Apical Meristem Organization

Introduction

The apical meristem includes the meristematic initials and their immediate derivatives at the apex of

a shoot or root. The apical meristem, thus corresponds approximately to the promeristem, and to

contrast with the partly developed derivatives of the promeristem, i.e., the protoderm, the ground

meristem, and the procambium. Apical meristem as consisting of the initiating cells only because

cells may be poorly differentiated from their most recent derivatives.

The terms shoot apex and root apex are more convenient to use instead of apical meristem of the

shoot and apical meristem of the root, respectively. In the similar way, the terms shoot apex and

root apex are more conveniently used as the substitutes of growing points.

Growth in the sense of cell division, which is characteristic of the meristematic state, is not restricted

to the so-called growing point but occurs abundantly — and may be even more intense — at some

distance from the apical meristem (Wardlaw, 1945; Goodwin and Stepka, 1945).

On the other hand, growth in the sense of increase in size of cells, tissues, and organs is most

pronounced, not in the apical meristem, but in its derivatives.

Initials and Derivatives of Apical Meristems:

An initial or initiating cell is a cell that remains within the meristem indefinitely with the addition of

cells to the plant body by combining self-perpetuation. The concept regarding meristematic initials

implies that a cell is an initial, not because of its inherent characteristics, but simply because of its

particular position in the meristem, a position that cannot be treated as permanent.

The number of initials in root and shoot apices is variable. In most of pteridophytes a single initial

cell occurs at the apex. In the lower vascular plants, as well as in the higher, several initials are

present. The single initial in its morphology is quite distinct from its derivatives and is commonly

known as the apical cell. If the initials are numerous, they are called apical initials.

Apical Meristems –Vegetative Shoot Apex:

Regions of Shoot Apex

Wardlaw (1957) has recognised five regions in the shoot apex. These are: -

1. Distal region- A group of initial cells arranged in one or more layers.

2. Sub- distal region- This represents the superficial layers of meristematic cells where growth

centres are situated.

3. Organogenic region- Region of leaf initiation and tissue differentiation

4. Sub apical region- This is characterized by continual cell division, cell elongation and cell

differentiation.

5. Region of Maturation- This represents the matured tissues having no division or

differentiation.

Types of Shoot Apices

According to Newman (1965), a group of permanent initial cells are always present in the shoot

apex. He used the term Continuing meristematic residue to refer to these cells. On the basis of

this meristem , Newman (1965) classified the shoot apex into the following types-

Monoplex type

This type of shoot apex is found in pteridophytes such as ferns. Here, the meristem is

represented by a single layer which is pointed inwardly and a single division is required for

growth.

Simplex type

This type is normally found in gymnosperms and is characterized by a parallel sided meristematic

residue limited to the superficial layer.

Duplex type

This is found in angiosperm. Here the meristematic residue in parallel sided but presents in at

least two layers superficial layer which divides anticlinally and inner layer dividing anticlinal as

well as periclinally.

The vegetative shoot apices vary in shape, size and cytohistologic structure, and in their relation to

the lateral organs. The shoot apex of Pinus and other conifers are commonly narrow and conical in

form. In Cycas (cycads) and Ginkgo they are usually broad and flat on the other hand the apical

meristem of a grass and some other monocotyledons remains elevated above the youngest leaf

primordium.

In many dicotyledons the apical meristem rises above the primordia, and in other cases it appears to

be sunken beneath them. The diameters of apices range from 90µ in some angiosperms to 3.5 mm

in Cycas revoluta (Foster, 1949). The size and shape of the apex marked by change during the plant

development.

Angiosperms:

There is tunica- corpus organization in the shoot apex of angiosperms. One to five layers of tunica

have been observed in the dicotyledons, and one to three layered in the monocotyledons. However,

tunica-corpus organiza­tion is not found in Saccharum officinarum.

To draw a clear cut de­marcation line in between tunica and corpus is not simple matter. In

ang­iosperms, the number of parallel periclinal layers in the shoot apex may vary during the

ontogeny of the plant body and under the influence of seasonal growth changes.

