annual rings can be used to
estimate the climate during the year of
formation. Climates covering a number of
centuries can be evaluated using this method
(called dendrochronology),. This is an
important tool in estimating climate change.
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Roots Roots have a meristem (growth region)
protected by a root cap. Just behind that is a
region of the root where cells elongate.
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In cross section, the center of the root contains
the vascular bundles (the vascular cylinder). It
is called the stele. In the center of the stele
is the xylem, usually star-shaped (i.e. having
projections outward) in dicots. Between the arms
of the star is the phloem. In monocots there is a
ring of vascular bundles, alternating xylem and
phloem.
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In both root forms the outer layer of the stele
is called the pericycle. This tissue is
meristematic, that is it can give rise to new
growth in the form of root branches. From the
surface epidermis project root hairs. They are
key to maximizing absorption of water and
minerals, enormously increasing the effective
surface area of the roots.
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Roots have differing patterns of growth in
different habitat conditions and among species.
The basic difference is between a taproot design
a thick principal root from which branches
develop, and a branched fibrous root system
many essentially equal diameter roots with
branching.
fibrous roots of barley
tap root of a dandelion
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By having differing types of roots and extension
to different depths, plants can reduce the
intensity of competition for water and nutrients.
On the prairies of central North America, some
plants have roots that go down lt 1m, while
others, e.g. Andropogon gerardii, the
characteristic grass of tall grass prairie,
extend down at least 4m, and Rosa suffulta, the
prairie rose, has roots that extend gt 7m.
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In addition to totally below ground roots, some
species have adventitious roots. These roots
originate on leaves and stems above ground. Corn
has prop roots that originate from the stem just
above ground. Banyan trees from Australia have
extensive aerial roots, reaching down to the soil
from far up in the branches of the tree.
Mangroves have extensive, spreading adventitious
roots above the surface of the water.
mangrove mangle
corn prop roots
banyan aerial roots
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Finally, roots act as storage organs of various
types in many species
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Leaves Leaves differ in shape, edge pattern, and
organization on the stem. First, leaf shape
Then, leaf edges
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Leaves differ in the pattern of veinationin
monocots the usual pattern is parallel veins in
dicots the pattern is more frequently net veins.
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Whether dicot or monocot, the basic organization
of leaf parts and their attachment to the stem
have many similarities
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And, finally, arrangement on the stem
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Leaves are the source of many adaptations plants
have made to drought
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• Flowers
• There are 4 basic parts of a flower organized
  into what are usually called four whorls.
• The whorl of sepals, collectively called the
  calyx
• The whorl of petals, collectively called the
  corolla
• The whorl of male structures, the androecium,
  made up of pollen-producing anthers, each
  supported by a filament
• The whorl of female structures, called the
  gynoecium, consisting of the carpel(s). A single
  carpel may consist of a simple pistil, or a fused
  compound pistil. Each carpel, of whatever type,
  is made up of a stigma (the receptive surface), a
  style (the supporting column), and an ovary.

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An inflorescence the set of flowers on a plant
can be organized in different ways
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Flowers can be perfect, having both male and
female parts, or can be imperfect, lacking
either male or female parts. If the male parts
are lacking, then the flowers are called
pistillate. If the female parts are lacking, they
are called staminate. If male and female flowers
are separately present on a single plant, the
plant is described as monoecious. If individual
plants have flowers of only one sex, the species
is described as dioecious. The arctic birch I
study is an example of a monoecious species.
Sugar maples, though they can switch sexes over
the course of their lives, are a dioecious
species, since each tree has only one sex of
flowers in any year.
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In arctic birch, male flowers appear before
female flowers each year. This picture is of
female flowers. Each inflorescence takes the form
of a catkin (like a cone) and the receptive
surfaces of individual flowers are visible as
red-purple, sticking out from the catkins.
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In plant taxonomy, one important characteristic
used in determining species is the position of
the ovary in relation to the remaining whorls of
the flower. If the sepals, petals and androecium
are inserted (connected) below the ovary, it (the
ovary) is described as superior and the flower is
described as hypogynous. If the other whorls are
inserted above the ovary, the ovary is described
as inferior and the flower as epigynous. Finally,
if the other whorls are inserted around the
gynoecium at the same level, the flower is called
perigynous.
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Here are diagrammatic images of the three
organizational patterns
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Now lets consider how the whole plant goes
together, comparing monocot to dicot morphology