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State the location and function of different types of Meristems.


(i) Apical meristems:

o Location:
These meristems occur at the tip of the roots and the tip of the shoots the shoot apical meristems mostly occupy the distant most region of the stem axis.

o Function:
They help in the elongation of the root and shoot. As they appear in the early life of the plant they contribute to the formation of the primary plant body.

(ii) Intercalary meristems:

o Location:
These meristems occur between mature tissues. They are also present in the axils of the leaf.

o Function:
They are capable of forming branches and flowers. In grasses they help in regenerating the parts removed by the grazing animals.

Both the above meristems are called primary meristems.

(iii) Secondary or Lateral meristems:

o Location:
They occur in the mature regions of roots and shoots especially in the woody axis.

o Function:
Their main function is to produce the secondary tissue.


Cork cambium forms tissues that form the cork. Do you agree with this statement? Explain.

Phellogen or cork cambium is a couple of layers thick. These layers are made of narrow, thin-walled and nearly rectangular cells. Cork cambium cuts off cells on both sides of these the outer cells differentiate into cork or phellem while the inner cells differentiate into secondary cortex or phelloderm. The Phellem and Phelloderm formed from the cork cambium or Phellogen are collectively called periderm.


Explain the process of secondary growth in the stems of woody angiosperms with the help of schematic diagrams. What is its significance?


The process and structures associated with secondary growth in dicot stems are,

(i) Formation of the cambial ring:
In dicot stems, an intrafascicular cambium is present between the xylem and phloem, which is primary in nature. The parenchymatous cells of each medullary ray, lying between the intrafascicular cambium of the adjacent bundles divide and become meristematic and form a strip of cambium called the interfascicular cambium. Both types of cambia join together and to form a complete ring of vascular cambium, which is partly primary and partly secondary meristematic in origin.

The cambial ring becomes active and begins to cut off new cells, both towards the inner and outer sides, by vertical or oblique divisions of the elongated fusiform initials. The derivatives, which cut-off towards the outer side get differentiated into the secondary phloem, whereas those produced on the inner side are differentiated into secondary xylem. The cambium is generally more active on the inner side than on the outer side. As a result, the xylem increases more rapidly in bulk than the phloem and soon forms a compact mass. This forms the main bulk of the plant body. Due to the continued formation of secondary xylem, both the primary and secondary phloem of the earlier years gets gradually crushed.

(ii) Annual rings:
The activity of the cambium is under the control of a series of physiological and environmental factors. The cell of the cambium divide rapidly and several layers of the undifferentiated cells become visible. Consequently, a larger volume of xylem tissue is produced, having comparatively large, thin-walled and light-staining components. One light and one dark coloured zone comprise one year’s growth and this is known as the annual ring or growth ring. Since each annual ring corresponds to the growth of wood in one year, one can estimate the age of the tree to some degree of accuracy by counting these rings.

Due to the addition of secondary phloem and secondary xylem elements, the outermost layer of the cortex becomes highly stretched and may crack open. During this process, a few layers of meristematic tissue arise in the cortex. This is called the cork cambium. The cells of the cork cambium are rectangular and these cut-off cells on both the sides. Those formed on the outer side become sub-raised and constitute the cork, which is impervious to water and air. The inner cells become parenchymatous and may contain chloroplasts. These constitute the secondary cortex. The cork cambium and the secondary cortex are collectively known as the periderm. Periderm cells are living and some of them take part in the conduction of the metabolites.

(iii) Lenticels:
These are openings or breaks in the cork tissue, which look like raised spots on the surface of the stem and permit the exchange of gases between the outer atmosphere and the internal tissues of the stem cells. These occur in most trees. Lenticels consist of a pore formed due to the rupture in the epidermal layer. Below the pore, a loose mass of thin-walled, rounded parenchymatous cells, which is known as the complementary tissue is visible. The lenticels may remain scattered or get arranged in longitudinal or vertical rows. Rows of lenticels may occur opposite the medullary rays, thus facilitating free exchange of gases.

(iv) Heartwood:
After certain years of growth, the xylem elements of the stem of a number of trees develop dark brown coloration, especially in the central or innermost layers. This region comprises of dead elements with highly lignified walls and is called heartwood.


Cut a transverse section of young stem of a plant from your school garden and observe it under the microscope. How would you ascertain whether it is a monocot stem or a dicot stem? Give reasons.


If the stem is a monocot stem it will show the following characters:

Internal structure of a monocotyledons stem:

This is the outermost layer of the stem, which is made of a single layer of cells. The epidermis does not bear unicellular or multicellular hairs.