In the angiosperms the segre­gation of apical-meristem zones is more definite than in lower groups.

There are two sets of initials, one above the other, which give rise to tunica and corpus. The tunica

has no or only rare periclinal divisions and ranges in thickness from several layers to one with two or

three layers probably most frequent. The number of layers in the tunica may vary even in an

individual plant.

Theories of Structural Development and Differentiation ofshoot Apical Meristems:

By several workers ,the view regarding the number, the arrangement, and the activity of the initial

cells and their derivatives in the apical meristems has undergone many changes since the shoot apex

was first recognized by Wolff in 1759 as an undeveloped region from which the growth of the plant

proceeded.

The Apical Cell Theory:

This theory was put forth by Nageli in 1858. Solitary apical cells occur in many of algae,

bryophytes and vascular cryptogams (pteridophytes).

The discovery of the apical cell in cryptogams led to the concept that such cells exist in

phanerogams (seed plants) as well. The apical cell was interpreted as a constant structural

and functional unit of apical meristems governing the whole process of growth.

However, this was confirmed by later researches that this theory may hold good for

cryptogams but is not applicable to the phanerogams. The apical cell theory was super-

ceded by the histogen theory.

The Histogen Theory:

This was introduced byHanstein(1870)who considered that the primordial meristem was

sharply separable into three distinct zones or histogens. Acc­ording to this theory the apical

meristem or growing region of the stem and root are composed of small mass of cells which

are all alike and are in a state of division.

These meristematic cells constitute promeri­stem. The cells of the promeristem soon

differentiate into three regions — dermatogen, periblem and plerome. Every zone consists

of a group of initials and is called a histogen or a tissue builder. Dermatogenis the single

outermost layer of the cells which later gives rise to the epidermis of the stem.Plerome gives

rise to the vasculature and pith, while periblem develops into the cortex

In the root it is also single layered, but at the apex it merges into the periblem and just

outside the periblem the dermatogen cuts off many new cells resulting into a small celled

tissue, the calyptrogen, which is also meristematic and gives rise to the root cap. Periblem

region is found internal to the dermatogen, and is the middle region of the apical meristem.

It is single layered at the apex but in central part it becomes multi-layered. It develops into

the cortex of the stem. In the roots it is also single layered at the apex and many layered in

the central portion. In the case of root, it also develops into the cortex. Pleromeis the

central meristematic region of stem apex and lies internal to the periblem.

It is also composed of thin walled isodiametric cells. Ultimately it develops and differentiates

into the central stele consisting of primary vascular tissues and ground tissues, such as,

pericycle, medullary rays, and medulla. In the roots the function of plerome is practically

same as in stem.

At a little distance behind the apex certain strands of cells show a tendency to elongate.

These strands of elongated cells make the procambium. The procambial strands ultimately

become differentiated, into vascular bundles. A portion, however, remains undifferentiated,

and it forms the cambium of the vascular bundle.

Recent investigations have revealed that there is no strict relationship between the

development of the histogens and various regions of plant body and the segmentation and

layering of the cells in the apical meristem. However, the distinction of these histogens in an

apex cannot be made in some plants, and in others the regions have no morphological

significance.

The Tunica-Corpus Theory:

This theory was put forth by Schmidt in 1924. The apical cell theory and the histogen theory

were developed with reference to both the root apex and the shoot apex. Later attention

became centred largely on shoot apices, and with the result the tunica corpus theory was

developed.

According to this theory, there are two zones of tissues in the apical meristems — the tunica

consisting of one or more peripheral layers of cells, and the corpus, a mass of cells

enclosed by the tunica. According to this theory different rates and method or growth in the

apex set apart two regions.

Tunica is the outer envelop whose cells are smaller and divide mainly anticlinally.

Surrounded by the tunica is the central core the corpus which divide in all planes producing

a mass of cells

The number of initials varies from few to many. For example, in small very slender apices,

such as those of grass seedlings, there may be only one or two in the tunica and about two

in the corpus.

