STUDY OF DIFFERENT TISSUE SYSTEMS
The morphologically most easily and clearly recognizable units of the plant
body are the cells. The united masses of cells are distinct from one another
structurally as well as functionally. Such groupings of cells may be referred to
as tissues which further may develop into a simpler or complex cellular
organization.
The arrangement of various tissues or tissue systems in the plant indicates its
specialized nature. For example, vascular tissues are mainly concerned with the
conduction of food and water, and for the efficient functioning; a complex
network is developed with the places of water intake, sites of food synthesis and
with areas of growth, development and storage. In the same way nonvascular
tissues are also continually arranged which indicates the specific
interrelationship of vascular tissues, storage tissues and supportive tissues. Plant
tissues are generally categorized in to two categories.
Ground Tissues
Vascular Tissues
Dermal tissues
Differences between Meristematic and Permanent tissues
In the plant body, the following three tissue systems can be distinguished.
a) Dermal tissue system: It represents the outer most part of the plant
which forms a protective covering line. It includes epidermis, periderm,
etc.
b) Vascular tissue system: It is concerned with trans-mission of material in
the plant and represents stelar structures like xylem and phloem.
c) Ground tissue system: It consists of simple cells which may be
strengthened by addition of thickened cells. It represents ground tissue
made up of parenchyma, collenchyma and sclerenchyma.
Dermal Tissue System
Epidermis
The epidermal tissue system is derived from the dermatogen of the apical
meristem and forms the epidermis (epi - upon, derma - skin) or outermost skin
layer, which extends over the entire surface of the plant body. Epidermis is the
outermost layer of the plant consisting normally of a single layer of flattened
cells. The walls may be straight, wavy or beaded and often covered with a layer
of cuticle made up of cutin.
Functions
1. The primary function of the epidermis is protection of the internal
tissues against mechanical injury, excessive heat or cold, fluctuations of
temperature, attacks of parasitic fungi and bacteria, and against the
leaching effect of rain. This is possible due to the presence of cuticle,
hairs, tannin, gum, etc.
2. Prevention of excessive evaporation of water from the internal tissues by
the development of thick cuticles, wax and other deposition, cutinized
hairs, scales, multiple epidermis, etc., is another important function of the
epidermis.
3. Strong cuticles and cutinized hairs, particularly a dense coating of hairs,
protect the plant against intense illumination (i.e. strong sunlight) and
excessive radiation of heat.
4. The epidermis also acts as a storehouse of water, as in desert plants.
5. The epidermis sometimes has some minor functions like photosynthesis,
secretion, etc.
Stomata
Stomata are minute openings usually found in the epidermis of the leaves as in
Digitalis, Senna, etc., or in young green stems as in Ephedra, in flower as in
clove and in fruit as in fennel, orange peel. These openings are surrounded with
a pair of kidney-shaped cells called guard cells. The term ‘stoma’ is often
applied to the stomatal arrangement, which consists of slit like opening along
with the guard cells. The epidermal cells surrounding the guard cells are called
neighbouring cells or subsidiary cells. These, in many cases, as in Digitalis
resemble the other epidermal cells, but in large number of plants they differ in
size, arrangement and shape from the other epidermal cells.
Types of stomatal arrangement: According to the arrangement of the
epidermal cells surrounding the stomata, they have been grouped as follows:
1. Diacytic or Caryophyllaceous (cross celled): The stoma is accompanied
by two subsidiary cells, the long axis of which is at right angles to that of
the stoma. This type of stoma is also, called the Labiatae type as it is
found in many plants of the family Labiatae such as vasaka, tulsi,
spearmint and peppermint.
2. Anisocytic or Cruciferous (unequal celled): The stoma is surrounded
by usually three subsidiary cells of which one is markedly smaller than
the others. This type of stoma is also called the Solanaceous type as it is
found in many plants of the family Solanaceae, such as Belladonna,
Datura, Hyoscyamus, Stramonium, Tobacco; it is also found in many
plants of the family Compositae.
3. Anomocytic or Ranunculaceous (irregular celled): The stoma is
surrounded by a varying number of cells in no way differing from those
of the epidermal cells as in Digitalis, eucalyptus, henna, lobelia, neem,
etc.