This is just below the epidermis it is made up of sclerenchymatous cells. It gives mechanical strength to the stem.

It is made up of conspicuous parenchymatous ground tissue.

Vascular bundles:
Vascular bundles are conjoint and closed. Peripheral vascular bundles are generally smaller than the centrally located ones. The phloem parenchyma is absent, and water-containing cavities are present within the vascular bundles.

If the stem is dicot stem it will show the following characters:

Internal structure of a dicotyledonous stem:

This is the outermost layer of the stem. It consists of a single layer of cells. The epidermis bears multicellular, uniseriate trichomes. A thin layer of cuticle is present on the epidermis as well as the tichomes.

The cortex consists of several layers of cells. This is divided into three sub-zones, namely hypodermis, general cortex and endodermis.

This is just below the epidermis and consists of 3 to 4 layers of collenchymatous cells. It gives mechanical strength to the stem. These cells are thickened at the corners and possess chloroplasts.

General cortex:
The cells of this zone are parenchymatous and multi-layered. Oil ducts, surrounded by a glandular parenchymatous layer, also occur copiously throughout this sub-zone.

It is the innermost layer of the cortex and consists of a layer of barrel-shaped cells. Since it is rich in starch, it is also referred to as the starch sheath.

It is in the form of semi lunar patches of sclerenchyma. Each patch associated with phloem of the vascular bundle, is called the hard bast.

Medullary rays:
In between the vascular bundles there are a few layers of parenchymatous cells, which constitute the medullary rays. These are slightly larger in size when compared to the other cortical cells. Usually, these are polygonal in shape and exhibit no intercellular spaces in between.

Vascular bundles:
The vascular bundles are arranged in a ring internal to the endodermis. Each vascular bundle is conjoint, collateral, endarch and open. It is composed of xylem, phloem and cambium.

This is situated on the outer side of the vascular bundle. The cells are thin-walled and polygonal. Phloem is a complex tissue and is composed of sieve tube elements, companion cells, the phloem parenchyma and phloem fibres.

The xylem tissue lies below the phloem. This is composed of vessels, tracheids, xylem parenchyma and fibres.

This is present in between the xylem and the phloem. It consists of 2-3 layers of thin-walled, rectangular cells.

This is the central portion of the stem. It consists of rounded, parenchymatous cells with plenty of intercellular spaces.


The transverse section of a plant material shows the following anatomical

(a) the vascular bundles are conjoint, scattered and surrounded by asclerenchymatous bundle sheaths. 

(b) phloem parenchyma is absent. What will you identify it as?

The transverse section of the plant material with the given anatomical features will be identified as monocot stem.


Why xylem and phloem are called complex tissues?


Xylem and phloem are complex tissues because it consist of various elements which are given below:

(a) Xylem:
It is a conducting tissue composed of four elements of different kinds:

(i) tracheids, (ii) vessels, (iii) wood fibres and (iv) wood parenchyma.

The function of xylem is to conduct water and mineral salts upwards from the root to the leaf and to give mechanical strength to the plant body.

(i) Tracheids:
A single tracheid is a highly elongated or tube-like cell with hard, thick and lignified walls and a large cavity. They are devoid of protoplast and are hence dead. The ends of the tracheids are tapering, blunt or chisel-like. These are constituents of the xylem of primitive plants. The cell wall is hard moderately thick and lignified. The secondary wall layers possess various kinds of thickenings in them and may be distinguished as annular, spiral, retuculate, scalar form or pitted. In a transverse section, they appear circular, polygonal or polyhedral in outline. On account of these structural adaptations, besides the mechanical support to the plant body, they transport water, hormones and solutes from the root to the stem, leaves and the floral parts. In gymnosperms, these are the chief water transporting elements.

(ii) Vessels:
A vessel is a long, cylindrical, tube-like structure with lignified walls and a wide central cavity. The cells are dead and without protoplast. These are arranged in longitudinal series in which the transverse walls are perforated and as such the entire structure looks like a water pipe. The perforations may be simple or multiple. The latter may be arranged in reticulate, scalariform or foraminate manners. Vessels have been found in a majority of angiosperms. They are also found in a few pteridophytes and gymnosperms. They serve as a more efficient mode of transportion of water and minerals as compared to tracheids, due to the presence of perforation plates. These also give mechanical support to the plant body.

(iii) Xylem fibres:
These are found in both the primary and secondary xylem and may possess simple or bordered pits, highly thickened walls and obliterated central lumen. These may either be septate or aseptate.

