Plus One Botany Notes Chapter 4 Anatomy of Flowering Plants is part of Plus One Botany Notes. Here we have given Kerala Plus One Botany Notes Chapter 4 Anatomy of Flowering Plants.
|Text Book||NCERT Based|
|Chapter Name||Anatomy of Flowering Plants|
|Category||Plus One Kerala|
Kerala Plus One Botany Notes Chapter 4 Anatomy of Flowering Plants
Study of internal structure of plants is called anatomy. Plants have cells as the basic unit, cells are organised into tissues and in turn the tissues are organised into organs. Different organs in a plant show differences in their internal structure. Within angiosperms, the monocots and dicots are also seen to be anatomically different.
A tissue is a group of cells having a common origin and usually performing a common function. A plant is made up of different kinds of tissues. Tissues are classified into two main groups, based on whether the cells being formed are capable of dividing or not. They are given below.
i. Meristematic tissues
Growth in plants is largely restricted to specialised regions of active cell division called meristems.
Classification of meristematic tissue
Meristem can be classified broadly based on three ways. Based on position, function and origin.
1. Classification based on position
The meristems which occur at the tips of roots and shoots and produce primary tissues are called apical meristems. Root apical meristem occupies the tip of a root while the shoot apical meristem occupies the distant most region of the stem axis. During the formation of leaves and elongation of stem, some cells ‘left behind’ ‘from shoot apical meristem, constitute the axillary bud. Such buds are present in the
axils of leaves and are capable of forming a branch or a flower.
The meristem which occurs between mature tissues is known as intercalary meristem. They occur in grasses and regenerate parts removed by the grazing herbivores.
The meristem that occurs in the mature regions of roots and shoots of many plants, particularly those that produce woody axis is called lateral meristem.
2. Classification based on origin
They appear early in life of a plant and contribute to the formation of the primary plant body. Both apical meristems and intercalary meristems are primary meristems.
The meristem that appears in later stage of development in the plant body. Fascicular vascular cambium, interfascicular cambium and cork-cambium are examples of lateral meristems. These are responsible for producing the secondary tissues.
3. Classification based on functions
It is the outermost portion of the primary meristem found at the apex of the stem and root. It develops into epidermis.
It develops into primary vascular tissues. It forms the isolated strands of elongated cells, very near to the central region.
It develops into the ground tissue. The cells are thin walled, living and isodiametric. In the later stages of growth, they become differentiated into hypodermis, cortex, endodermis, pericycle, medullary rays and pith
ii. Permanent tissue.
The meristematic cells gradually differentiate and become mature or permanent. The permenent tissues actually composed of cells in which the growth has stopped. Permanent tissues can be divided into two types.
1. Simple permanent tissues
A simple tissue is made of only one type of cells. Common simple tissues are,
It forms the major component within organs. The cells of the parenchyma are generally isodiametric. They may be spherical, oval, round, polygonal or elongated in shape. Their walls are thin and made up of cellulose. They may either be closely packed or have small intercellular spaces. The parenchyma performs various functions like photosynthesis, storage, secretion.
Parenchyma with air spaces helps in buoyancy in water plants.
It occurs in layers below the epidermis in dicotyledonous plants. It is found either as a homogeneous layer or in patches.
It consists of cells which are much thickened at the corners due to a deposition of cellulose, hemicellulose and pectin. Collenchymatous cells may be oval, spherical or polygonal and often contain chloroplasts. They provide mechanical support to the growing parts of the plant such as young stem and petiole of a leaf.
It consists of long, narrow cells with thick and lignified cell walls having a few or numerous pits. They are I usually dead and without protoplasts. These are commonly found in the fruit walls of nuts, I pulp of fruits like guava, pear and sapota, seed coats of legumes and leaves of tea. Scleren- I chyma provides mechanical support to organs, On the basis of variation in form, structure, origin and development, sclerenchyma may be either fibres or sclereids.
|Thick walled elongated and pointed cells||Spherical, oval or cylindrical and highly thickened dead cells with narrow cavities|
|Found in various parts of the plant||Found in the fruit walls of nuts, pulp of fruits like guava, pear and sapota|
2. Complex permanent tissues
The complex tissues are made of more than one type of cells and these work together as a unit. Xylem and phloem constitute the complex tissues in plants.
Xylem functions as a conducting tissue for water and minerals from roots to the stem and leaves. It also provides mechanical strength to the plant parts. It is composed of four different kinds of elements. They are given below.
It are elongated or Tracheids tube like cells with thick and lignified walls and tapering ends, These are dead and are without protoplasm. The inner layers of the cell walls have thickenings which vary in form. The tracheids H conduct water and dissolved w mineral elements from roots to leaves. They also provide mechanical support.