In vascular plants, the differentiation of the zones of stem-apex follows more or less definite

patterns that seem to be characteristic of the major groups. These patterns show increasing

complexity from the lower to the higher groups and appear to represent a series in

specialization from simplicity to complexity.

As regards the concept of tunica and corpus, there may be several types which are found in

the stem apices of several vascular plants.

The types may be as follows:

The primitive type of stem apex having no distinction of tunica and corpus:

Lycopodium, Isoetes, Selaginella (pteridophytes) and cycads (gymnosperms) belong to this group.

They have simple apices with surface initials and no distinction of tunica and corpus. In Lycopodium,

the initiating layer is weakly defined, having uniseriate surface area which divides freely both

anticlinally and periclinally.

Here all the cells of the layer are morphologically alike. The anticlinal divisions increase the area of

the surface layer, whereas the periclinal divisions form the inner core.

The stem apices with weak tunica-corpus demarcation:

The demarcation of tunica and corpus layers begins in some of the lower conifers. In Abies and Pinus

(Coniferales), the initials make an apical uniseriate group. These initials further give rise to a central

core and an enveloping uniseriate layer by both periclinal and anticlinal divisions.

The uniseriate layer that envelops the central core suggests a tunica in appearance, but there is no

clear-cut demarcation between the tissues of two regions.

The stem apices with distinct tunica and corpus:

In angiosperms the demarcation of meristematic zones of apical region is usually more distinct and

definite than in lower groups. There are two sets of initials, one above the other, which give rise to

tunica and corpus which seem to be completely independent. The tunica has no or only rare

periclinal divisions. It ranges in thickness from one to several layers. Usually there occur two or three

layers.

The larger numbers of tunica layers occur more frequently in the dicotyledons. A single-layered

tunica occurs in the grasses. However, in monocotyledons the number of tunica layers is one to

three. In Zea, tunica divides periclinally, which shows an exceptional condition. The number of the

layers in tunica may vary even in an individual plant. The corpus varies from a large complex type to

a slender, simple type.

Significance of the tunica corpus theory:

The tunica-corpus theory served well in the establishment of meristematic patterns of the shoot

apices of seed plants. The position, number and behaviour of the initiating cells in seed-plant stems,

and early stages in the development of primary body of the shoot are now much better understood.

The tunica-corpus theory is of topographical value in studies of detailed development. The lateral

organs of the stem, i.e., leaves, branches and floral organs, arise near the apex and studies of tunica

and corpus have added greatly to a knowledge of origin and early development of these organs.

Other Theories of Shoot Apex Organization:

In support of shoot apex organ­ization other theories have also been propounded. Dermen (1947)

put forth his Histogenic Layer Concent. Acc­ording to him there is no distinct layer of apical

meristems. He named the different layers of apical meristem as L1, L2, L3, etc. He recognized these

layers on the basis of their origin.

However, this concept did not get any support. Popham and Chan (1950) put forth Mantle Core

Concept. This concept is comparable to tunica corpus theory. They used the term mantle instead of

tunica and core in place of corpus

Apical Meristems –Root Apex:

During the later stages of development of embryo, the cells at the root pole become

arranged in a pattern characteristic of the species. This group of cells comprises the apical

meristem of the primary root. The cells of this region are all relatively undifferentiated and

meristematic, densely protoplasmic and with large nuclei and they all undergo active

division.

The tissues of the mature root are eventually derived from a number of these cells of the

apical meristem, which are termed initials.

In contrast to the apical meristem of the shoot, that of the root produces cells not only

toward the axis but also away from it, for it initiates the root cap and because of the

presence of root cap the root meristem is not terminal but sub-terminal in its position, in the

sense that it is located beneath the root cap.

According to Clowes (1961), there is a inactive centre at the tip of the root, called the

quiescent centre. In this region, the cells are not meristematic. Only cells outside the

quiescent centre are active in division.