(iv) Xylem parenchyma:
The cell walls of the primary xylem parenchyma are thin and made up of cellulose. These store food materials in the form of starch or fat and sometimes tannins and other substances. The ray parenchyma cells also participate in the radial conduction of water.

The first formed xylem elements are described as protoxylem and consist of annular, spiral and scalariform vessels and lie towards the centre of the stem. The latter formed xylem is described as metaxylem and it consists of some tracheids along with reticulate and pitted vessels. In the stem, it lies away from the centre and its vessels have much bigger cavities when compared to those of the protoxylem.

(b) Phloem:
The phloem is yet another conducting tissue.

It is composed of four elements: (i) sieve cells or sieve tube elements, (ii) companion cells, (iii) phloem parenchyma and (iv) Phloem fibres.

The main function of the phloem is to conduct prepared food materials from the leaf to the storage organs and growing regions of the plant.

(i) Sieve tube elements:
These occur as long, slender tube-like structures, arranged in longitudinal series and are associated with companion cells. Their end walls are perforated in a sieve-like manner to form the sieve plate. These may either be simple or compound and at maturity they become impregnated with cellulose. They are devoid of nucleus at maturity. However, they possess a peripheral cytoplasm as well as a large vacuole. The uniqueness of the sieve tube is that although without nucleus, it is living and the nucleus of the companion cells controls its functional activities. Distinct proteinaceous inclusions, the P-proteins are seen evenly distributed throughout the lumen of the sieve tube. During wounding, along with cellulose, P-protein helps in sealing.

(ii) Companion cells:
These are specialized parenchyma cells, which are closely associated with the sieve tube elements in their origin, position and function. These originate from the same meristematic cells that give rise to the sieve tube elements.

The sieve tube elements and companion cells are connected by pit field presenting their longitudinal walls, which is a common wall for both and with the death of one, the other cell also dies. The companion cells play an important role in the maintenance of a pressure gradient in the sieve tubes.

(iii) Phloem parenchyma:
This is made up of elongated, tapering to broadly cylindrical, living cells, which have a dense cytoplasm and nucleus. The cell wall is composed of cellulose, with pits, interconnection axial parenchyma cells and ray cells. The phloem parenchyma stores organic food materials and other substances such as resins, mucilage, latex, etc.

(iv) Phloem fibres:
These are much elongated, unbranched and have pointed, needle-like apices. Their cell wall is quite thick with simple or slightly bordered pits. At maturity, these fibres lose their protoplast and die. These occur in groups, as sheets or cylinders.

The outer portion of the phloem, consisting of narrow-tube elements constitutes the protophloem. The inner portion is made up of broader sieve tube elements, which make the metaphloem.


What is stomatal apparatus? Explain the structure of stomata with a labeled diagram.

The stomatal apparatus is present in the leaves of both monocot and dicot leaf. In dicot leaf stomata are present more in the abaxial epidermis and less in the adaxial epidermis.

Structure of Stomata
The stomata consist of guard cells, which open into an air cavity in the mesophyll tissue, which is made up of parenchymatous cells. The air cavity helps in the exchange of air.


Name the three basic tissue systems in Plants. Give the tissue names under each system.


There are three basic types of tissues found in Plants.

(i) Dermal plant tissue system.

(ii) Vascular plant tissue system – xylem and phloem.

(iii) Ground or fundamental tissue system.

These three tissues perform various functions for the smooth working of plants. Each tissue contributes to the functioning of an organ. For example, leaf is an organ. It has the following tissues - epidermis for the protection and exchange of gases, mesophyll for carrying out photosynthesis and vascular tissue for the conduction of food, water and minerals.

In plants, mainly two types of tissues are found,

(i) meristematic and

(ii) permanent tissues.

(i) Meristematic tissues:
A meristematic tissue is a group of cells that are in a continuous state of division and thus produces new cells. They are found in the growing regions of a plant's shoot tip and root tip and also in lateral regions. According to their position, meristems are of three types: (a) apical meristems, which are present at the tips of the stems, roots and branches, (b) lateral meristems, which are present along the side of the stems, and (c) the third type of meristem is intercalary meristems, which are found at the bases of internodes and leaf sheaths of monocotyledonous plants.

