The study of internal structure of plants is called plant anatomy. Tissue is a group of similar cells which performs one or more particular functions. Depending upon the dividing capacity, the tissues are classified into two types:

1.Meristematic   and   2. Permanent

Types of Tissues:

1. Meristematic Tissues:

Fig: meristematic  tissue

It can be defined as a group of immature cells which has the capacity of division. They are living cells, small and thin walled. The cells are compactly arranged and lack intercellular spaces. They are usually isodiametric, spherical, oval or polygonal in shape.

Classification of Meristems:

1.Classification based on origin and development:

a.Primary meristem: They are found at the tips of root, stem and appendages. These meristems retain power of division throughout the life of the plant.

b.Secondary meristem: Some primary permanent tissues become meristematic and the resulting meristems are called secondary meristems. They develop to heal up the wounded portion or to effect secondary growth ssuch as vascular cambium of the root,etc

2.Classification based on position:

a.Apical meristem: The meristems loacated at the apices or the growing points of main and lateral shoots and roots are called apical meristems. They include both promeristems and primary meristems.

b.Intercalary meristems: The meristem present between the regions of permanent tissues  is called intercalary meristem.Intercalary meristems are found at the base of leaves in Pinus.

c.Lateral meristems: Lateral meristems lie along the lateral sides of roots and stems. They are responsible for the increase in thickness of the plant part.

3.Classification based on function: On the basis of function they are classified into three categories;

1.Protoderm: This is the outermost layer of meristem which divides periclinally and forms the outer epidermal layer.

b.Procambium: It is located inside the protoderm. It consists of narrow , elongated meristematic cells, etc.

c.Ground meristematic: It consists of large thin walled cells that differentiate into ground tissue system.

The shoot apex: The shoot apex is a dome-shaped mass of cells and is covered by young leaves and can be seen only after their removal.

Theories of structural organization of root and shoot:

a.The apical cell theory: This theory was put forwarded by Nageli(1858). This theory is applicable to algae as well as to most of the bryophytes and pteridophytes.

b.Histogen theory: This theory was proposed by Hanstein(1870). According to this theory, the apical meristem of the root and stem soon  differentiates into three regions;

1.Dermatogen: This is the outermost single layer of the cells. The dermatogens gives rise to a single layered epidermis. These are called calyptrogens.

2.Periblem: It is the middle region composed of isodiametric cells. It forms the cortex.

3.Plerome: It is the central or inner meristematic region.

c.Tunica-Corpus theory: It is proposed by  Schmidt(1924) and according to this, the apical meristem consists of two distinct regions;

Tunica: It is outer zone of the cell which may be one or several cell and  are comparatively smaller in size. It give rise to epidermis or protoderm.

Corpus: It is innermass of the cells which is several layers and the cells are comparatively larger in size. It gives rise to cortex, vascular bundle and pith.

1. Permanent tissues:

Fig: L.s of root apex

Root apex is found at the tip of main root and its branches. It is covered by protective  root  cap. It is further differentiated in to five region.

1.Protoderm :

It forms epidermis or epiblema and root cap in dicots.

2.Calyptrogen:

It is found in monocot and is responsible for the formation of root cap.

3.Procambium:

It gives rise to vascular tissues.

4.Ground meristem:

It takes parts in the formation of pith, cortex, indodermis and pericycle.

5.Quiescent center:

In this region cells divides at a very slow rate.

Histogen theory gave the idea that as in shoot apex, root apex has also three histogens.

a.Dermatogen:

It forms epiblema and root cap in dicots.

b.Periblem:

It forms cortex and endodermis.

c.Plerome:

It forms pericycle, vascular tissues, pith etc.