The root apex also differs from the shoot meristem in that it forms no lateral appendages

comparable to the leaves, and no branches. The root branches are usually initiated beyond

the region of most active growth, and they arise endogenously. It also produces no nodes,

and internodes, and therefore, the root grows more uniformly in length than the shoot, in

which the internodes elongate much more than the nodes.

Theories of Root apical meristem

Apical Cell Theory:

This theory was put forth by Nageli. In the roots of vascular cryptogams (pteridophytes), e.g.,

Dryopteris, a single tetrahedral apical cell is present; it is generally thought that by its division this

gives rise to all the tissues of the root. However, the apical cell theory was super-ceded by the

histogen theory.

Histogen Theory

Hanstein (1868) who proposed histogen concept to shoot apex organization applied it to root apex

also. According to him, the root apex has three distinct regions- Dermatogen (Gives rise to

epidermis), Periblem (gives rise to cortex) and Plerome (gives rise to the vasculature). Haberlandt

(1914) while agreeing with Hanstein proposed the names -protoderm, ground meristem and

procambium to the three regions.

Korper- Koppe theory

According to schenepp (1917) the cells of root apex divide in two planes. The zone with ‘T’ type of

division is given the name Korper(cap) and the other with straight ‘T’ type of division is called

Koppe(body). However this theory also doesnot satisfactorily explain the root apex structure.

Albugo

Class: Phycomycetes

Order: Peronosporales

Family: Albuginaceae

Genus: Albugo

Albugo. It is an obligate parasite distributed all over the world.

It attacks mostly crucifers like turnip, mustard, radish, cabbage, cauliflower etc

The disease caused by Albugo is commonly known as white rust because it appears

in the form of shiny, white, smooth irregular patches (pustules) or blisters on the

leaves, stems and other aerial parts of the plant.

With this several other effects are also produced. Increase in the size of the cells

(hypertrophy) and organs takes place. It results in the formation of large galls on the

various parts of the host.

Severe infection causes proliferation of the lateral buds, discoloration of flowers,

malformation of floral parts and sterile gynoecium.

The flowers and fruits when infected become distorted and hypertrophied. Such

deformed inflorescences are known as Stag heads. Gradually these patches turn

powdery

and hence the name White rust. The infection occurs through stomata.

Mycelium:

Thallus is eucarpic and mycelial. Hyphae are intercellular, coenocytic, aseptate and

profusely branched.

Cell wall is composed of cellulose not chitin. The protoplasm contains a large number

of nuclei distributed in the cytoplasm.

Some mycelium is intracellular in the form of knob-like haustoria for the absorption

of food material from the host cell.

Each haustorium has two parts. They are neck and body.

Reproduction:

The fungus reproduces both by asexual and sexual methods.

Asexual Reproduction:

When mycelium has reached certain stage of maturity it produces pads of hyphae at

certain places below the epidermis.

The hyphae produce short, thick walled, un-branched and club shaped structures called

conidiophores or sporangiophores.

After reaching a certain stage of maturity, the apical portion of conidiophore gets swollen

and is ready is cut off a conidium / sporangium .

A chain of sporangia or conidia is formed above each conidiophore in basipetal

succession. (youngest at the base and oldest at the tip)

The sporangia are cut off by the deepening of constriction at the terminal region and

gradually they meet and form transverse septum. Just beneath this another sporangium is

cut off in the same manner

A gelatinous pad is developed between conidia and they tend to stick to one another.

The basipetal arrangement of sporangium is helpful in ready dispersal of older

sporangium and nourishment of younger ones.

Each sporangium is nearly spherical, lemon shaped structure with smooth surface.

The vigorous growth and production of conidia exert pressure on epidermis which later

ruptures exposing the white crust.

As the sporangia are exposed, the gelatinous pad dries off to separate the sporangia.

Germination of the sporangia:

The sporangium may either directly germinate into new mycelium or may produce

zoospores which gives rise to new mycelium.

In the second method multinucleate protoplast divides to form 4-8 uninucleate

daughter protoplasts.

Each daughter protoplast is transformed into kidney shaped biflagellate zoospores.