(ii) Permanent tissue:
Permanent tissues do not divide. They are composed of mature cells and after undergoing complete growth, they have assumed a definite shape, size and function and have lost the power of division. Permanent tissues are of the following types,

These are one cell thick tissues, which are found converging and protecting the plant surface. In the aerial parts of the plant, a waxy substance may cover this tissue. This is called the cuticle. Cuticle protects the plant from water loss, mechanical injury and invasion of pathogens. The epidermis may have hair or glands on it. The epidermis of leaves and green stems may have a large number of openings called stomata.

This tissue has polygonal cells and unevenly thickened walls, which are prominent at the corners. The main function of this tissue is to give strength to the plant parts.

This tissue is composed of dead cells. The main functions of this tissue are to give mechanical support to the organs in which it is found. Sclerenchyma consists of two types of cells - the fibres and sclereids. These two differ in shape and size. Fibres are elongated, flexible cells with tapering ends. Some sclerids are called stone.

These are thin-walled, many sided and of different shapes. They retain the ability to divide at maturity. Their main function is storage.

When Parenchyma is situated at the periphery of the organ and contains chloroplasts, it is called collenchyma. These cells perform photosynthesis. In aquatic plants, the parenchyma cells having regular system of intercellular air spaces are present. This is called parenchyma.

Vascular tissues:
This tissue is called conduction tissue and is composed of xylem and phloem. Xylem is responsible for the transport of water and minerals and phloem is responsible for the translocation of organic food materials.


How is the study of plant anatomy useful to us?

Study about the internal structure of plants is called plant anatomy. It is usually done by dissection and microscope examination of the plant tissue. By studying the internal structure we can see several similarities as well as differences in plant kingdom.

In all plants cells form the basic unit. Cells are organized into tissues and in turn tissues into organs. The simple type in plant kingdom is the unicellular form, and the progressive evolution in plants has resulted in the increasing complexity of structures. In higher plants roots, stem, leaves, and flowers carry out different function. Due to these divisions of labour the cells of the plants are differentiated to form different tissues.

Different organs in the plants have different structures. Within angiosperms the monocots and dicots show a great difference in their anatomy of root, stem, and leaf. By studying the anatomy of plants we can classify a part of the plant according to its division, phylum etc.

Anatomy of the plants also shows the adaptations exhibited by the plants to diverse environment.


Describe the internal structure of a dorsiventral leaf with the help of labeled diagrams.

Internal structure of the leaf of a dicotyledonous plant:

Upper and lower surfaces of the leaf are bound by the epidermis, known as the lower and upper epidermis respectively. The upper epidermis generally comprises of a single layer of cells. A thick cuticle is usually present on the outer walls of these cells. The stomata are absent and if present, they are few in number when compared to those on the lower epidermis. The lower epidermis is also made up of a single layer of cells bearing a large number of stomata and this is covered by a thin layer of cuticle. Each stoma is surrounded by two guard cells, which contains chloroplasts and opens into a large space below, called the sub-stomatal chamber. On account of the presence of chloroplasts, they are able to carry out photosynthesis to a limited extent.

The tissue between the upper and lower epidermis is called the mesophyll. The mesophyll consists of two regions - the palisade parenchyma and the spongy parenchyma. The palisade parenchyma is vertically elongated cells, which may be organised into one or more layers. The palisade cells are compactly arranged through out and show only little intercellular spaces. They process abundant chloroplasts. The palisade cells carry out photosynthesis in the presence of sunlight. The spongy parenchyma is components of the remaining portions of the mesophyll tissue, which is extended from below the palisade layer to the lower epidermis. Its cells are oval or rounded, enclosing numerous large air spaces and air cavities. These spaces open towards the outside through the stomata and help in the diffusion of gases. Spongy parenchyma cells contain chloroplasts to perform photosynthesis, like those of the palisade parenchyma.

Vascular bundles:
The vascular bundles are conjoint, collateral and endarch. Each vascular bundle is surrounded by a layer of thick-walled cells arranged compactly and is known as the bundle sheath. The xylem is situated towards the upper epidermis and the phloem lies towards the lower epidermis.


What is periderm? How does periderm formation take place in the dicot stems?

The Phellem and Phelloderm formed from the cork cambium or Phellogen are collectively called periderm.

Formation of different layers of periderm
As the stem continues to increase in grith due to the activity of vascular cambium, the outer cortical and epidermal layers get broken which has to be replaced by new protective layers. The protective layers formed are merismatic tissues called cork cambium. Phellogen or cork cambium is a couple of layers thick. These layers are made of narrow, thin-walled and nearly rectangular cells. Cork cambium cuts off cells on both sides of these the outer cells differentiate into cork or phellem while the inner cells differentiate into secondary cortex or phelloderm.

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