Study of internal structure is called Anatomy. In this Chapter the internal structure and functional organisation of higher plants.

The Tissues

A tissue is a group of cells having a common origin and usually performing a common function.

In Plants it is of two types :1. Meristematic tissue                                    

2. Permanent tissue

1.Meristematic Tissues

• largely restricted to specialised regions of active cell division called meristems (Gk. meristos: divided).
• It is of two types- (a) Primary meristematic     
  •   (b) Secondary meristematic
• Primary Meristematic is of two types (i) Apical   (ii) Intercalary

(i) Apical Meristematic

The meristems which occur at the tips of roots and shoots and produce primary tissues.

• Root apical meristem occupies the tip of a root while the shoot apical meristem occupies the distant most region of the stem axis.
• Formation of leaves and elongation of stem, some cells ‘left behind’ from shoot apical meristem, constitute the axillary bud.
• Such buds are capable of forming a branch or a flower.
• Specific regions of the apical meristem produce dermal tissues, ground tissues and vascular tissues.

(ii) Intercalary Meristematic

The meristem which occurs between mature tissues.

Occur in grasses and regenerate parts removed by the grazing herbivores.

Both apical meristems and intercalary meristems are contribute to the formation of the primary plant body.

(b) Secondary/Lateral meristematic

Occurs in the mature regions of roots and shoots of many plants, particularly those that produce woody axis and appear later than primary meristem.

• Responsible for producing the secondary tissues. (NEET 2010,2013)
• Example of Lateral  - Fascicular vascular cambium, interfascicular cambium and cork- cambium

Permanent Tissue

After divisions of cells in both primary and as well as secondary meristems, the newly formed cells become structurally and functionally specialised and lose the ability to divide. Such cells are termed permanent or mature cells and constitute the permanent tissues.

• It is of two types :-

(a) Simple tissue - having all cells similar in structure and function. Three types

 (i) Parenchyma                (ii) Collenchyma               (ii) Sclerenchyma

 (b) Complex tissue - having many different types of cells. Two types

(i) Xylem                        (ii) Phloem

Simple Tissues

(i) Parenchyma - forms the major component within organs.

• cells are generally isodiametric and may be spherical, oval, round, polygonal or elongated in shape.
• Cell walls are thin and made up of cellulose.
• May either be closely packed or have small intercellular spaces.
• Functions like photosynthesis, storage, secretion.

(ii) Collenchyma :- occurs in layers below the epidermis in most of the dicotyledonous plants.

• Found either as a homogeneous layer or in patches.
• Cells which are much thickened at the corners due to a deposition of cellulose, hemicellulose and pectin.
• Shape of cells may be oval, spherical or polygonal and often contain chloroplasts Intercellular spaces are absent.
• Function - provide mechanical support to the growing parts of the plant such as young stem and petiole of a leaf

Sclerenchyma :- consists of long, narrow cells with thick and lignified cell walls having a few or numerous pits and usually dead and without protoplasts. Sclerenchyma provides mechanical support to organs

It is of two types       • fibres         • sclereids

(a) Fibres :- are thick-walled, elongated and pointed cells, generally occurring in groups, in various parts of the plant.

(b) Sclereids:- are spherical, oval or cylindrical, highly thickened dead cells with very narrow cavities (lumen) commonly found in the fruit walls of nuts; pulp of fruits like guava, pear and sapota; seed coats of legumes and leaves of tea.

Complex Tissues

Made of more than one type of cells and these work together as a unit.

(i) Xylem

Functions as a conducting tissue for water, minerals, some organic nitrogen and hormones from roots to the stem and leaves. (NEET 2019)

Also provides mechanical strength to the plant parts.

☆ Composed of four different kinds of elements :-

• Tracheids     • Vessels       • Xylem fibres         • Xylem parenchyma.

Tracheids - Elongated or tube like cells with thick and lignified walls and tapering ends are dead and are without protoplasm. (NEET 2014)

• Inner layers of the cell walls have thickenings which vary in form.
• In flowering plants, tracheids and vessels are the main water transporting elements.

Vessel - Long cylindrical tube-like structure made up of many cells called vessel members, each with lignified walls and a large central cavity.

• Vessel cells are also devoid of protoplasm.
• Vessel members are interconnected through perforations in their common walls.
• Presence of vessels is a characteristic feature of angiosperms.
• Gymnosperms lack vessels in their xylem. (NEET 2010)

Xylem fibres - Highly thickened walls and obliterated central lumens may either be septate or aseptate.