4. Paracytic or Rubiaceous (parallel celled): The stoma is surrounded
usually by two subsidiary cells, the long axis of which are parallel to that
of stoma as in Senna and many Rubiaceous plants.
5. Actinocytic (radiate celled): The stoma is surrounded by circle of
radiating cells, as in Uva ursi.
Functions and distributions of stomata: Stomata perform the function of
gaseous exchange and transpiration in the plant body. They are most abundant
in the lower epidermis of a dorsiventral leaf and less abundant on the upper
epidermis. In isobilateral leaves, stomata remain confined to the upper
epidermis alone; in submerged leaves no stoma is present. In Buchu and Neem,
stomata are present only on lower surface, while in case of Belladonna, Datura,
Senna, etc.; stomata are present on the both surfaces. The distribution of stoma
shows great variation between upper and lower epidermis. In desert plants and
in those showing xerophytic adaptations, e.g. Ephedra, Agave, Oleander, etc.,
stomata are situated in grooves or pits in the stem or leaf. This is a special
adaptation to reduce excessive evaporation, as the stomata sunken in pits are
protected from gusts of wind.
Trichomes
Trichomes are more elongated outgrowths of one or more epidermal cells, and
consist of two carts, a foot or root embedded in the epidermis and a free
projecting portion termed as body. Trichomes usually occur in leaves but are
also found to be present on some other parts of the plant as in Kurchi, Nux
vomica and Strophanthus seeds, Andrographis and Belladonna stem, Cummin,
and Lady’s finger fruits, etc. Trichomes are rarely present on the leaves of
Bearberry, Buchu, Henna, etc., and are absent in glabrous leaves like Coca,
Hemlock, Savin, etc.
Functions of trichomes: Trichomes or hairs are adapted to many different
purposes. A dense covering of trichomes prevents the damage by insects and the
clogging of stomata due to accumulation of dust. Trichomes also aid the
dispersion of seeds of Milkweed (Asclepias) and Madar (Calotropis), which are
readily scattered by wind. In Peppermint, Rosemary, Tulsi, etc., trichomes
perform the function of secreting volatile oil.
Types of trichomes: Broadly, the trichomes are classified as:
1. Covering trichomes or clothing hairs or nonglandular trichomes and
2. Glandular trichomes
Depending upon the structure, shape and number of cells, they are further
classified as follows:
[A] Covering trichomes
a) Unicellular trichomes
1) Linear, strongly waved, thick walled trichomes— Yerba santa
2) Linear, thick walled and warty trichomes— Damiana
3) Short. conical trichomes—Tea
4) Short, conical, warty trichomes—Senna
5) Large, conical, longitudinally striated trichomes— Lobelia
6) Long, tubular, flattened and twisted trichomes— Cotton
7) Lignified trichomes—Nux vomica, strophanthus
8) Short, sharp, pointed, curved, conical trichomes— Cannabis
9) Unicellular, stellate trichomes—Deutezia scabra
b) Multicellular unbranched trichornes
1) Uniseriate, bicellular, conical—Datura
2) Biseriate—Calendula officinalis
3) Multiseriate—Male fern
c) Multicellular branched trichomes
1. Stellate (star shaped)—Hamamelis, Kamala
2. Peltate (shield-like structure)—cascarilla
3. Candelebra (branched)—Rosemary, Verbascum thapsus
4. T-shaped trichomes—Pyrethrum
Covering trichomes
[B] Glandular trichomes
a) Unicellular glandular trichomes
1. Sessile trichomes—Without stalk - Piper betel, Vasaka
b) Multicellular glandular trichomes
1) Unicellular stalk with single spherical secreting cell at the apex—
Digitalis purpurea
2) Uniseriate, multicellular stalk with single spherical cell at the apex—
Digitalis thapsi
3) Uniseriate stalk and bicellular head—Digitalis purpurea
4) Multicellular, uniseriate stalk and multicellular head—Hyoscyamus
5) Biseriate stalk and biseriate secreting head— Santonica
6) Short, unicellular stalk and head formed by a rosette of two to eight clubshaped cells—Mentha
7) Multiseriate, multicellular cylindrical stalk and a secreting head of about
eight radiating club-shaped cells—Cannabis
Glandular
trichomes
Periderm
In the stem and root of mature plant, the layers immediately below the
epidermis (phellogen) divide and redivide. On the outside they form cork or
phellem and on the inner side they form phelloderm.