Permanent tissues:

A permanent tissues is a group of cells that have lost the powe of cell division and the growth. The permanent tissues can be classified into two types:

A.Simple permanent tissue: These tissues are the group of cells which are similar in origin, form and  function. These are also mainly three types;

1.Parenchyma:

 It is most common permanent tissues. They are thin walled living cells which are oval, spherical or polygonal in shape. They contain large vacuole. The cells have intercellular spaces in between them which may small or big. This tissue is generally present in almost all the organs. They are of following types;

a.Prosenchyma: In some cases the parenchymatous cell become long and taper at either end such tissues is called prosenchyma.

b.Aerenchyma: Some cells have wide air spaces between them called aerenchyma.

c.Chlorenchyma: In the leaves, the palisade cells are long and contain chloroplast in them. They help in photosynthesis and are essentially parenchyamatous due to their contain chlorophyll.

Functions of parenchyma:

1.Parenchymatous cells help in photosynthesis. e.g palisade cells of leaf.

2.It servers mainly for the storage of food materials in the form starches, proteins, oils and fats.

3.In the fleshy stems of leaves, it helps in store water.

B.Collenchyma:

Collenchyma are the living cells which are somewhat elongated. Cells of this tissues possess localized thickening on the cell wall, particularly at the corner. It occurs in the herbaceous dicotyledons e.g. sunflower, gourd, etc.

Funcations:

1.It gives mechanical strength to the organs.

2.It also manufacture sugar and starch.

3.In some plants, collenchyma cell regain the power to divide.

C.Sclerenchyma:

It consists of long, narrow, thick walled and lignified cells which are dead cells and do not perform any metabolic activities. They are found abundantly in plants and occurs in patches or in definite layers.

Functions:

1.They give strength and rigidity to plant body.

2.They also enable it to withstand mechanical strength.

Sclerenchyma are of two types;

1.Fibres: Fibres are long and tapering at ends which are thick walled cells. The fully developed fibres are always dead. They are present in the xylem of stems and roots, testa of various seeds. Its main function is to give mechanical strength.

2.Sclereids: They are extremely thick walled and may be spherical, oval, cylindrical, etc. They are generally found in hard parts of plant body sometimes in the pulp of fruits. They are be classified into four main types on the basis of their shapes. Its main function is to provide local mechanical strength.

Complex or Compound permanent tissues:

There are two types of complex tissues. They are xylem and phloem.

1.Xylem: It is a complex tissues that is mainly responsible for the conduction of water and saps inside the plant from the root upto the top of the plant. It also provides mechanical strength. Xylem is commonly known as wood which is composed of 4 kinds of cells respectively, tracheids, vessels, wood parenchyma and wood fibres.

Xylem are of two types they are protoxylem and metaxylem.

Protoxylem: It is early formed xylem which bears narrow vessels. It is retained only for a short period where fibres are absent or rare.

Metaxylem: It is late formed xylem which bears large vessels. It is retain for long period where fibres often occurs.

2.Phloem: It is living, non lignified tissue also called bast. Its main function is to transport organic food inside the plant body from the leaves to the storage organ and from the storage organ to various parts according to the need of the organ. It consists of sieve tubes, companion cells, phloem parenchyma, bast fibres. Phloem are of two types primary and secondary phloem.

Primary phloem: It is develop from procambium which is differentiated into protophloem and metaphloem.

Secondary phloem: It is developed from cambium during secondary growth which shows radial differentiation.

Vascular bundles:

Vascular bundles are a strand of conducting tissue which consist of xylem and phloem. On the basis of arrangement of xylem and phloem, they are of Radial, conjoint and amphivasal.

Function of vascular bundles are;

1.they are responsible for transport of  food, minerals and water within the plant.

2.they provide mechanical support to the plants.

Anatomy of Dicot Root and Stem:

The internal structure of a dicot root can be studied from transverse section of a young root of sunflower, gram and pea. It shows the following arrangement of tissues from the periphery to the centre.

1.Epiblema or piliferous layer: Ebliblema is the outermost single layer. It is composed of thin- walled, closely packed parenchymatous cells without intercellular spaces. Unicellular root hairs are present in epiblema.