Xylem parenchyma - cells are living and thin-walled, and their cell walls are made up of cellulose.

• Store food materials in the form of starch or fat, and other substances like tannins.
• Radial conduction of water takes place by the ray parenchymatous cells.

Primary Xylem is of two types a. Protoxylem          b. Metaxylem

• Protoxylem - first formed primary xylem elements are called protoxylem
• Metaxylem - later formed primary xylem is called metaxylem.
• Endarch - In stems, the protoxylem lies towards the centre (pith) and the metaxylem lies towards the periphery of the organ.
• Exarch - In roots, the protoxylem lies towards periphery and metaxylem lies towards the centre.

(ii) Phloem

Transports food materials, usually from leaves to other parts of the plant.

• In angiosperms is composed of sieve tube elements, companion cells, phloem parenchyma and phloem fibres.
• Gymnosperms have albuminous cells and sieve cells and lack sieve tubes and companion cells. (NEET 2019)

Sieve tube elements - Long, tube-like structures, arranged longitudinally and are associated with the companion cells. (NEET 2012)

• End walls are perforated in a sieve-like manner to form the sieve plates.
• A mature sieve element possesses a peripheral cytoplasm and a large vacuole but lacks a nucleus.
• Functions of sieve tubes are controlled by the nucleus of companion cells.

▪ Companion cells - Specialised parenchymatous cells, which are closely associated with sieve tube elements.

• Sieve tube elements and companion cells are connected by pit fields present between their common longitudinal walls.
• Help in maintaining the pressure gradient in the sieve tubes.

▪ Phloem parenchyma - Made up of elongated, tapering cylindrical cells which have dense cytoplasm and nucleus.

• Cell wall is composed of cellulose and has pits through which plasmodesmatal connections exist between the cells.
• Stores food material and other substances like resins, latex and mucilage.
• Absent in most of the monocotyledons.

▪ Phloem fibres (bast fibres) - Made up of sclerenchymatous cells and generally absent in the primary phloem but are found in the secondary phloem.

• Much elongated, unbranched and have pointed, needle like apices.
• Cell wall of phloem fibres is quite thick at maturity, these fibres lose their protoplasm and become dead.
• Phloem fibres of jute, flax and hemp are used commercially.
• Protophloem - First formed primary phloem consists of narrow sieve tubes.
• Metaphloem - Later formed phloem has bigger sieve tubes.

The Tissue System

 Tissue structure and function also be dependent on location. Basis of their structure and location, there are three types of tissue systems. These are the  

1. Epidermal tissue system      2. Ground or fundamental tissue system

3. Vascular or conducting tissue system

1. Epidermal tissue system

• m forms the outer-most covering of the whole plant body.
• Contain • Epidermis •Stomata
• Epidermal appendages – the trichomes and hairs.

• Epidermis -
• Outermost layer of the primary plant body made up of elongated, compactly arranged cells, which form a continuous layer and usually single layered.
• Epidermal cells are parenchymatous with a small amount of cytoplasm lining the cell wall and a large vacuole.
• Cuticle :-
• Outside of the epidermis is often covered with a waxy thick layer which prevents the loss of water.
• Cuticle is absent in roots.

Stomata :-Present in the epidermis of leaves and regulate the process of transpiration and gaseous exchange.

• Each stoma is composed of two bean shaped cells known as guard cells which enclose stomatal pore.
• In grasses, the guard cells are dumb-bell shaped. (NEET 2018)
• Outer walls of guard cells (away from the stomatal pore) are thin and the inner walls (towards the stomatal pore) are highly thickened.
• Guard cells possess chloroplasts and regulate the opening and closing of stomata.
• 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. (NEET 2016)
• Root hairs - Unicellular elongations of the epidermal cells and help absorb water and minerals from the soil.
• Trichomes - Stem epidermal hairs are called trichomes.
• Trichomes in the shoot system are usually multicellular.
• May be branched or unbranched and soft or stiff may even be secretory.
• Trichomes help in preventing water loss due to transpiration.

2. The Ground Tissue System

• All tissues except epidermis and vascular bundles constitute the ground tissue. (NEET 2011)
• 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. The Vascular Tissue System

The vascular system consists of complex tissues, the phloem and the xylem.