After some time the sporangial wall ruptures to release zoospores in a vesicle.

The released zoospores swim in water for some time and settle down on the host.

The flagella are retracted and a wall is secreted. It germinates to produce new

mycelium.

The mycelium enters the host through the stomata and ramify in the host tissue.

Sexual reproduction:

1. It is of oogamous type. Male sex organ is called Antheridium and female sex organ

is called oogonium . They are developed in the intercellular spaces of tissues. The sex

organs arise on separate hyphae. They are called male and female hyphae.

Antheridium: This is elongate club shaped and multinucleate structures. The tip of

the male hyphae swells to form club shaped structure by accumulating cytoplasm and

nuclei which is soon cut off by a transverse septum. They develop on the male hyphae

close to the oogonium.

Oogonium: The tip of female hyphae swells to form a globose structure called

oogonium. A young oogonium contains numerous nuclei and vacuolated cytoplasm.

They are evenly distributed. As the oogonium matures the nuclei undergo mitosis and

increase in number. Later cytoplasm is differentiated into two distinct regions. They

are central dense ooplasm and peripheral periplasm. Now the nuclei except one

migrate to periplasm and become arranged in a ring. Only one nucleus survives and

remaining will degenerate. The uninucleate ooplasm serves as female gamete. In

some species mature oogonium posses many functional nuclei which are fertilized

with male gametes and shows many zygotes. This is known as compound oospores.

Fertilization:

Plasmogamy takes place by gametangial contact.

The antheridium comes in contact with oogonium to the side.

A portion of the oogonium grows into papilla like outgrowth called receptive papilla

The antheridium forms a slender fertilization tube that penetrates into the periplasm

and grows deeply until it reaches the egg prior to this a granular cytoplasmic body

appears in the center called coenocentrum.

Single female nucleus is attached towards it and become attached to a point. The

fertilization tube finally reaches coenocentrum and ruptures to release male gametes.

The male and female gametes fuse to form diploid oospore or zygote.

Germination of oospore:

The oospore germinates with the onset of favourable conditions

During germination the diploid nucleus undergo meiosis to produce haploid nuclei.

These haploid nuclei undergo repeated mitotic divisions to produce many daughter

nuclei.

Each daughter nucleus gathers a bit of cytoplasm to form many daughter protoplasts

is transformed into a biflagellate, kidney shaped zoospore.

The thick wall cracks to release zoospores in a vesicle

The zoospore germinates on the suitable host by producing a germ tube.

The germ tube enters through the stoma and extends into the intercellular spaces of

the host.

LIFE CYCLE OF ALBUGO

Albugo

Class: Phycomycetes

Order: Peronosporales

Family: Albuginaceae

Genus: Albugo

Albugo. It is an obligate parasite distributed all over the world.

It attacks mostly crucifers like turnip, mustard, radish, cabbage, cauliflower etc

The disease caused by Albugo is commonly known as white rust because it appears

in the form of shiny, white, smooth irregular patches (pustules) or blisters on the

leaves, stems and other aerial parts of the plant.

With this several other effects are also produced. Increase in the size of the cells

(hypertrophy) and organs takes place. It results in the formation of large galls on the

various parts of the host.

Severe infection causes proliferation of the lateral buds, discoloration of flowers,

malformation of floral parts and sterile gynoecium.

The flowers and fruits when infected become distorted and hypertrophied. Such

deformed inflorescences are known as Stag heads. Gradually these patches turn

powdery

and hence the name White rust. The infection occurs through stomata.

Mycelium:

Thallus is eucarpic and mycelial. Hyphae are intercellular, coenocytic, aseptate and

profusely branched.

Cell wall is composed of cellulose not chitin. The protoplasm contains a large number

of nuclei distributed in the cytoplasm.

Some mycelium is intracellular in the form of knob-like haustoria for the absorption

of food material from the host cell.

Each haustorium has two parts. They are neck and body.

Reproduction:

The fungus reproduces both by asexual and sexual methods.

