Materials to practical classes 4
VASCULAR AND MECHANICAL TISSUES OF PLANTS.
COMPLEX TISSUES OF PLANTS – XYLEM AND
PHLOEM.
VASCULAR BUNDLES
There are several types of plant tissues.
Ground tissue system: tissues derived from the ground
meristem. All are simple tissues composed of a single type of cell, which is named after the tissue. The
tissues of the ground tissue system include:
parenchyma:
tissues composed of cells with thin primary cell wall.
Types include:
chlorenchyma: contains
chloroplasts and functions in photosynthesis.
aerenchyma: contains large intracellular air
spaces and functions in gas exchange.
endodermis: characterized by a suberized
Casparian strip; regulates transport of materials into the vascular bundles of
most roots and some leaves and stems.
storage parenchyma: characterized by
large accumulations of storage products such as starch, protein, oil,
hemicellulose, or water.
collenchyma: tissues composed of cells with unevenly thickened primary cell walls that
strengthen growing organs. Types are classified according to the arrangement of
the wall thickenings and include.
angular collenchyma: cell wall is thickest in the corners.
lamellar collenchyma: cell wall is thickest on two opposite sides.
lacunar collenchyma: cell wall is thickest in the corners, intercellular
air spaces present.
sclerenchyma: tissues composed of cells with thick, secondary cell wall that are usually
lignified. Types are classified according to cell shape and include:
fiber: long, straight and thin, often
occurring in bundles. Sometimes called "extraxylary
fibers" to distinguish them from xylary fibers, which look
similar, but have a different evolutionary origin.
sclereids: variable in shape, but not like fibers.
Types are classified according to shape and include:
brachysclereids: also called stone cells, length and width nearly equal.
astrosclereids: star shaped, with several projecting arms.
trichosclereids: hair-like, similar to a
fibers, except branched.
macrosclereids: column shaped, longer than
wide.
osteosclereids: bone shaped, elongated with
swollen ends.
secretory
structures:
hydathode: a structure in the margins of
leaves that secretes water.
oil cavities: a cavity lined with cells that
secrete oils.
resin duct: a tube lined with cells that
secrete resin.
laticifer: a secretory structure that produces
latex. latex: a milky fluid of unspecified
composition.
Dermal tissue system: Tissues derived from the
protoderm or cork cambium that cover the surface of
the plant body. The dermal tissues are complex (composed of several cell types)
and include:
epidermis: a complex tissue that is usually
a single cell layer thick and composed of the following cell types.
pavement cells: the least specialized cells of
the epidermis (i.e. cells that are NOT specialized as guard cells, root hairs,
trichomes, etc.). May secrete a cuticle.
guard cells: cells that surround and control
the size of stomatal pores.
stomate (plural: stomata): an opening defined by
pairs of guard cells that controls gas exchange and water loss.
subsidiary cells: cells adjacent to guard
cells that are distinct in appearance from ordinary epidermal cells.
trichomes: cells that project from the surface of
the epidermis. Types include:
unicellular
trichome:
consists of one cell.
multicellular trichome: consists of several cells.
secretory (glandular) trichome: secretes a substance.
root hair: specialized unicellular trichome found in roots.
Vascular tissue system: Tissues derived from the
procambium or vascular cambium that transport water
and photosynthate. The vascular tissues are complex (composed of several cell
types) and include:
xylem:
the water-conducting tissue of plants.
*vessel element: a tracheary element with
perforation plates.
perforation plate: the end wall of a vessel element where the secondary
cell wall was not deposited and the primary cell wall has been digested.
vessel: a long tube of vessel elements
connected by perforation plates
tracheid: a tracheary element that lacks perforations plates, water flows from
between tracheids through pits.
*tracheary element: a conducting cell of the xylem,
characterized by an elongated shape and lignified secondary cell wall.
*libriform fiber: a cell in the xylem that is very
long and thin and has simple pits, sometimes called "xylary
fibers" to distinguish them from extraxylary fibers, which look
similar, but have a different evolutionary origin;
*parenchyma cells
phloem: the photosynthate-conducting tissue of plants.
*sieve element: a conducting cell in the phloem.
sieve-tube member: a sieve element with
perforation plates,
characteristic of angiosperms.
sieve plate: the end wall of a sieve-tube element that is
perforated by sieve plate pores.
sieve plate pore: an enlarged plasmodesma that perforates a sieve plate.
sieve tube: a long tube of sieve elements (also called sieve tube members)
connected by sieve plates.
*companion cells: a cell in the phloem that is connected to a sieve-tube member by numerous
plasmodesmata.
*Fiber elements
*parenchyma cells
Let us start with the differences
between the plant and animal
tissues, plant tissue consist of a cell wall and it has
chlorophyll, whereas animal tissue do not have chlorophyll, unless it is a
bacteria (Unicellular) and it consist of a cell membrane. There are mainly 2 types of plant tissues, simple
tissue and complex tissue and they derive from meristem. Protoderm, procambium
and ground meristem are the primary meristem which forms long lasting plant
tissue. Plants usually are composed of 3 tissue systems, ground tissue system,
dermal tissue system and vascular tissue system.
Today we shall study VASCULAR
and MECHANICAL tissues of plants.
Tissues
that support plant are called mechanical tissues.
They belong to ground simple tissues, becouse include only one tipe of
cells. There are two types of MECHANICAL TISSUES: collenchyma and sclerenchyma. VASCULAR and MECHANICAL tissues of plants contain 90 % of wood in trees.