• Open Vascular bundles - In dicotyledonous stems, cambium is present between phloem and xylem because of the presence of cambium possess the ability to form secondary xylem and phloem tissues. (NEET 2011,2018)
• Closed Vascular bundles - In the monocotyledons, the vascular bundles have no cambium present in them and they do not form secondary tissues. (NEET 2012,2015)
• Radial Vascular bundles - When xylem and phloem within a vascular bundle are arranged in an alternate manner along the different radii. (common in roots).
• Conjoint Vascular bundles - the xylem and phloem are jointly situated along the same radius of vascular bundles.( common in stems and leaves).
• Conjoint vascular bundles usually have the phloem located only on the outer side of xylem.

▪ Anatomy of Dicotyledons and Monocotyledons Plants.

• For Roots, stems and leaves.

Dicotyledons Root

• The outermost layer is epiblema. (Many cells in the form of unicellular root hairs).
• The cortex consists of several layers of thin-walled parenchyma cells with intercellular space.
• The innermost layer of the cortex is called endodermis a single layer of barrel-shaped cells without any intercellular spaces.
• The tangential as well as radial walls of the endodermal cells have a deposition of water- impermeable, waxy material suberin in the form of casparian strips. (NEET 2018)
• Next to endodermis lies a few layers of thick-walled parenchyomatous cells referred to as pericycle.
• Lateral roots and vascular cambium during the secondary growth takes place in these cells. The pith is small or inconspicuous.
• The parenchymatous cells which lie between the xylem and the phloem are called conjuctive tissue.
• Usually two to four xylem and phloem patches.
• A cambium ring develops between the xylem and phloem.
• All tissues on the innerside of the endodermis such as pericycle, vascular bundles and pith constitute the stele.

Monocotyledons Root

• It has epidermis, cortex, endodermis, pericycle, vascular bundles and pith.
• Usually more than six (polyarch) xylem bundles in the monocot root. (NEET 2012)
• Pith is large and well developed.

• Do not undergo any secondary growth.

Dicotyledons Stem

The epidermis is the outermost protective layer of the stem and covered with a thin layer of cuticle, it may bear trichomes and a few stomata.

• Cortex -  Cells arranged in multiple layers between epidermis and pericycle.

Three sub-zones in Cortex.

 i. Outer hypodermis        ii. Middle Cortical layer     iii. Inner Endodermis

1. Hypodermis -  consists of a few layers of collenchymatous cells just below the epidermis, which provide mechanical strength to the young stem.
2. Cortical layers - below hypodermis consist of rounded thin walled parenchymatous cells with conspicuous intercellular spaces.
3. Endodermis - The cells of the endodermis are rich in starch grains and the layer is also referred to as the starch sheath.
4. Pericycle is present present on the inner side of the endodermis and above the phloem in the form of semi-lunar patches of sclerenchyma.
5. Between the vascular bundles there are a few layers of radially placed parenchymatous cells, which constitute medullary rays.
6. A large number of vascular bundles are arranged in a ring ; the ‘ring’ arrangement of vascular bundles is a characteristic of dicot stem.
7. Each vascular bundle is conjoint, open, and with endarch protoxylem.
8. Large number of rounded, parenchymatous cells with large intercellular spaces which occupy the central portion of the stem constitute the pith.

Monocotyledons Stem

• 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. (NEET 2009)
• 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.

Dicotyledons (Dorsiventral) Leaf

Lamina shows three main parts (i) epidermis        (ii) mesophyll (iii) vascular system.

(i) Epidermis- Covers both the upper surface (adaxial epidermis) and lower surface (abaxial epidermis) of the leaf has a conspicuous cuticle.

• Abaxial epidermis bears more stomata.
• Adaxial epidermis may even lack stomata.

(ii) Mesophyll- The tissue between the upper and the lower epidermis which possesses chloroplasts and carry out photosynthesis, is made up of parenchyma.

It has two types of cells :–   (a) Palisade parenchyma (b) Spongy parenchyma.

(a)Palisade parenchyma is made up of elongated cells, which are arranged vertically and parallel to each other.

(b) Spongy parenchyma - Oval or round and loosely arranged spongy parenchyma is situated below the palisade cells and extends to the lower epidermis and numerous large spaces and air cavities between these cells.

(iii) Vascular system - Includes vascular bundles, which can be seen in the veins and the midrib.

• The size of the vascular bundles are dependent on the size of the veins.
• The veins vary in thickness in the reticulate venation of the dicot leaves.
• Vascular bundles are surrounded by a layer of thick walled bundle sheath cells.