Phellem + Phellogen + Phelloderm = Periderm
Periderm
The cork cells are rectangular brick shaped or polygonal; phelloderm cells are
mostly parenchymatous in nature. Lenticels are present in the periderm,
especially in the bark of old plants which are similar in function to stomata.
These are open pores with absence of guard cells. The cork cells are
impregnated with a layer of suberin. The various types of cork cells are shown
bellow.
Various types of cork cell
Vascular Tissue System
This system consists of a number of vascular bundles which are distributed in
the stele. The stele is the central cylinder of the stem and the root surrounded by
the endodermis. It consists of vascular bundles, pericycle, pith and medullary
rays. Each bundle is made up of xylem and phloem, with a cambium in
dicotyledonous stems, or without a cambium in monocotyledonous stems, or
only one kind of tissue xylem or phloem, as in roots.
Function
The function of this system is to conduct water and raw food material from the
roots to the leaves, and prepared food material from leaves to the storage organs
and the growing regions.
The vascular bundle of a dicotyledonous stem, when fully formed, consists of
three well-defined tissues:
1.
Xylem or wood
2.
Phloem or bast, and
3.
Cambium.
[1] XYLEM
Xylem or wood is a conducting tissue and is composed of elements of different
kinds, viz. (a) tracheids, (b) vessels or tracheae, (c) wood fibres and (d) wood
parenchyma. Xylem, as a whole, is meant to conduct water and mineral salts
upwards from the root to the leaf to give mechanical strength to the plant body.
(a) Tracheids: These are elongated, tube-like cells with hard, thick and
lignified walls and large cell cavities. Their ends are tapering, either rounded or
chisel-like and less frequently, pointed. They are dead, empty cells and their
walls are provided with one or more rows of bordered pits. Tracheids may also
be annular, spiral, scalariform or pitted (with simple pits). In transverse section,
they are angular— either polygonal or rectangular. Tracheids (and not vessels)
occur alone in the wood of ferns and gymnosperms, whereas in the wood of
angiosperms, they are associated with the vessels. Their walls being lignified
and hard, their function is conduction of water from the root to the leaf.
(a) Tracheids with bordered pits (b) Scalariform tracheid
(b) Vessels or tracheae: Vessels are cylindrical, tube-like structures. They are
formed from a row of cells placed end to end, from which the transverse
partition walls break down. A vessel or trachea is, thus, a tube-like series of
cells, very much like a series of water pipes forming a pipeline. Their walls are
thickened in various ways, and vessels can be annular, spiral, scalariform,
reticulate, or pitted, according to the mode of thickening. Associated with the
vessels are often some tracheids. Vessels and tracheids form the main elements
of the wood or xylem of the vascular bundle. They serve to conduct water and
mineral salts from the roots to the leaves. They are dead, thick-walled and
lignified, and as such, they also serve the mechanical function of strengthening
the plant body.
Different kinds of vessels
(c) Xylem (wood) fibres: Sclerenchymatous cells associated with wood or
xylem are known as wood fibres. They occur abundantly in woody dicotyledons
and add to the mechanical strength of the xylem and of the plant body as a
whole.
(d) Xylem (wood) parenchyma: Parenchymatous cells are of frequent
occurrence in the xylem, and are known as wood parenchyma. The cells are
alive and generally thin walled. The wood parenchyma assists, directly or
indirectly, in the conduction of water, upwards, through the vessels and the
tracheids. It also serves to store food.
[2] PHLOEM
The phloem or bast is another conducting tissue, and is composed of the
following elements: (a) sieve tubes, (b) Companion cells, (c) phloem
parenchyma and (d) bast fibres (rarely). Phloem, as a whole, is meant to conduct
prepared food materials from the leaf to the storage organs and growing regions.