Collenchyma cells are elongate (up to
Types are classified according to
the arrangement of the wall thickenings and include.
angular collenchyma: cell wall is thickest in the corners.
·
lamellar collenchyma: cell wall is thickest on two opposite sides.
·
lacunar collenchyma: cell wall is thickest in the
corners, intercellular air
Summary: Collenchyma tissues are mainly
found under the epidermis in young stems in the large veins of leaves. The
cells are composed of living, elongated cells running parallel to the length of
organs that it is found in. Collenchyma cells have thick cellulose cell walls
which thickened at the corners or two opposite side/ The cells
contain living protoplasm and they sometimes contain chloroplasts. Functions: the collenchyma serve as supporting and strengthening tissue. In collenchyma with chloroplasts
photosynthesis takes place.
Sclerenchyma cells are rigid and have thick, nonstretchable secondary cell walls. They support and
strengthen nonextending regions of plants such as mature stems, and are
usually dead at maturity. Sclerenchyma also makes up the hard outer covering of seeds and nuts.
Tissues composed of cells with
thick, secondary cell wall that are usually lignified. Types are classified
according to cell shape and include:
§
fiber:
long, straight and thin, often occurring in bundles. Usually fiber cells are much longer than they are wide
and have a very tiny cavity in the center of the cell. Currently, fibers from
over 40 different plant families are used in the manufacture of textiles,
ropes, string and canvas goods to name a few.
§
§
sclereids: variable in shape, but not like fibers. Types are classified according to
shape and include:
brachysclereids: also called stone cells, length
and width nearly equal.
astrosclereids: star shaped, with several projecting arms.
trichosclereids: hair-like, similar to a fibers, except branched.
macrosclereids: column shaped, longer than wide.
osteosclereids: bone shaped, elongated with swollen ends.
VASCULAR
TISSUES are specialized for long-distance. Xylem and phloem are the two most important complex tissues in a
plant, as their primary functions include the transport of water, ions and
soluble food substances throughout the plant. While some complex tissues are
produced by apical meristems, most in woody plants are produced by the vascular
cambium and is often referenced as vascular tissue.
Other
complex tissues include the epidermis and the periderm. The epidermis consists
primarily of parenchyma-like cells and forms a protective covering for all
plant organs. The epidermis includes specialized cells that allow for the
movement of water and gases in and out of the plant, secretory glands, various
hairs, cells in which crystals are accumulated and isolated, and other cells
that increase absorption in the roots. The periderm is mostly cork cells
and therefore forms the outer bark of woody plants.
Vascular tissues derived from the procambium or vascular cambium that
transport water and photosynthate. The vascular tissues are complex
(composed of several cell types). XYLEM conducts water and dissolved minerals from the roots
to all the other parts of the plant. So xylem is the water-conducting
tissue of plants. Wood is xylem.
Xylem is a complex tissue composed of xylem vessels
(or tracheids), fibres and parenchyma
cells. In Angiosperms, most of the water travels in
the xylem vessels.
1) Vessel
element: a
tracheary element with perforation plates. There are vessel element where the secondary cell wall
was not deposited and the primary cell wall has been digested. Vessel is a long tube of vessel elements
connected by perforation plates. Their
diameter may be as large as
2) Tacheids: a tracheary element that lacks
perforations plates, water flows from between tracheids through pits. The xylem of ferns and conifers contains only tracheids.
3) Libriform fiber: a cell in the xylem that is very long and thin and has simple pits,
sometimes called "xylary fibers" to distinguish them from extraxylary
fibers, which look similar, but have a different evolutionary origin.
4)
Xylem parenchyma cells. The thin-walled
parenchyma cells have large vacuoles
and distinct intercellular spaces.
Xylem is an important plant tissue as it is part of the ‘plumbing’
of a plant. Think of bundles of pipes running along the main axis
of stems and roots. It carries water and dissolved substances throughout and
consists of a combination of parenchyma cells, fibers, vessels and tracheids. Vessel
members and tracheids are dead at maturity. Tracheids have thick secondary cell
walls and are tapered at the ends. They do not have end openings such as the
vessels. The tracheids ends overlap with each other, with pairs of pits
present. The pit pairs allow water to pass from cell to cell. In trees, and
other woody plants,
there are ray will radiate out from the center of stems
and roots and in cross-section will look like the spokes of a wheel.
PHLOEM is the photosynthate-conducting
tissue of plants. Phloem is a
complex tissue composed of sieve elements, companion cells, fiber elements and parenchyma cells. The
main components of phloem are sieve
elements and companion cells.
1) Sieve element: a conducting cell in the
phloem. Sieve elements have no
nucleus and only a sparse collection of other organelles. They depend on the
adjacent companion cells for many functions. Sieve-tube
membe is a sieve element with perforation plates,characteristic
of Angiosperms. Sieve tube: a long tube of sieve
elements (also called sieve tube members) connected by sieve plates.
2)Companion cells are a cell in the phloem that is
connected to a sieve-tube member by numerous plasmodesmata. They have a nucleus.
3) Fiber elements are long and thin (over
4) Phloem parenchyma cells.
Phloem is an equally important plant tissue as it also is part of
the ‘plumbing’ of a plant. Primarily, phloem carries dissolved food substances
throughout the plant. This conduction system is composed of sieve-tube member
and companion cells, that are without secondary walls. The parent cells
of the vascular cambium produce both xylem and phloem. This usually also
includes fibers and
parenchyma cells.