Asexual Reproduction:

When mycelium has reached certain stage of maturity it produces pads of hyphae at

certain places below the epidermis.

The hyphae produce short, thick walled, un-branched and club shaped structures called

conidiophores or sporangiophores.

After reaching a certain stage of maturity, the apical portion of conidiophore gets swollen

and is ready is cut off a conidium / sporangium .

A chain of sporangia or conidia is formed above each conidiophore in basipetal

succession. (youngest at the base and oldest at the tip)

The sporangia are cut off by the deepening of constriction at the terminal region and

gradually they meet and form transverse septum. Just beneath this another sporangium is

cut off in the same manner

A gelatinous pad is developed between conidia and they tend to stick to one another.

The basipetal arrangement of sporangium is helpful in ready dispersal of older

sporangium and nourishment of younger ones.

Each sporangium is nearly spherical, lemon shaped structure with smooth surface.

The vigorous growth and production of conidia exert pressure on epidermis which later

ruptures exposing the white crust.

As the sporangia are exposed, the gelatinous pad dries off to separate the sporangia.

Germination of the sporangia:

The sporangium may either directly germinate into new mycelium or may produce

zoospores which gives rise to new mycelium.

In the second method multinucleate protoplast divides to form 4-8 uninucleate

daughter protoplasts.

Each daughter protoplast is transformed into kidney shaped biflagellate zoospores.

After some time the sporangial wall ruptures to release zoospores in a vesicle.

The released zoospores swim in water for some time and settle down on the host.

The flagella are retracted and a wall is secreted. It germinates to produce new

mycelium.

The mycelium enters the host through the stomata and ramify in the host tissue.

Sexual reproduction:

2. It is of oogamous type. Male sex organ is called Antheridium and female sex organ

is called oogonium . They are developed in the intercellular spaces of tissues. The sex

organs arise on separate hyphae. They are called male and female hyphae.

Antheridium: This is elongate club shaped and multinucleate structures. The tip of

the male hyphae swells to form club shaped structure by accumulating cytoplasm and

nuclei which is soon cut off by a transverse septum. They develop on the male hyphae

close to the oogonium.

Oogonium: The tip of female hyphae swells to form a globose structure called

oogonium. A young oogonium contains numerous nuclei and vacuolated cytoplasm.

They are evenly distributed. As the oogonium matures the nuclei undergo mitosis and

increase in number. Later cytoplasm is differentiated into two distinct regions. They

are central dense ooplasm and peripheral periplasm. Now the nuclei except one

migrate to periplasm and become arranged in a ring. Only one nucleus survives and

remaining will degenerate. The uninucleate ooplasm serves as female gamete. In

some species mature oogonium posses many functional nuclei which are fertilized

with male gametes and shows many zygotes. This is known as compound oospores.

Fertilization:

Plasmogamy takes place by gametangial contact.

The antheridium comes in contact with oogonium to the side.

A portion of the oogonium grows into papilla like outgrowth called receptive papilla

The antheridium forms a slender fertilization tube that penetrates into the periplasm

and grows deeply until it reaches the egg prior to this a granular cytoplasmic body

appears in the center called coenocentrum.

Single female nucleus is attached towards it and become attached to a point. The

fertilization tube finally reaches coenocentrum and ruptures to release male gametes.

The male and female gametes fuse to form diploid oospore or zygote.

Germination of oospore:

The oospore germinates with the onset of favourable conditions

During germination the diploid nucleus undergo meiosis to produce haploid nuclei.

These haploid nuclei undergo repeated mitotic divisions to produce many daughter

nuclei.

Each daughter nucleus gathers a bit of cytoplasm to form many daughter protoplasts

is transformed into a biflagellate, kidney shaped zoospore.

The thick wall cracks to release zoospores in a vesicle

The zoospore germinates on the suitable host by producing a germ tube.

The germ tube enters through the stoma and extends into the intercellular spaces of

the host.