2.Cortex: It lies below the epiblema and consists of many layers of thin walled rounded or polygonal parenchymatous cells. Cotex cells store food and conduct water from epiblema to the inner tissues.

3.Endodermis: It is the innermost layer, made up of a single layer of barrel shaped compact parenchyamatous cells without intercellular spaces. Cells of endodermis lying opposite the protoxylem elements are thin walled and known as passage cells because they allow the passage of water from roots to the xylem.

3.Pericycle: It lies internal to the endodermis and composed of single layer of thin walled parenchymatous cells containing abundant protoplasm.

5.Conjunctive tissue: The parenchyma lying in between xylem and phloem bundles constitutes the conjunctive tissue.

6.Vasculare bundles: These are arranged in an ring but xylem and phloem form an equal number of separate bundles placed on different radii.

a.Xylem: It appears conical in shape and are thick walled. Protoxylem lies towards the periphery, so the xylem is exarch while metaxlyem vessels meet in the centre and pith gets obliterated. Xylem parenchyma and fibres are absent and a few tracheids are present around the vessels.

b.Phloem: It lies alternate to xylem patches. The patches are smaller and consists of sieve tubes, companion cells and phloem parenchyma. The phloem fibres are absent. The outerpart of this tissue next to pericycle is the protophloem and inner is the metaphloem.

7.Pith: It occupies small area in the centre and consists of a few compactly arranged, thin- walled parenchymatous cells without any intercellular space.

Internal structure of dicot stem:

T.S. of a Dicot Stem (Sunflower)

The internal structure of a dicot stem can be studied from transverse section of a young sunflower.

1.Epidermis: It is the outermost layer and consists of a single layer of parenchymatous cells. The outer walls are greatly thickened and heavily cutinized. It protects the underlying tissues from mechanical injury and prevents the entry of harmful organisms.

2.Hypodermis: This layer is below the epidermis and is composed of 4 or 5 layers of collenchymatous cells. The cells are living and may contain few chloroplasts. It provides mechanical strength and elasticity to the peripheral portion of the stem. It also acts as storage of food.

3.General cortex: It lies internal to the hypodermis and consists of a few layers of thin walled, large, rounded, oval, living parenchymatous cells , having conspicuous intercellular spaces. They serve for storage of food.

4. Endodermis: It is the innermost layer of the cortex and separates the cortex from stele. They are barrel shaped and compactly arranged having no intercellular spaces. They serve as food reserve.

5.Stele: Part of stem inside of cortex is known as the stele. It consists of four regions they are the pericycle, the vascular bundle region, pith and the medullary rays.

6.Pericycle: The region between the vascular bundles and the cortex is known as thye pericycle. Parenchymatous pericycle is present outside the medullary rays which serves to store food.

7.Vascular bundle: These are arranged in a ring around the central pith and inner to pericycle. These are conjoint, collateral, open and wedge- shaped. Each bundle contain patch of xylem towards centre and phloem towards periphery and a strip of cambium in between them. There lies a layer of meristematic cell between the xylem and the phloem which is known as the cambium.

8.Pith:The centre of the stem is pith or medulla which is composed of parenchymatous cell. Food is stored in this region.

Anatomical structure of monocot root:

The internal structure of a monocot root (maize) can be described as follows;

1.Epidermis: The epidermis or outermost layer of the root is known as rhizodermis which is uniseriate and composed compact cells having no inter cellular spaces. Epidermis hairs absorb water and minerals salt.

2.Cortex: The epidermis or massive cortex lies consisting of thin walled parenchyma cell having intercellular spaces. It help in conduction of water and minerals salts from root hairs to inner tissues and storage of food.

3.Endodermis: It is innermost layer of cortex which is composed of barrel shaped, compact cells having no intercellular spaces. Young endodermal cell have internal strip of suberin and lignin. Endodermis is to regulate flow of fluid both inward as well as outward.