The Phloem
The other vascular tissue is the phloem. Its principal function is the
conducting of assimilates and food. It is composed of the sieve elements, of which two
types can be distinguished, sieve
cells and sieve-tube members. Phloem
elements do typically have sieve
plates instead of final
walls. Sieve-tube members of angiosperms are associated by living companion cells.
The phloem is the principal food-conducting tissue of vascular plants.
Its elements are elongated, just like those of the xylem. In contrast to
tracheids and wood vessels, mature phloem elements contain a protoplast and
sometimes even a nucleus. Phloem elements may be of primary or secondary origin
though the early primary phloem, the protophloem,
is frequently destroyed during elongation of the resepctive organ.
The main conducting elements of the phloem are the sieve
elements, of which there are two different types: sieve cells and sieve-tube
members. Sieve cells have narrow pores, their sieve areas are quite uniform
in structure, and they are distributed evenly on all walls. One of the
principal differences between sieve cells and sieve-tube members is the
presence of sieve plates in sieve-tube members, that are absent
in sieve cells. Sieve cells are the only type of food-conducting cells in most
seedless vascular plants and gymnosperms, whereas in angiosperms only
sieve-tube members are present. Sieve-tube members occur end-on-end in
longitudinal series called sieve tubes. They are in contact via plasmodesmata. Typically, the final walls
are interspersed with primary pit areas (groups of plasmodesmata), that later
on develop into sieve plates. Sieve tubes in the phloem of angiosperms are
flanked by one or several plasma-rich, nucleated companion cells, that do not
occur in gymnosperms.
Much less is known about the phylogeny
of sieve elements than about
that of xylem members. One cause are the cell walls of sieve elements, that are
strengthened entirely by cellulose and are therefore not as resistant as
lignified ones, another is the transitory working order of the single sieve
element. After loss of function the cells are reorganized and lose their
typical structural properties. Good fossils are thus rare.
Despite these obstacles it was possible to reveal in comparative plant
anatomical studies (of monocots) definite phylogenetic trends (V. I. CHEADLE,
1948, CHEADLE et al.,
1941, 1948, K. ESAU et al.,
1953, ESAU, K., 1964/69):
1.
The originally scattered pits are
concentrated in 'sieve areas' (accumulations of pores). The diameter of a pore is around 0.1
- 15 µm.
2. Specialized
sieve areas are concentrated in the final walls.
3. A
gradual change of orientation of these final walls from very sloping to right
angles occurs.
4. The
change is followed by a stepwise transition from compound to simple sieve
plates.
5. The
activity of the sieve areas in the side walls is reduced. This leads to
canalizing of the flow of assimilates in longitudinal direction and the
long-distance transport rates are enhanced.
Phloem elements of the roots are normally organized in a more progressive
and thus more efficient way than their equivalents in the shoots. Callose is
deposited in the sieve plates at regular intervals and can be detected with
special reagents. Resorcin blue gives a blue, aniline blue a bright yellow
staining. The amount of callose increases with cell age, continuously reducing
the diameter of the pores. Sieve elements that have lost their function are
blocked by thick plugs of callose. The question, what is cause and what result
remains. Are the cells inactivated by the callose plugs or do these form as a
result of the loss of function?
Active sieve tubes contain huge
amounts of so-called P-proteins (phloem - proteins). It has often been
speculated, whether they have an active part in assimilate transport. No answer
has been found up until now.
The sieve tube elements of angiosperms (mono- and dicots), but not those
of pteridophytes and gymnosperms are associated with companion cells. In Ginkgo and other gymnosperms their function
is taken over by specialized parenchyma cells, that resemble companion cells in
structure: the albuminous
cells. They work out the contacts between phloem and the surrounding
transfusion parenchyma and can also be found as mediators between ray
parenchyma cells of the bark and mature sieve cells. Albuminous cells
participate in loading and unloading of these cells. The process has been
studied in detail in Pinus.
Companion cells and sieve elements are of meristematic origin, their
development is similar to that of the xylem. Primary
phloem develops by
longitudinal division and subsequent elongation of meristematic cells. The
cells do often divide unequally. The bigger daughter cell differentiates into a
sieve element, the smaller after one or two further longitudinal divisions into
two to four companion cells. Consequently several companion cells may belong to
one sieve cell. The exact number is usually typical for the respective tissue,
but it may even vary within a single plant.
VASCULAR
BUNDLES are classified
according to special relationships of xylem and phloem. There are several types
of vascular bundles:
·
collateral bundles have xylem on one side and phloem on the
other side. There are close and open collateral bundles;
·
bicollateral
bundles have phloem on both sides of the xylem; they
also have cambium.
concentric bundles which
divides to:
o
amphicribral
- phloem surrounding the xylem;
o
amphivasal - xylem surrounding the phloem;
·
radial bundles xylem
occurs in radial directions, and phloem takes place between them.
·
All
these types are typical for some divisions, classes, families and plant organs.
For
example:
closed collateral vascular bundle – for stems and leaves of
monocot plants;
v
opened
collateral vascular bundle - for dicot and gymnosperm plant;
v bicollateral vascular bundle – only for some families
of dicot plants (Cucurbitaceae (pumpkin family);
v
concentric vascular bundles are common only for rhizomes: with
phloem inside - for monocot Angiosperm, with xylem inside - for ferns.
v
radial
polyarch vascular bundle is typical only for roots of monocot plants, dicot
have radial tetraarch or tryarch vascular bundle.
|
Tissue System |
Component Tissues |
Location of Tissue Systems |
|
Dermal Tissue System |
Epidermis |
|
|
Ground Tissue System |
Parenchyma tissue Secretory structures Collenchyma tissue |
|
|
Vascular Tissue System |
Xylem tissue Phloem tissue |
Plant cells are basic building
blocks
•
Can specialize in form and
function
•
By working together, forming
tissues, they can support each other and survive
•
Levels of organization
atoms > molecules > cells
> tissues > organs > whole plant > pop.