LIFE CYCLE OF ALBUGO

ComplexPermanent tissues

The complex tissues are heterogeneous in nature, being com­posed of different types of cell

elements. The latter remain contiguous and form a struc­tural part of the plant, adapted to

carry on a specialised function.

Xylem and phloem are the complex tissues which constitute the component parts of the

vascular bundle. They are also called vascular tissues.

The vascular system occupies a unique position in the plant body, both from the point of

view of prominence and physiological importance. The term ‘vascular plants’ has been in

use since a long time.

Vascular bundles form a continuous and inter­connected system in the different organs of

the plants.They are primarily responsible for transport of water and solutes and elaborated

food matters.

Xylem:

Xylem is a complex tissue forming a part of the vascular bundle. It is primarily

ins­trumental for conduction of water and solutes, and also for mechanical support..

As a complex tissue it consists of different types of cells and elements, living and

non-living. The term xylem was introduced by Nageli (1858) and is derived from the

Greek word, Xylos meaning wood.

Xylem is mainly classified into primary and secondary xylem. Primary xylem

originates from the procambium of apical meristem, and secondary xylem from the

fascicular cambium during secondary growth.

The tissues composing xylem. Of the above mentioned elements only the

parenchyma cells are living and the rest are dead. A term hadrome was once used

for xylem. It included the elements excepting the fibres.

Thickening of Cell wall

The wall is hard, moderately thick and usually lignified.The walls are thickened due to

deposition of lignin which are of various types.Deposition of lignin is not uniform so that

the thickenings may be different types .

1. Annular- Lignin deposition is in form of annuli or rings. These rings are

unconnected. The space between the ring is unthickened.

2. Spiral- Lignification occur in the form of a spiral band.

3. Scalariform (ladder like)- wall thickening is in the form of transverse rods, very

much resembling the rungs of ladder.

4. Reticulate-Lignin is deposited in the form of networks or reticulum. The un

thickened areas of the walls are irregular in shape.

5. Pitted- Lignin deposition is almost uniform over the wall, except for small pores

here and there. These pores are called pits.

Structure of Pits

Pits are formed either side of walls. In any area there will be two pits lying

opposite to each other..These are usually termed as pit pairs.

There is a space inside each pit called pit cavity or pit chamber.

There is a thin membrane which separates the pit cavities of the opposite pits. It

is called pit membrane.

The opening of pit is called pit aperture.

There are two types of pits. Simple pits and bordered pits. A simple pit is just an

opening in the wall, whereas in a bordered pit, the pit aperture is overarched by

the projection of the secondary wall.

Simple pits generally occur in parenchyma cells, medullary rays, phloem fibres,

companion cells, and also in tracheids. Simple pits may be oval, polygonal or

irregular in shape.

In a bordered pit, the pit membrane becomes thick at centre. The thickening is

known as torus. It occurs in the vessels of Angiosperm and in the tracheids of

many conifer.

Elements of Xylem

Xylem is composed of different kinds of elements like tracheids, tracheae or

vessels, fibres, called xylem fibres or wood fibres, and parenchyma, referred to as

xylem or wood parenchyma.

a) Tracheids

A tracheid is a very much elongate cell occurring along the long axis of the organ.

They are the fundamental type of xylem elements present in pteridophytes and

gymnosperm where they constitute chief conducting elements.

The wood of ancient vascular plants was exclusively made of tracheids. This is the

only type of element found in the fossils of seed-plants.

The cells are devoid of protoplast, and hence dead.

A tracheid has a fairly large cavity or lumen without any contents and tapering blunt

or chisel-like ends.

The end walls usually do not uniformly taper in all planes. Tracheids are round or

polyhedral in cross-section.

Besides help in conduction, tracheids also give mechanical support to the plant.

b) Vessels

They are long tubes consisting of a series of drum-shaped cells placed one above

the other with their walls perforated or dissolved.

Vessels are syncytes formed by the fusion of cells. These are mainly evolved from

tracheidsnd are present mainly in Angiosperms, pteridophytes (selaginella, pteris)

and gymnosperm.