It is a long cylindrical tube-like structure made up of many cells called vessel members, each with lignified walls and a large central cavity. The vessel cells are also devoid of protoplasm. Vessel members are interconnected through perforations in their common walls. The presence of vessels istic feature of angiosperms.
iii. Xylem fibres.
The sclerenchyma cells associated with the xylem are called xylem fibres. These fibres have lignified cell walls. The thickness of the walls varies considerably, but they are usually thicker than the walls of the tracheids in the same wood. They provide me chanical strength.
iv. Xylem parenchyma.
Their cells are living and thin-walled, and their cell walls are made up of cellulose. They store food materials in the form of starch or fat, and other substances like tannins. The radial conduction of water takes place by the ray parenchymatous cells. The xylem paranchyma can be sub-divided into two types namely protoxylem and metaxylem.
Phloem transports food materials, usually from leaves to other parts of the plant. Phloem in angiosperms is composed of sieve tube elements, companion cells, phloem parenchyma, and phloem fibres. ‘Gymnosperms have albuminous cells and sieve cells. They lack sieve tubes and companion cells.
i. Sieve elements.
These are the conducting elements of phloem. Their end walls are transverse or oblique and contain a number of pores and are called sieve plates. These are living cells without nucleus.
ii. Companion cells.
The small thin-walled elongated cells associated with sieve elements are called companion cells. These are nucleated cells.
iii. Phloem parenchyma.
These are living, and their function is storage and conduction of food. They are present in all gymnosperms and dicots.
iv. Phloem fibres.
The sclerenchyma cells that occur in phloem are known as phloem fibres or bast fibres. Their function is to give strength and rigidity to the organs.
A group of tissues performing a similar function, irrespective of its position in the plant body, is regarded to constitute a tissue systern. On the basis of their structure and location, there are three types of tissue systems.
1. Epidermal tissue system
The epidermal tissue system forms the outermost covering of the whole plant body and comprises epidermis, stomata and the epidermal appendages.
Outermost layer of the plant body. Made up of elongated compactly arranged cells. Outer wall is covered with cuticle which prevents the loss of water. Cuticle is absent in roots.
These are minute openings found on the epidermis. Abundant on the lower surface of leaves of terestrial plants. Stomata regulate the process of transpiration and gaseous exchange. Each stoma is composed of two beanshaped cells known as guard cells, a few epidermal cells, in the vicinity of the guard cells become specialised in their shape and size and are known as subsidiary cells. The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus.
iii. Epidermal appendages.
The epidermis of most plants often bear outgrowth known as epidermal appendages or epidermal outgrowths. They are of following two types. Tricornes and emergences. The epidermal hairs present on the stem are called trichomes. Emergence are multicellular, stiff and sharp epidermal outgrowths containing some inner tissues.
iv.Ground or fundamental tissue system
All tissues except epidermis and vascular bundles constitute the ground tissue. It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma. Parenchymatous cells are usually present in cortex, pericycle, pith and medullary rays, in the primary stems and roots. In leaves, the ground tissue consists of thin-walled chloroplast containing cells and is called mesophyll.
3. Vascular or conducting tissue system
It consists of complex tissue, the phloem and the xylem. Xylem and phloem are always organised in groups called vascular bundles. When cambium is present in a vascular bundles. It is known as open bundle, e.g., dicot stem. When cambium is absent in a vascular bundle, it is known as closed bundle,
e.g., monocot stem.
When xylem and phloem within a vascular bundle are arranged in alternate manner on different ‘radii.
Xylem and phloem are situated at the same radius of vascular bundles. Such vascular bundles are common in stems and leaves. The conjoint vascular bundles usually have the phloem located only on the outer side of xylem.
Anatomy of dicotyledonous and monocotyledonous plants
i. Dicotyledonous root
- The outermost layer is epidermis.
- Cortex consists of several layers of thin- walled parenchyma cells with intercellular spaces.
- Cortex consists of several layers of thin- walled parenchyma cells with intercellular spaces.
- The innermost layer of the cortex is called endodermis.
- Next to endodermis lies a few layers of thick- walled parenchyomatous cells referred to as pericycle.
- The parenchymatous cells which lie between the xylem and the phloem are called conjuctive tissue.
- Protoxylem and metaxylem is present. The protoxylem lies towards the periphery and metaxylem towards the centre. This condition is said to exarch.
- The pith is small and conspicuous.
ii. Monocotyledonous root
- It has epidermis, cortex, endodermis, pericycle, vascular bundles and pith.
- They have fewer xylem bundles, there are usually more than six (polyarch) xylem bundles in the monocot root.