A mature vascular plant (any plant other than mosses and liverworts),
contains several types of differentiated cells. These are grouped together in
tissues. Some tissues contain only one type of cell. Some consist of several.
The main function of meristematic tissue is mitosis. The cells are small, thin-walled, with no central vacuole and no specialized features.
Meristematic tissue is
located in
- the apical
meristems at
the growing points of roots and stems.
- the secondary meristems (lateral
buds) at the nodes of stems (where branching occurs) [View], and in some plants,
- meristematic tissue, called the cambium, that is
found within mature stems and roots.
The cells produced in the meristems soon become
differentiated into one or another of several types.
Protective tissue covers the surface of leaves and the
living cells of roots and stems. Its cells are flattened with their top and
bottom surfaces parallel. The upper and lower epidermis
of the leaf are examples of protective tissue
Vi
The cells of parenchyma are large, thin-walled, and
usually have a large central vacuole. They are often partially separated from
each other and are usually stuffed with plastids.
In areas not exposed to light, colorless plastids
predominate and food storage is the main function. The cells of the white
potato are parenchyma cells. [View]
Where light is present, e.g., in leaves, chloroplasts predominate and photosynthesis is the main function.
Supporting Tissues
The development of stable
supporting elements has been an important prerequisite for the evolution of
large terrestrial organisms. Animals have endo- or exoskeletons that correspond
in function to the woody stems or trunks of plants. The architectural design of
the plant's body of vegetation is very complex. Thin petioles carry heavy and
flat laminas, stems support leaves, flowers and fruits. All plant organs are
exposed to mechanical strains. Organs above ground follow the wind's drift.
Their high elasticity lets them either return to their original position, or it
makes them swing around an imaginative axis. Trunks are stable enough to resist
the wind's pulling. They withstand pressure and are inflexible, although their
projecting treetops provide the wind with a large target. The wind makes the
upper plant organs and the trunk act like a lever, a large part of the force is
hence exerted onto the roots, that anchor the plant in the soil. Other
functions of the root are water and nutriment uptake.
The strength of tissues
protects also against enemies. The hard shell of many seeds prevents a chewing
to pieces or puncturing by animals and avoids that parasites like fungi or
bacteria force their way into them.
The preceding topic mentioned
the high water-content of plant cells that lends a
high tension to plant tissues and is caused by the turgor. It supplies plant tissues with a
certain stability. Its actual importance is seen best in wilting leaves or
flowers after their water supply has been stopped. Extensive specialized
supporting tissues exist only in vascular
plants. Despite the existence of huge marine brown algae (seaweeds, like Macrocystis, Laminaria), not a
single terrestrial alga, whose thallus raises more than a few cell layers above
ground, is known. Vascular plants have up to three types of supporting tissue:
1. The collenchyma,
a tissue of living cells,
2. the sclerenchyma,
a tissue of nearly always dead cells, and
3. the vascular tissue consisting of both living and dead
cells. It is responsible for the transport and dispersal of water, nutriments
and assimilates.
All three types are reviewed
below.
The larger a vessel plant is,
the higher is its content of dead cells. Dead cells are exceptions among
bryophytes, but very common in flowering plants. They are usually elongated
(prosenchymatous) cells, in parallel to the axis of the respective organ and
often combined in sheaves, the fibres.
The botanist H. v. MOHL from Tübingen recognized
already in the 30th of the 19th century that all these fibres spring from normal,
living cells.
Supporting tissues reside
generally in the periphery of plant organs. If the cells are combined in
layers, tubes, whose stability is much greater than that of sticks of the same
diameter are formed. The supporting tissues of ribbed or edged stems are
concentrated in these ribs or edges. In submerse living vascular plants, the
supporting tissue is reduced to a minimum.
The Collenchyma
The collenchyma is the typical
supporting tissue of the primary plant body and growing plant parts. Its
prosenchymatous cells are living at maturity and are always kept in a primary
state, which means that they are never lignified. Collenchyma walls are
interspersed with groups of pits that tend to be organized in special areas.
The name collenchyma derives from the Greek word "kolla", meaning
"glue", which refers to the thick, glistening appearance of the walls
in fresh tissues.
The collenchyma is the
typical supporting tissue of the primary plant body and growing plant parts,
though it is kept with unaltered structure and function even in outgrown organs
like stems, petioles, laminae or roots. In cross-sections of stems, the
collenchyma commonly appears as discrete strands or as a peripheral cylinder
that lies, depending on the species, either directly beneath the epidermis or
is separated from it by several layers of parenchyma. The cylinder is usually
composed of several layers. Collenchyma is also found bordering the veins of dicot
leaves. It forms fibres in edgy stems that run along the edges or ribs. Often
either phloem or xylem of the vascular bundles is
associated with collenchyma cells.
Many transitions prove the
collenchyma's origin from the parenchyma. The differentiation is
reversible, a degeneration to meristematic states has often been observed. The
walls of collenchyma cells are strengthened by the deposit of cellulose and the
coating with pectin. These strengthenings are often
restricted to single parts or edges of the cell. The walls of parenchyma cells
are opened by pits that are often arranged in special areas.