(a) Sieve tubes: Sieve tubes are slender, tube-like structures, composed of
elongated cells which are placed end to end. Their walls are thin and made of
cellulose. The transverse partition walls are, however, perforated by a number
of pores. The transverse wall then looks very much like a sieve, and is called the
sieve plate. The sieve plate may sometimes be formed in the side (longitudinal)
wall. In some cases, the sieve plate is not transverse (horizontal), but inclined
obliquely, and then different areas of it become perforated. A sieve plate of this
nature is called a compound plate. At the close of the growing season, the sieve
plate is covered by a deposit of colourless, shining substance in the form of a
pad, called the callus or callus pad. This consists of carbohydrate, called
callose. In winter, the callus completely clogs the pores, but in spring, when the
active season begins, it gets dissolved. In old sieve tubes, the callus forms a
permanent deposit. The sieve tube contains no nucleus, but has a lining layer of
cytoplasm, which is continuous through the pores. Sieve tubes are used for the
longitudinal transmission of prepared food materials—proteins and
carbohydrates—downward from the leaves to the storage organs, and later
upward from the storage organs to the growing regions. A heavy deposit of food
material is found on either side of the sieve plate with a narrow median portion.
A sieve tube in longitudinal section
(b) Companion cells: Associated with each sieve lube and connected with it by
pores is a thin-walled, elongated cell known as the companion cell. It is living
and contains protoplasm and an elongated nucleus. The companion cell is
present only in angiosperms (both dicotyledons and monocotyledons). It assists
the sieve tube in the conduction of food.
(c) Phloem parenchyma: There are always some parenchymatous cells
forming a part of the phloem in all dicotyledons, gymnosperms and ferns. The
cells are living, and often cylindrical. They store up food material and help to
conduct it. Phloem parenchyma is, however, absent in most monocotyledons.
(d) Bast fibres: Sclerenchymatous cells occurring in the phloem or bast are
known as bast fibres. These are generally absent in the primary but occur
frequently in the secondary phloem.
[3] CAMBIUM
This is a thin strip of primary meristem lying between the xylem and phloem. It
consists of one or a few layers of thin-walled and roughly rectangular cells.
Although cambial cells look rectangular in transverse section, they are very
elongated, often with oblique ends. They become flattened tangentially, i.e. at
right angles to the radius of the stem.
Types of Vascular Bundles
According to the arrangement of xylem and phloem, the vascular bundles are of
the following types:
(A) Radial vascular bundle: When the xylem and phloem form separate
bundles which lie on different radii, alternat-ing with each other, as in roots.
The radial vascular bundle is the most primitive type of vascular bundles.
(B) Conjoint vascular
bundle: When the xylem and phloem combine into one bundle, it is called as
conjoint vascular bundle. There are different types of conjoint vascular
bundles.
(1) Collateral: When the xylem and phloem lie together on the same radius, the
xylem being internal and the phloem external is called collateral. When
cambium is present in collateral as in all dicotyledonous stems, the bundle is
said to be open collateral, and when the cambium is absent, it is said to be
closed collateral, as in monocotyledonous stems
(2) Bicollateral:
When the both phloem and cambium occur twice in a collateral bundle—once
on the outer side of the xylem and again on the inner side of it, is called as
bicollateral. The sequence is outer phloem, outer cambium, xylem, inner
cambium and inner phloem. Bicollateral bundles are characteristics of
Cucurbitaceae. They are also often found in Solanaceae, Apocynaceae,
Convolvulaceae, Myrtaceae, etc. A bicollateral bundle is always open.
(C) Concentric vascular bundle: When one kind of vascular tissue (xylem or
phloem) is surrounded by the other is called as concentric vascular bundle.
Evidently, there are two types, according to whether one is central or the other
one is so. When the phloem lies in the centre and is surrounded by xylem, as in
some monocotyledonous, the concentric bundle is said to be amphivasal
(leptocentric). When, on the other hand, the xylem lies in the centre and is
surrounded by phloem, the concentric bundle is said to be amphicribral
(Hadrocentric). A concentric bundle is always closed.
