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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’.
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)
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.
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?
Tissue type
Characteristics
Location
Function
Parenchyma
living cells, primary
cell walls only,
usually spherical
or only somewhat
elongated
cortex and basic
pith of
metabolism
roots, stems;
xylem, phloem
leaf mesophyll
Collenchyma living cells,
elongated
uneven cell walls
leaf petioles, support
young stems,
petals
Sclerenchyma non-living at
maturity,
elongate or various
thickened secondary
cell walls
fibers:
support
xylem, phloem,
cortex; sclereids:
cortex, pith,
mesophyll
Parenchyma
Collenchyma
Sclerenchyma
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
There is important structure within the phloem and xylem
bundles, and they’re different.
Xylem is comprised of tracheids, vessel elements, fibers, and
some parenchyma.
Vessel
elements
indicate that
this is an
Angiosperm
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.
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.
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.
You’ve 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.
In a monocot stem:
xylem
bundle
sheath
phloem
In a dicot stem:
xylem
parenchyma
(parenchyma and
fibers)
phloem
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.
Roots
Roots have a meristem (growth region) protected by a root
cap. Just behind that is a region of the root where cells
elongate.
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.
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.
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
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 < 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
> 7m.
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
Finally, roots act as storage organs of various types in many
species:
Leaves
Leaves differ in shape, edge pattern, and organization on the
stem. First, leaf shape:
Then, leaf edges:
Leaves differ in the pattern of veination…in monocots the
usual pattern is parallel veins; in dicots the pattern is more
frequently net veins.
Whether dicot or monocot, the basic organization of leaf parts
and their attachment to the stem have many similarities…
And, finally, arrangement on the stem:
Leaves are the source of many adaptations plants have made
to drought…
Flowers
There are 4 basic parts of a flower organized into what are
usually called four whorls.
1. The whorl of sepals, collectively called the calyx
2. The whorl of petals, collectively called the corolla
3. The whorl of male structures, the androecium, made up of
pollen-producing anthers, each supported by a filament
4. 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.
An inflorescence – the set of flowers on a plant – can be
organized in different ways…
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.
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 redpurple, sticking out from the
catkins.
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.
Here are diagrammatic images of the three organizational
patterns:
Now let’s consider how the whole plant goes together,
comparing monocot to dicot morphology…