The unevenly thickened cell
walls led the German botanist C. MÜLLER (1890) to distinguished between
different collenchyma types:
1. Angular
Collenchyma. A thickening of the cell's edges can
be seen in cross-section. Longitudinal sections show the elongated shape of
both cell and thickening. A cross-section through the stem of Begonia rex or related species is the typical
specimen used in botanical microscopic courses. Angular collenchyma occurs also
in species of the following genera: Ficus,
Vitis, Ampelopsis, Polygonium, Beta, Rumex, Boehmeria, Morus, Cannabis,
Pelargonium and others.
2. Tangential
Collenchyma. The tangential walls of this
collenchyma type are thicker than the radial walls. Examples: Sambucus nigra, species of the
genera Sanguisorba, Rhoeo,
Eupatoria.
3. Lacunar
Collenchyma. While hardly any intercellular spaces
exist in the two types above, are those of this type very large. Clear gaps can
be recognized between the cells. Occurrence: species of the generaLactuca,
Salvia, Prunella and the
Composite-family.
The cell walls of collenchyma
cells are distortable when stretched. Shape and arrangement of the cells cause
a high mechanic stability with a capacity of 10-12 kg/mm2. This
quality is especially advantageous in growing plant organs. It enables the
collenchyma cells to stretch in synchrony with the other cells without spoiling
the toughness of the tissue. The new state is stabilized by the simultaneous
working-in of additional wall material.
The Sclerenchyma
The other true supporting tissue is the sclerenchyma. Two groups of
sclerenchyma cells exist: fibres and sclereids. Their walls consist of cellulose and/or lignin. Sclerenchyma cells are the
principal supporting cells in plant parts that have ceased elongation.
Sclerenchyma fibres are of great economical importance, since they constitute
the source material for many fabrics (flax, hemp, jute, ramie).
Contrary to the collenchyma,
mature sclerenchyma is composed of dead cells with extremely thick cell walls (secondary walls) that make up to 90% of the
whole cell volume. The term "sclerenchyma" is derived from the Greek
"scleros", meaning "hard". It is their hard, thick walls
that make sclerenchyma cells important strengthening and supporting elements in
plant parts that have ceased elongation. The difference between fibres and
sclereids is not always clear. Transitions do exist, sometimes even within one
and the same plant.
Fibres are generally long, slender, so-called prosenchymatous cells,
usually occuring in strands or bundles. Such bundles or the totality of a
stem's bundles are colloquially called fibres. Their high load-bearing capacity
and the ease with which they can be processed has since antiquity made them the
source material for a number of things, like ropes, fabrics or mattresses. The
fibres of flax (Linum
usitatissimum) have been
known in Europe and Egypt since more than 3000 years, those of hemp (Canabis sativa) in China for just as long. These
fibres, and those of jute (Corchorus
capsularis) and ramie (Boehmeria
nivea, a nettle), are extremely soft and elastic and are especially well
suited for the processing to textiles. Their principal cell wall material is
cellulose.
Contrasting are hard fibres
that are mostly found in monocots. Typical examples are the fibres of many
Gramineae, Agaves (sisal: Agave sisalana), lilies (Yucca or Phormium tenax), Musa textilis
and others. Their cell walls harbour, besides cellulose, a high proportion of
lignin. The load-bearing capacity of Phormium
tenax is as high as 20-25
kg/mm2 and is thus the
same as that of good steel wire (25 kg/ mm2). But the fibre tears as
soon as it is put too great a strain on it, while the wire distorts and tears
not before a strain of 80 kg/mm2. The thickening of a cell wall has
been studied in Linum.
Starting at the centre of the fibre are the thickening layers of the secondary
wall deposited one after the other. Growth at both tips of the cell leads to
simultaneous elongation. During development do the layers of secondary material
seem like tubes, of which the outer one is always longer and older than the
next. After completion of growth the missing parts are supplemented, so that
the wall is evenly thickened up to the tips of the fibres.
Fibres stem usually from meristematic tissues. Cambium and procambium are
their main centers of production. They are often associated with the xylem of
the vascular bundles. The fibres of the xylem are always lignified.
Reliable evidence for the fibre cells' evolutionary origin of tracheids exists.
During evolution the strength of the cell walls was enhanced, the ability to
conduct water was lost and the size of the pits reduced. Fibres that do not
belong to the xylem are bast (outside the ring of cambium) and such fibres that
are arranged in characteristic patterns at different sites of the shoot.
Sclereids are variable in
shape. The cells can be isodiametric, prosenchymatic, forked or fantastically
branched. They can be grouped into bundles, can form complete tubes located at
the periphery or can occur as single cells or small groups of cells within
parenchyma tissues. But compared with most fibres sclereids are relatively
short. Characteristic examples are the stone
cells (called stone cells
because of their hardness) of pears (Pyrus
communis) and quinces (Cydonia oblonga) and those of the shoot of the wax
plant (Hoya carnosa). The
cell walls fill nearly all the cell's volume. A layering of the walls and the
existence of branched pits is clearly visible. Branched pits such as these are
called ramiform pits. The shell of many seeds like those of nuts as well as
the stones of drupes like cherries or plums are made up from sclereids.
Sclerenchyma
The walls of these cells are very thick and built up
in a uniform layer around the entire margin of the cell. Often, the cell dies
after its cell wall is fully formed. Sclerenchyma cells are usually found
associated with other cells types and give them mechanical support.
Sclerenchyma is found in stems and also in leaf veins.