Ground
Tissue
System
Ground
tissue
system is represented by the cortex, hypo-dermis, pith, mesophyll and portion of
midrib of leaves and comprises of the following tissues.
(a) Parenchyma
The parenchyma consists of a collection of cells which are more or less
isodiametric, that is, equally expanded on all sides. Typical parenchymatous
cells are oval, spherical or polygonal. Their walls are thin and made of
cellulose. They are usually living. Parenchymatous tissue is of universal
occurrence in all the soft parts of plants. Its main function is storage of food
material. When parenchymatous tissue contains chloroplasts, it is called chlorenchyma. Its function is to manufacture food material. A special type of
parenchyma develops in many aquatic plants and in the petiole of banana. The
wall of each such cell grows out in several places, like rays radiating from a star
and is, therefore, stellate or star-like in general appearance. These cells leave a
lot of air cavities between them, where air is stored up. Such a tissue is often
called aerenchyma.
(a) Parenchyma, (b) Chlorenchyma and (c) Aerenchyma
(b) Collenchyma
This tissue consists of somewhat elongated, parenchymatous cells with oblique,
slightly rounded or tapering ends. The cells are much thickened at the corners
against the intercellular spaces. They look circular, oval or polygonal in a
transverse section of the stem. The thickening is due to a deposit of cellulose,
hemicellulose and protopectin. Although thickened, the cells are never lignified.
Simple pits can be found here and there in their walls. Their thickened walls
have a high refractive index and, therefore, this tissue in section is very
conspicuous under the micro-scope. Collenchyma is found under the skin
(epidermis) of herbaceous dicotyledons, e.g. sunflower, gourd, etc., occurring
there in a few layers with special development at the ridges, as in gourd stem. It
is absent from the root and the monocotyledon, except in special cases. The
cells are living and often contain a few chloroplasts. Being flexible in nature,
collenchyma gives tensile strength to the growing organs, and being extensible,
it readily adapts itself to rapid elongation of the stem. Since it contains
chloroplasts, it also manufactures sugar and starch. Its function is, therefore,
both mechanical and vital.
(a) Collenchyma in transaction and (b) Collenchyma in longitudinal
section
(c) Sclerenchyma
Sclerenchyma (scleros means hard) consists of very long, narrow, thick and
lignified cells, usually pointed at both ends. They are fibre-like in appearance
and hence, they are also called sclerenchymatous fibres, or simply fibres. Their
walls often become so greatly thickened that the cell cavity is nearly obliterated.
They have simple, often oblique, pits in their walls. The middle lamella is
conspicuous in sclerenchyma. They are dead cells and serve a purely
mechanical function, i.e. they give the requisite strength, rigidity, flexibility and
elasticity to the plant body and thus enable it to withstand various strains.
Sclereids: Sometimes, special types of sclerenchyma develop in various parts
of the plant body to meet local mechanical needs. They are known as Sclereids
or Stone cells. They may occur in the cortex, pith, phloem, hard seeds, nuts,
stony fruits, and in the leaves and stems of many dicotyledons and also
gymnosperms. The cells, though very thick-walled, hard and strongly lignified
(sometimes cutinized or suberized), are not long and pointed like sclerenchyma,
but are mostly isodiametric, polyhedral, short-cylindrical, slightly elongated, or
irregular in shape. Usually, they have no definite shape. They are dead cells,
and have very narrow cell cavities, which may be almost obliterated, owing to
excessive thickness of the cell wall. They may be somewhat loosely arranged or
closely packed. They may also occur singly. They contribute to the firmness and
hardness of the part concerned.
(a) Sclerenchymatous fi bres and (b) Sclereids (Stone cells)
Cell Differentiation and Development
Cell differentiation is only part of the larger picture of plant development. As
plant organs develop (the process of organogenesis), the precursors of the tissue
systems form in response to positional signals. Then, within each tissue system
precursor, cell types must be specified in the proper spatial pattern. For
instance, the spacing of trichomes and stomates within the protoderm must be
specified before their precursor cells begin differentiation. Exchange of signals
among neighboring cells is an important aspect of the processes of spatial
patterning and cell differentiation. In addition, long distance signals are required
so that the strands of xylem and phloem cells within the leaf vascular bundles
connect perfectly with those in the stem.