- Pith is large and well developed.
- Monocotyledonous roots do not undergo any secondary growth.
iii. Dicotyledonous stem
- Epidermis is the outermost protective layer of the stem, which is covered with a thin layer of cuticle.
- The cells arranged in multiple layers between epidermis and pericycle constitute the cortex.
- It consists of hypodermis, cortical layer and endodermis.
- Large number of vascular bundles are arranged in a ring. The ‘ring’ arrangement of vascular bundles is a characteristic of dicot stem.
- A large number of rounded, parenchymatous cells with large intercellular spaces which occupy the central portion of the stem constitute the pith.
iv. Monocotyledonous stem
- The monocot stem has a sclerenchymatous hypodermis, a large number of scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath, and a large, conspicuous parenchymatous ground tissue.
- 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 vascular bundles.
v. Dorsiventral (Dicotyledonous ) leaf
- The vertical section of a dorsiventral leaf through the lamina shows three main parts, namely, epidermis, mesophyll and vascular system.
- The epidermis which covers both the upper surface (adaxial epidermis) and lower surface (abaxial epidermis) of the leaf has a conspicuous cuticle.
- The tissue between the upper and the lower epidermis is called the mesophyll.
- Mesophyll, which possesses chloroplasts and carry out photosynthesis, is made up of parenchyma. It has two types of cells – the palisade parenchyma and the spongy parenchyma.
Isobilateral (Monocotyledonous) leaf
- The mesophyll is not differentiated into palisade and spongy parenchyma.
- In grasses, certain adaxial epidermal cells along the veins modify themselves into large, empty, colourless cells. These are called bulliform cells.
- Stomata are seen on both the surfaces of the epidermis.
The growth of the roots and stems in length with the help of apical meristem is called the primary growth.
Most dicotyledonous plants exhibit an increase in girth. This increase is called the secondary growth. The tissues involved in secondary growth are the two lateral meristems. They are vascular cambium and cork cambium.
i. Vascular cambium
The meristematic layer that is responsible for cutting off vascular tissues, xylem and pholem is called vascular cambium. In the young stem it is present in patches as a single layer between the xylem and phloem. Later it forms a complete ring.
Activity of the cambial ring
- In dicot stems, the cells of cambium present between primary xylem and primary phloem is the intrafascicular cambium.
- The cells of medullary cells, adjoining these intrafascicular cambium become meristematic and form the interfascicular cambium.
- The cambial ring becomes active and begins to cut off new cells, both towards the inner and the outer sides. The cells cut off towards pith, mature into secondary xylem and the cells cut off towards periphery mature into secondary phloem.
- The cambium forms a narrow band of parenchyma, which passes through the secondary xylem and the secondary phloem in the radial directions. These are the secondary medullary rays.
Spring wood and autumn wood
In the spring season, cambium is very active and produces a large number of xylary elements having vessels with wider cavities. The wood formed during this season is called spring wood or early wood. In winter, the cambium is less active and forms fewer xylary elements that have narrow vessels, and this wood is called autumn wood or late wood.
Heartwood and sapwood
In living trees, after certain years of growth, the central region has no living cells and it ceases to conduct water. This region is full of tannis, oils, gums, resins etc. It looks darker in colour and is hard and durable. This region is known as heart wood.
The outer region of the secondary wood in living trees contains living cells and reserve materials. This peripheral light-coloured living wood involved in conduction of water and minerals
ii. Cork cambium
- The stem continues to increase in girth due to the activity of vascular cambium, the outer cortical and epidermis layers get broken and need to be replaced to provide new protective cell layers. Hence, sooner or later, another meristematic tissue called cork cambium or phellogen develops.
- Phellogen cuts off cells on both sides. The outer cells differentiate into cork or phellem while the inner cells differentiate into secondary cortex or phelloderm.
- The cells of secondary cortex are parenchymatous. Phellogen, phellem, and phelloderm are collectively known as periderm.
- Bark is a non-technical term that refers to all tissues exterior to the vascular cambium, therefore including secondary phloem.
- Bark that is formed early in the season is called early or soft bark. Towards the end of the season late or hard bark is formed.
- Parenchymatous cells soon rupture the epidermis, forming a lens shaped openings called lenticels. Lenticels permit the exchange of gases between the outer atmosphere and the internal tissue of the stem. These occur in most woody trees.
Secondary growth in roots
In the dicot root, the vascular cambium is completely secondary in origin. It originates from the tissue located just below the phloem bundles, a portion of pericycle tissue, above the protoxylem forming a complete and continuous wavy ring, which later becomes circular. Secondary growth also occurs in stems and roots of gymnosperms. However, secondary growth does not occur in monocotyledons.
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