[View] Sclerenchyma also makes up the hard outer covering
of seeds and nuts.
Collenchyma cells have thick walls that are especially
thick at their corners. These cells provide mechanical support for the plant.
They are most often found in areas that are growing rapidly and need to be
strengthened. The petiole("stalk")
of leaves is usually reinforced with collenchyma [View].
Xylem conducts water and dissolved minerals from the
roots to all the other parts of the plant.
|
Link to
discussion of water and mineral transport in the xylem. |
In angiosperms, most of the water travels in the xylem
vessels. These are thick-walled tubes that can extend vertically through
several feet of xylem tissue. Their diameter may be as large as 0.7 mm. Their
walls are thickened with secondary deposits of cellulose and are usually
further strengthened by impregnation with lignin. The secondary
walls of the xylem vessels are deposited in spirals and rings and are usually
perforated by pits. [View]
Xylem vessels arise from individual cylindrical cells
oriented end to end. At maturity the end walls of these cells dissolve away,
and the cytoplasmic contents die. The result is the xylem vessel, a continuous
nonliving duct.
Xylem also contains tracheids. These are
individual cells tapered at each end so the tapered end of one cell overlaps
that of the adjacent cell. Like xylem vessels, they have thick, lignified walls
and, at maturity, no cytoplasm. Their walls are perforated so that water can
flow from one tracheid to the next. The xylem of ferns and conifers contains only tracheids.
In woody plants, the older xylem ceases to participate
in water transport and simply serves to give strength to the trunk. Wood is
xylem. When counting the annual rings of a tree, one is counting rings of xylem
[View].
The main components of phloem are
- sieve
elements and
- companion
cells.
Sieve elements are so-named because their end walls are
perforated. This allows cytoplasmic connections between vertically-stacked
cells. The result is a sieve tube that conducts the products
of photosynthesis — sugars and amino
acids — from the
place where they are manufactured (a "source"), e.g., leaves, to the
places ("sinks") where they are consumed or stored; such as
- roots
- growing tips of stems and leaves
- flowers
- fruits, tubers, corms, etc.
Sieve elements have no nucleus and only a sparse
collection of other organelles. They depend on the adjacent companion cells for
many functions.
Companion cells move sugars, amino acids and a variety of
macromolecules into and out of the sieve elements. In "source"
tissue, such as a leaf, the companion cells use transmembrane proteins to take
up — by active transport — sugars and other organic molecules from the
cells manufacturing them. Water follows by osmosis. These materials then move into adjacent sieve
elements through plasmodesmata. The pressure created by osmosis drives the flow of
materials through the sieve tubes.
SUMMARY
Plant Tissues
Plants are composed of three major
organ groups: roots, stems and leaves. As we know from other areas of biology,
these organs are comprised of tissues working together for a common goal
(function). In turn, tissues are made of a number of cells which are made of
elements and atoms on the most fundamental level. In this section, we will look
at the various types of plant tissue and their place and purpose within a
plant. It is important to realize that there may be slight variations and
modifications to the basic tissue types in special plants.
Plant tissues are characterized and
classified according to their structure and function. The organs that they form
will be organized into patterns within a plant which will aid in further
classifying the plant. A good example of this is the three basic tissue
patterns found in roots and stems which serve to delineate between woody dicot,
herbaceous dicot and monocot plants. We will look at these classifications
later on in the tutorial.
Meristematic Tissues
Tissues where cells are constantly
dividing are called meristems or meristematic tissues. These regions produce
new cells. These new cells are generally small, six-sided boxlike structures
with a number of tiny vacuoles and a large nucleus, by comparison. Sometimes
there are no vacuoles at all. As the cells mature the vacuoles will grow to
many different shapes and sizes, depending on the needs of the cell. It is
possible that the vacuole may fill 95% or more of the cell’s total volume.
There are three types of meristems:
1.
Apical Meristems
2.
Lateral Meristems
3.
Intercalary Meristems
Apical meristems are located at or
near the tips of roots and shoots. As new cells form in the meristems, the
roots and shoots will increase in length. This vertical growth is also known as
primary growth. A good example would be the growth of a tree in height. Each
apical meristem will produce embryo leaves and buds as well as three types of
primary meristems: protoderm, ground meristems, and procambium. These
primary meristems will produce the cells that will form the primary tissues.
Lateral meristems account for
secondary growth in plants. Secondary growth is generally horizontal growth. A
good example would be the growth of a tree trunk in girth. There are two types
of lateral meristems to be aware of in the study of plants.
The vascular cambium, the first type
of lateral meristem, is sometimes just called the cambium. The cambium is a
thin, branching cylinder that, except for the tips where the apical meristems
are located, runs the length of the roots and stems of most perennial plants
and many herbaceous annuals. The cambium is responsible for the production of
cells and tissues that increase the thickness, or girth, of the plant.
The cork cambium, the second type of
lateral meristem, is much like the vascular cambium in that it is also a thin
cylinder that runs the length of roots and stems. The difference is that it is
only found in woody plants, as it will produce the outer bark.
Both the vascular cambium and the
cork cambium, if present, will begin to produce cells and tissues only after
the primary tissues produced by the apical meristems have begun to mature.
Intercalary meristems are found in
grasses and related plants that do not have a vascular cambium or a cork
cambium, as they do not increase in girth. These plants do have apical
meristems and in areas of leaf attachment, called nodes, they have the third
type of meristematic tissue. This meristem will also actively produce new cells
and is responsibly for increases in length. The intercalary meristem is
responsible for the regrowth of cut grass.