Each cell appears circular, oval or sometimes polygonal with a very wide lumen

which later becomes lignified and dead.

Vessels like tracheids possess various types of thickenings like scalariform,

reticulate and

They are the chief conducting tissues of vascular plants, particularly in

angiosperms. They translocate water and minerals from roots to the leaves.

They also provide mechanical support to the plants.

c) Xylem parenchyma

These are living parenchymatous cells present as component of the xylem, both in

primary and secondary xylem.

Wood parenchyma and ray parenchyma are two types of parenchyma present in

secondary xylem.

The wood parenchyma is formed from fusiform cambium initials whereas ray

parenchyma is formed from ray initials of the cambium.

Both of them have thin walls and living protoplasm.

Their main function is storage of food in the form of starch or fats.

They also help in radial conduction of water and minerals.

d) Xylem fibres

The xylem fibers develop from the same meristematic tissues as the other xylem

cells.

They have lignified secondary walls and narrow cell lumen.

They are usually longer than the tracheids of the same plant and present both in

primary as well as secondary xylem.

They originate from the derivatives of the cambium and have varied shape.

In wood fibres, known as labriform fibres, the wall is extremely thick and lumen is

almost absent.

It conducts water and minerals from root to leaves and also provides mechanical

support to the plant.

Phloem:

It is the chief food-conducting tissue of vascular plants responsible for translocation

of organic solutes.

It is a complex tissue which transports organic food inside the body of the plant.

Phloem is also called bast (= bass, a vague term).

Like xylem, phloem is mainly classified into primary and secondary phloem. Primary

phloem originates from the procambium of apical meristem, and secondary phloem

from the fascicular cambium during secondary growth.

The phloem is composed of four components; sieve elements, companion cells,

phloem parenchyma and phloem fibers.

Unlike xylem, all the elements of phloem (except phloem fibre) are living.

A fifth kind of cell type, the transfer cell has recently been reported from the

phloem. Occasionally sclereids, laticiferous cells and idioblasts are found in phloem.

Fig: Phloem elements

a. Sieve elements:

These are the main components of the phloem which are placed one above the

other forming sieve tubes.

They are long tubular structures which consist of living cells without nucleus,

endoplasmic reticulum, mitochondria, plastids etc.

Cytoplasm occurs in the form of thin lining enclosing a big central vacuole which is

filled with albuminous substance.

There is presence of sieve areas (group of pores present in walls) and sieve plates

(the portion of cross wall with sieve areas) in sieve elements.

The sieve plates may be simple or compound.

The sieve tubes are syncytes (formed by fusion of cells) and allow free diffusion of

organic substances.

They help in translocation of organic solutes (prepared food) from leaves to different

body parts.

b. Companion cells:

These are living cells, each cell always associated with one sieve tube or sometimes

more.

The cell consists of thin cellulose cell wall and active protoplast with all important

cellular components; nucleus, plastid, endoplasmic reticulum, ribosomes etc.

The common wall between companion cell and sieve tube shows presence of fine

pits which are traversed by plasmodesmata.

They are present in angiosperms (both monocots and dicots) but are absent in

pteridophytes and gymnosperms. In gymnosperm albuminous cells are present in

place of companion cell.

They assist the sieve tubes in the process of translocation of solutes.

c. Phloem parenchyma:

They are living parenchymatous cells which are elongated with rounded ends and

primary cellulose cell walls.

They are present in most of the dicots and absent in monocots.

They are no different from other ordinary parenchymatous cell.

In shape, the cell may be elongated, pointed, broadly cylindrical or even

subspherical.

The cells store food in the form of starch and fats. Sometimes they contain resins

and tannins, mucilage and latex.

They also help in the translocation of organic solutes.

d. Phloem fibers:

They are non-living cells also called bastfibers and mostly occur in secondary

phloem.

These fibers have both cellulose and lignified thickenings.

Secondary phloem consists of elongated lignified cells with simple pits.

The ends of these cells may be pointed needle-like or blunt.

They provide mechanical support and give strength and rigidity to the plant parts.