Monocotyledons (Isobilateral) Leaf

• the stomata are present on both the surfaces of the epidermis; and 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 called bulliform cells.
• When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface is exposed.
• When they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss.
• The parallel venation in monocot leaves is reflected in the near similar sizes of vascular bundles (except in main veins) as seen in vertical sections of the leaves.

Secondary Growth

• Most dicotyledonous plants exhibit an increase in girth called the secondary growth.
• Tissues involved in secondary growth are the two lateral meristems:

               1. Vascular cambium                      2. Cork cambium.

1. Vascular Cambium

• Meristematic layer that is responsible for cutting off vascular tissues – xylem and pholem.
• In the young stem it is present in patches as a single layer between the xylem and phloem. Later it forms a complete ring.

• Formation of Cambial Ring

• Intrafascicular Cambium - In dicot stems, the cells of cambium present between primary xylem and primary phloem.
• Interfascicular Cambium - The cells of medullary rays, adjoining these intrafascicular cambium become meristematic and form the interfascicular cambium.

-Thus, a continuous ring of cambium is formed.

Activity of the cambial ring -

• The cambial ring becomes active and begins to cut off new cells, both towards the inner and the outer sides.
• Cells cut off towards pith, mature into secondary xylem and the cells cut off towards periphery mature into secondary phloem.
• The cambium is generally more active on the inner side than on the outer, So the amount of secondary xylem produced is more than secondary phloem. (NEET 2017)
• The primary and secondary phloems get gradually crushed due to the continued formation and accumulation of secondary xylem.
• The primary xylem however remains more or less intact, in or around the centre.
• At some places, 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

• Activity of cambium is under the control of many physiological and environmental factors.
• In temperate regions, the climatic conditions are not uniform through the year so form annual ring by early and late wood. (NEET 2019)

• Spring / Early wood - In the spring season, cambium is very active and produces a large number of xylary elements having vessels with wider cavities.

- wood is lighter in colour and has a lower density.

• Autumn / Late wood - In winter, the cambium is less active and forms fewer xylary elements that have narrow vessels, and this wood is autumn wood or late wood.

- Autumn wood is darker and has a higher density.

• Annual Ring - The two kinds of woods that appear as alternate concentric rings, constitute an annual ring. Annual rings seen in a cut stem give an estimate of the age of the tree. (NEET 2013)

▪ Heartwood and Sapwood

• Heartwood - In old trees, the greater part of secondary xylem is dark brown due to deposition of organic compounds like tannins, resins, oils, gums, aromatic substances and essential oils in the central or innermost layers of the stem. These substances make it hard, durable and resistant to the attacks of microorganisms and insects. This region comprises dead elements with highly lignified walls and is called heartwood. (NEET 2017)

- Heartwood does not conduct water but it gives mechanical support to the stem.

• Sapwood - The peripheral region of the secondary xylem, is lighter in colour. it is involved in the conduction of water and minerals from root to leaf. (NEET 2010)

2. Cork Cambium

• As 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, usually in the cortex region.
• Phellogen is a couple of layers thick made of narrow, thin-walled and nearly rectangular cells.
• Phellogen cuts off cells on both sides.
• Outer cells differentiate into cork or phellem
• Inner cells differentiate into secondary cortex or phelloderm.
• Cork is impervious to water due to suberin deposition in the cell wall.
• Cells of secondary cortex are parenchymatous.
• Phellogen, phellem, and phelloderm are collectively known as periderm.

Due to activity of the cork cambium, pressure builds up on the remaining layers peripheral to phellogen (phellem and phelloderm)and ultimately these layers die and slough off. (NEET 2017)

Bark  - All tissues exterior to the vascular cambium, therefore including secondary phloem.

• Bark refers to a number of tissue types, viz., periderm and secondary phloem.
• Early Bark - formed early in the season is called early or soft bark.
• Late Bark - end of the season, late or hard bark is formed. Name the various kinds of cell layers which constitute the bark.

Lenticels - the phellogen cuts off closely arranged parenchymatous cells on the outer side instead of cork cells. These parenchymatous cells soon rupture the epidermis, forming a lens shaped openings.

Lenticels permit the exchange of gases between the outer atmosphere and the internal tissue of the stem occur in most woody trees. (NEET 2013)

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.
• Further events are similar to those already described above for a dicotyledon stem.
• Secondary growth also occurs in stems and roots of gymnosperms.