4.Stele: It consists of pericycle and vascular tissues where pericycle lies just below the endodermis and is composed single layered sclerenchymatous cells and in other side vascular tissues consists of alternating strands of xylem and phloem. The bundles are numerous and referred as  polyarch.

Anatomical structure of monocot stem:

                                             Fig; Structure of monocot stem

The internal structure of a monocot stem (maize) can be studied as following internal structure;

1.Epidermis: It is single outermost layered which is composed of small thin walled parenchymatous cells. A few stomata are present in the epidermis.

2.Cortex: It is not well differentiated into distinct region but is composed of hypodermis and ground tissues regions. Hypodermis lies just below the epidermis and ground tissue lies below the hypodermis. Vascular bundles are irregularly embedded in this region.

3.Vascular bundles: They are conjoint, collateral , closed and irregulary scattered in the ground tissue. It consists of xylem and phloem.In this type xylem is Y-shaped bearing two large metaxylem and phloem lies outside the xylem which is composed of sieve elements and companion cells.

Anatomy of leaf: 

Internal structure of dorsiventral leaf:

Fig; Internal structure of dorsiventral leaf

These types of leaves are found in dicotyledons e.g mango which shows following characters ;

1.Epidermis: The leaf compose of upper and lower epidermal layer. Upper epidermis is made up of thin walled compactly arranged cells and lower epidermis is made up of single layer cutinized cells.

2.Mesophyll: Mesophyll tissue lies between upper and lower epidermis which is composed of two types of chlorenchymatous cells ( palisade and spongy).

3.Vascular bundles: It consists of xylem and phloem. Xylem is towards the upper epidermis and phloem towards the lower epidermis where phloem lies below the xylem.

Internal structure of isobilateral leaf:

Fig; Internal structure of isobilateral leaf

Isobilateral leaf are found in monocotyledon ( maize) which are usually erect. Internal structure consists of following characters:

1.Epidermis: Leaf is bounded by thickly cuticularised epidermis in both sides.

2.Mesophyll: It is found between the two epidermal layers which is composed of large oval, spongy parenchyma of varied shape and size.

3.Vascular bundle: These bundles are arranged in a parallel series which is differ in their size. Bundle is conjoint, collateral and closed type.

Secondary growth in Dicot Stem:

On the basis of the activities of cambium and cork cambium it can be discussed under following two types;

A.Activity of cambium:

1.Formation of cambium ring in dicot stem cambium is present between xylem and phloem. During secondary growth the cambium, some cells of medullary ray also becomes active form in intrafasicular cambium. Intra and inter fasicular cambium unite together to form cambium ring.

2.Formation of secondary tissues: When the cambium ring active as it starts cutting of new cell, both internally and externally. The cut off cells get differentiated into elements of phloem and inner side is modified into the elements of xylem.

3.Secondary medullary ray: Certain cell of cambium instead of forming secondary xylem and phloem form some narrow bands of living parenchyma cells. These form 2 or 3 layers thick radical rows passing through secondary xylem and phloem are called secondary medullary rays.

4.Annual ring: The age of the plant can be approximately determined by cutting the number of annual rings where dendrochronology is the branch of anatomy which deals with determining the age of tree.

5.Wood: In the old tress, the secondary wood is differentiated into a centrally situated darker and harder called heart wood and outer light coloured zone called sap wood.  

Fig; digram showing stages in the sec. growth of a dicot stem.

B.Activity of the cork-cabium: The marked increased in the diameter in the stem brought about by the secondary thickening exerts great pressure on the outer tissues cause rupture of the cortex and epidermis as a result the outermost layer of the cortex next to epidermis become meristematic and begins to divide the process known as cork- cambium.

Significance of secondary growth:

1.It provides support to increasing weight of aerial growth.

2.It produces a corky bark around the tree, protects from heat, cold and infections.

3.It adds new conducting tissues for replacing old non functioning ones.