There are other tissues in plants
that do not actively produce new cells. These tissues are called
nonmeristematic tissues. Nonmeristematic tissues are made of cells that are
produced by the meristems and are formed to various shapes and sizes depending
on their intended function in the plant. Sometimes the tissues are composed of
the same type of cells throughout, or sometimes they are mixed. There are
simple tissues and complex tissues to consider, but we will start with the
simple tissues for the sake of discussion.
Simple Tissues
There are three basic types, named
for the type of cell that makes up their composition.
1.
Parenchyma cells form parenchyma tissue. Parenchyma cells are the
most abundant of cell types and are found in almost all major parts of higher
plants (we will discuss higher plants later in the tutorial). These cells are
basically sphere shaped when they are first made. However, these cells have
thin walls, which flatten at the points of contact when many cells are packed
together. Generally, they have many sides with the majority having 14 sides.
These cells have large vacuoles and may contain various secretions including
starch, oils, tannins, and crystals. Some parenchyma cells have many
chloroplasts and form the tissues found in leaves. This type of tissue is
called chlorenchyma. The chief function of this type of tissue is
photosynthesis, while parenchyma tissues without chloroplasts are generally
used for food or water storage. Additionally, some groups of cells are loosely
packed together with connected air spaces, such as in water lilies, this tissue
is called aerenchyma tissue. These type of cells can also develop irregular
extensions of the inner wall which increases overall surface area of the plasma
membrane and facilitates transferring of dissolved substances between adjacent
cells. Parenchyma cells can divide if they are mature, and this is vital
in repairing damage to plant tissues. Parenchyma cells and tissues comprise
most of the edible portions of fruit.
2.
3.
Collenchyma cells form collenchyma tissue. These cells have a living protoplasm, like parenchyma cells, and may also
stay alive for a long period of time. Their main distinguishing difference from
parenchyma cells is the increased thickness of their walls. In cross section,
the walls looks uneven. Collenchyma cells are found
just beneath the epidermis and generally they are elongated and their walls are
pliable in addition to being strong. As a plant grows these cells and the
tissues they form, provide flexible support for organs such as leaves and
flower parts. Good examples of collenchyma plant cells are the ‘strings’ from
celery that get stuck in our teeth.
4.
Sclerenchyma cells form sclerenchyma tissue. These cells have
thick, tough secondary walls that are imbedded with lignin. At maturity, most
sclerenchyma cells are dead and function in structure and support. Sclerenchyma
cells can occur in two forms:
1.
Sclereids
are sclerenchyma cells that are randomly distributed throughout other tissues.
Sometimes they are grouped within other tissues in specific zones or regions.
They are generally as long as they are wide. An example,
would be the gritty texture in some types of pears. The grittiness is due to
groups of sclereid cells. Sclereids are sometimes called stone cells.
2.
Fibers
are sometimes found in association with a wide variety of tissues in roots,
stems, leaves and fruits. Usually fiber cells are much longer than they are
wide and have a very tiny cavity in the center of the cell. Currently, fibers
from over 40 different plant families are used in the manufacture of textiles,
ropes, string and canvas goods to name a few.
Secretory Cells and Tissues
As a result of cellular processes,
substances that are left to accumulate within the cell can sometimes damage the
protoplasm. Thus it is essential that these materials are either isolated from
the protoplasm in which they originate, or be moved outside the plant body.
Although most of these substances are waste products, some substances are vital
to normal plant functions. Examples: oils in citrus, pine resin, latex, opium,
nectar, perfumes and plant hormones. Generally, secretory cells are derived
from parenchyma cells and may function on their own or as a tissue. They
sometimes have great commercial value.
Complex Tissues
Tissues composed of more than one
cell type are generically referred to as complex tissues. Xylem and phloem are
the two most important complex tissues in a plant, as their primary functions
include the transport of water, ions and soluble food substances throughout the
plant. While some complex tissues are produced by apical meristems, most in
woody plants are produced by the vascular cambium and is often referenced as
vascular tissue. Other complex tissues include the epidermis and the periderm.
The epidermis consists primarily of parenchyma-like cells and forms a
protective covering for all plant organs. The epidermis includes specialized
cells that allow for the movement of water and gases in and out of the plant,
secretory glands, various hairs, cells in which crystals are accumulated and
isolated, and other cells that increase absorption in the roots. The
periderm is mostly cork cells and therefore forms the outer bark of woody
plants. It is considered to be a complex tissue because of the pockets of
parenchyma cells scattered throughout.
Xylem
Xylem is an important plant tissue as
it is part of the ‘plumbing’ of a plant. Think of bundles of pipes
running along the main axis of stems and roots. It carries water and dissolved
substances throughout and consists of a combination of parenchyma cells,
fibers, vessels, tracheids and ray cells. Long tubes made up of
individual cells are the vessels, while vessel members are open at each end.
Internally, there may be bars of wall material extending across the open space.
These cells are joined end to end to form long tubes. Vessel members and
tracheids are dead at maturity. Tracheids have thick secondary cell walls and
are tapered at the ends. They do not have end openings such as the vessels. The
tracheids ends overlap with each other, with pairs of pits present. The pit
pairs allow water to pass from cell to cell. While most conduction in the xylem
is up and down, there is some side-to-side or lateral conduction via rays. Rays
are horizontal rows of long-living parenchyma cells that arise out of the
vascular cambium. In trees, and other woody plants, ray will radiate out from
the center of stems and roots and in cross-section will look like the spokes of
a wheel.