Examples of Cell Differentiation
Trichomes: The distinctive branched unicellular trichomes of plants such as
Arabidopsis differentiate from undistinguished precursor cells in the protoderm.
These precursor cells initiate the differentiation pathway by undergoing
deoxyribonucleic acid (DNA) synthesis without accompanying cytokinesis, so
that trichome precursors typically have eight or sixteen times the amount of
DNA of adjacent pavement cells. Next, trichome precursors begin cell
expansion in the plane perpendicular to the epidermis, forming a tubular
extension. Once this stalk is formed, the nucleus migrates from the base of the
stalk to its tip, using the cell's cytoskeleton to pull it to a new location. The
trichome then undergoes an unusual pattern of cell wall growth, in which the
cell wall balloons out at three locations, forming the
Trichrome differentiation.
three trichome branches. When the trichome cell has reached its full size and
shape, it adds thickness to its cell wall and deposits sharp crystals of calcium
oxalate on the surface of the trichome, adding to its effectiveness in defense
against herbivores (see Figure above).
Vessel Elements. Vessel elements differentiate from cells of the procambium.
Vessel elements are first differentiated from other procambial cells because they
expand more than their neighbors. Vessel element precursors next begin to
deposit the thickened, lignified parts of their cell walls in the ringlike, helical,
netlike, or pitted pattern. The pattern can be predicted by the location of
elements of the cytoskeleton within the cytoplasm that help guide wall
precursor to the proper location. When cell wall synthesis is complete, special
wall-degrading enzymes attack the end walls of the cell, forming the perforation
between adjacent elements in a vessel. Finally, the vessel elements undergo
programmed cell death. The cell makes protease enzymes and nuclease
enzymes that reduce proteins and nucleic acids to their simple building blocks.
Surrounding parenchyma cells absorb these small molecules, leaving an empty
vessel (see Figure below).
Vessel element differentiation.
Bundle Sheath Cells. In most plants, the cells of the photosynthetic ground
tissue are uniform in size, shape, and chloroplast development. Two types of
photosynthetic parenchyma cells are sharply differentiated in plants that have
the C4 photosynthetic pathway, however. These two cell types, the mesophyll
and bundle sheath cells, begin differentiation as similar appearing ground
meristem cells. During leaf expansion, the bundle sheath cells begin to enlarge
first. The cell wall becomes thickened and impermeable to the diffusion of
gases. Their plastids replicate, grow, and become asymmetrically placed within
the cell. In contrast, the mesophyll cells undergo a minimal amount of
enlargement and have thin, permeable cell walls. The number of plastids is low
and the plastids remain small. During cell differentiation the genes encoding the
enzymes of the C4 biochemical pathway are expressed exclusively in the
mesophyll cells, whereas the genes encoding the enzymes of the C3 pathway
are expressed only in the bundle sheath cells (see Figure below).
Bundle sheath cell differentiation
Hormonal Influences
Many aspects of differentiation
are controlled by hormones. The
hormone auxin, for example, plays an important role in the differentiation of
vessel elements, both in intact and wounded plants. This role was first
demonstrated in experiments where small incisions were made in stem
internodes that cut though the phloem and xylem of a single vascular bundle.
Auxin produced by the apical meristem and young leaves above the wound
induces parenchyma cells to regenerate the damaged vascular tissue.
Parenchyma cells undergo transdifferentiation.
Although they already had differentiated as parenchyma cells from ground
meristem precursors, they now repeat the steps that procambial cells take when
they differentiate as vessel elements. Cells are induced to do this in a chainlike
pattern, so that a new continuous strand of vascular tissue is formed as a detour
around the original incision. Scientists know that auxin is involved, since
transdifferentiation is blocked when the sources of natural auxin (young leaves
and buds) are removed or when auxin transport inhibitors are applied. If natural
sources of auxin are removed, and artificial sources added, transdifferentiation
of parenchyma cells will occur, regenerating the vascular bundle.