Phloem
Phloem is an equally important plant
tissue as it also is part of the ‘plumbing’ of a plant. Primarily, phloem
carries dissolved food substances throughout the plant. This conduction system
is composed of sieve-tube member and companion cells, that are without
secondary walls. The parent cells of the vascular cambium produce both
xylem and phloem. This usually also includes fibers, parenchyma and ray cells.
Sieve tubes are formed from sieve-tube members laid end to end. The end walls,
unlike vessel members in xylem, do not have openings. The end walls, however,
are full of small pores where cytoplasm extends from cell to cell. These porous
connections are called sieve plates. In spite of the fact that their cytoplasm
is actively involved in the conduction of food materials, sieve-tube members do
not have nuclei at maturity. It is the companion cells that are nestled between
sieve-tube members that function in some manner bringing about the conduction
of food. Sieve-tube members that are alive contain a polymer called callose.
Callose stays in solution as long at the cell contents are under pressure. As a
repair mechanism, if an insect injures a cell and the pressure drops, the
callose will precipitate. However, the callose and a phloem protein will be
moved through the nearest sieve plate where they will for a plug. This prevents
further leakage of sieve tube contents and the injury is not necessarily fatal
to overall plant turgor pressure.
Epidermis
|
The epidermis
is a dermal tissue that is usually a single layer of cells covering the
younger parts of a plant. It secretes a waxy layer called the cuticle that
inhibits water loss. |
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Epidermal
hairs lower water loss by decreasing the flow of air over the plant surface,
which in turn, slows the loss of water from the plant. |
Glandular
hairs prevent herbivory by storing substances that are harmful to insects. |
Root hairs
increase water uptake by increasing the surface area of the cell. |
|
In older
stems and roots, the epidermis may be replaced by the periderm, which
provides protection while permitting gas exchange.
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The epidermis is also a complex plant
tissue, and an interesting one at that. Officially, the epidermis is the
outermost layer of cells on all plant organs (roots, stems, leaves). The
epidermis is in direct contact with the environment and therefore is subject to
environmental conditions and constraints. Generally, the epidermis is one cell
layer thick, however there are exceptions such as tropical plants where the
layer may be several cells thick and thus acts as a sponge. Cutin, a fatty
substance secreted by most epidermal cells, forms a waxy protective layer
called the cuticle. The thickness of the cuticle is one of the main
determiners of how much water is lost by evaporation. Additionally, at no extra
charge, the cuticle provides some resistance to bacteria and other disease
organisms. Some plants, such as the wax palm, produce enough cuticle to have
commercial value: carnauba wax. Other wax products are used as polishes,
candles and even phonographic records. Epidermal cells are important for
increasing absorptive surface area in root hairs. Root hairs are essentially
tubular extensions of the main root body composed entirely of epidermal cells.
Leaves are not left out. They have many small pores called stomata that are
surrounded by pairs of specialized epidermal cells called guard cells. Guard
cells are unique epidermal cells because they are of a different shape and
contain chloroplasts. They will be discussed in detail later on in the
tutorial. There are other modified epidermal cells that may be glands or hairs
that repel insects or reduce water loss.
Periderm
In woody plants, when the cork
cambium begins to produce new tissues to increase the girth of the stem or root
the epidermis is sloughed off and replaced by a periderm. The periderm is made
of semi-rectangular and boxlike cork cells. This will be the outermost layer of
bark. These cells are dead at maturity. However, before the cells die,
the protoplasm secretes a fatty substance called suberin into the cell walls.
Suberin makes the cork cells waterproof and aids in protecting tissues beneath
the bark. There are parts of the cork cambium that produce pockets of
loosely packed cork cells. These cork cells do not have suberin imbedded in
their cell walls. These loose areas are extended through the surface of the
periderm and are called lenticels. Lenticels function in gas exchange between
the air and the stem interior. At the bottom of the deep fissures in tree bark
are the lenticels.
Use
information from the table to answer the questions below it.
Literature
1. Botany / Randy Moore, W.Denis
Clark, Kingsley R.Stern, Darrell Vodopich. - Dubuque, IA, Bogota, Boston, Buenos
Aires, Caracas,Chicago, Guilford, CT, London, Madrid, Mexico City, Sydney, Toronto: Wm.C.Brown Publishers.- 1994.-
2.
Kindsley R. Stern. Introductory plant biology-
3. Gulko R.M. Explanatory Dictionary of Medicinal Botany-
Lviv: LSMU, 2003.-200 p.
4. ßêîâëåâ
Ã.Ï., ×åëîìáèòüêî Â.À. Áîòàíèêà: Ó÷åá. äëÿ ôàðìàö. èíñòèòóòîâ è ôàðìàö. ôàê.
ìåä. âóçîâ / Ïîä ðåä. È.Â. Ãðóøâèöêîãî. – Ì.: Âûñø. øê., 1990. – 367 ñ.
5. Âàñèëüåâ À.Å., Âîðîíèí Í.Ñ., Åëåíåâñêèé À.Ã.,
Ñåðåáðÿêîâà Ò.È. Áîòàíèêà. Àíàòîìèÿ è ìîðôîëîãèÿ ðàñòåíèé. – Ì.: Ïðîñâåùåíèå,
1978. – 478 ñ.
6. Òêà÷åíêî
H.M., Cep6ií A..Ã .
Áîòàí³êà.- Xapê³â: Ocíîâa, 1997.
Prepared by ass. prof. Shanayda M.I.