THEORY
Anatomy
Anatomy is the study of internal structure of organisms. Plant anatomy includes organization and structure of tissues. Tissue is a group is cells having a common origin and usually performing a common function.
The Tissue
A group of cells having a common origin and usually performing common function are called tissues.
There are two types of tissues (i) Meristematic (ii) Permanent.
Meristematic Tissues: The meristematic tissue is made up of the cells which have the capability to divide. Meristems in plants are restricted to a specialized regions and responsible to the growth of plants.
Permanent Tissues: The permanent tissues are derived from meristematic tissue, are composed of cells, which have lost the ability to divide and have become structurally and functionally specialised.
Meristematic tissues:
There are three types of Meristem:
- Apical meristem
- Intercalary meristem
- Lateral meristem
Apical meristem: Meristematic tissue is a simple tissue composed of group of similar and immature cells which can divide and form new cells. The meristem which occurs at tips of roots and shoots are called apical meristem.
Intercalary meristem: Intercalary meristem occurs between mature tissues especially in grasses. Both apical meristems and intercalary meristems are primary meristems because they appear early in life of a plant and help to form the primary plant body.
Lateral meristem: The meristem which occurs on the sides and takes part in increasing girth of the plants are called Lateral meristem. Intrafascicular cambium in the primary lateral meristem. Vascular cambium, cork cambium are secondary meristem.
Axillary bud: The buds which are present in the axils of leaves (Consist of cells left behind from shoot apical meristem) and are responsible for forming branches of flowers.
Permanent tissues The permanent tissues are derived from meristematic tissue, are composed of cells, which have lost the ability to divide and have become structurally and functionally specialized.
Parenchyma: Parenchyma is a simple permanent living tissue which is made up of thin-walled isodiametric cells. Each cell encloses a large central vacuole and peripheral cytoplasm containing nucleus. They are found in non-woody and soft areas of stem, root, leaves, fruits and flowers. They store the food and provide turgidity to softer parts of plant.
Collenchyma: Collenchyma consists of cells which are much thickened at corner due to cellulose, hemicellulose and pectin. Oval, spherical or polygonal often contain chlorophyll. They provide mechanical support to the growing parts of the plants like young stem.
Sclerenchyma: Sclerenchyma are supportive tissue having highly thick-walled cells with little or no protoplasm due to deposition of cellulose or lignin. They are of two types: fibers and sclereids. They provide mechanical support to mature plant organs to tolerate bending, shearing, compression etc
Complex tissue: Permanent tissues having all cells similar in structure and function are called simple permanent tissues and those having different kinds of cells are called complex tissue.
Xylem: Xylem consists of tracheid’s vessels, xylem fibers and xylem parenchyma. It conducts water and minerals from roots to other parts of plant.
Phloem: Phloem consists of sieve tube elements, companion cells, phloem fibers and phloem parenchyma; Phloem transports the food material from leaves to various parts of the plant.
Endarch: Primary xylem is of two types- protoxylem and metaxylem. In stem, protoxylem lies in centre and metaxylem towards periphery. This type of primary xylem is called endarch.
Exarch: In roots, protoxylem lies in periphery and metaxylem lies towards the center. This type of primary xylem is called exarch.
Tissue System
The tissue system is divided into three categories based on a division of labor. Each system usually consists of a combination of tissue organizations that perform specific functions.
Epidermal Tissue System
- It forms the outermost covering of whole plant body, which consists of epidermal cells, stomata, epidermal appendages (trichomes and hairs).
- Epidermis is single layered, parenchymatous with waxy thick layers of cuticle to prevent water loss.
- Stomata is present in epidermis of leaves. It regulates the transpiration and gaseous exchange. In dicots, stomata are bean-shaped having two guard cells closing the stomatal pore. In monocots, stoma is dumbbell shaped. Guard cells contain chloroplasts and help in opening and closing of stomata.
- Guard cells are surrounded by subsidiary cells. The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus.
- Epidermis also contains a number of hairs. Root hairs are unicellular elongation of epidermal cells. Trichomes are present on stems, which are multicellular, branched or un-branched preventing water loss due to transpiration.
The ground Tissue System
- All the tissue between epidermis and vascular bundle forms the ground tissues. It consists of simple permanent tissues. Parenchyma is present in pericycle, cortex, pith and medullary rays in stem and roots.
- In leaves the mesophyll, chloroplast containing cell, forms the ground tissues.
The vascular tissue system
It includes vascular bundles which are made up of xylem and phloem.
Anatomy of Root
| Dicot Root | Monocot Root |
| Cortex is comparatively narrow. | Cortex is very wide. |
| Endodermis is less thicknened casparian strips are more prominent. | Endodermal cells are highly thickened Casparian strips are visible only in young roots. |
| The xylem and phloem bundles varies from 2 to 5. | Xylem and phloem are more than 6 (polyarch). |
| Pith is absent or very small. | Well developed pith is present. |
| Secondary growth takes place. | Secondary growth is absent. |
Casparian Strips
The tangential as well as radial walls of endodermal cells of dicot roots have deposition of water impermeable, waxy material, suberin in the form of casparian strips.
Anatomy of Stem
| Dicot Stem | Monocot Stem |
| The ground tissue is differentiated into cortex, endodermis, pericycle and pitch. | The ground tissue is made up of similar cells |
| The vascular bundles are arranged in a ring. | The vascular bundles are scattered throughout the ground tissue. |
| Vascular bundles are open, without Surrounded bundle sheath and wedge-shaped outline. | Vascular bundles are closed, by sclerenchymatous bundle sheath, oval or rounded in shape. |
| The stem shows secondary growth due to presence of cambium between xylem and phloem. | Secondary growth is absent. |
Secondary growth dicot stem
An increase in the girth (diameter) in plants. Vascular cambium and cork cambium (lateral meristems) are involved in secondary growth.
- Formation of cambial ring: Intrafascicular cambium + interfascicular cambium.
- Formation of secondary xylem and secondary phloem from cambial ring.
- Formation of spring wood and autumn wood.
- Development of cork cambium(phellogen).
Secondary growth in dicot roots
Secondary growth in dicot roots occur with the activity of secondary meristems (vascular cambium). This cambium is produced in the stele and cortex, and results in increasing the girth of dicot roots.
Spring wood
It is also called early wood in which cambium is active Vessels with wide cavities & Light in colour, low density presence of Xylary elements more.
Autumn Wood
Also called late wood in which cambium is less active presence of Xylary elements is less Vessels are narrow & Dark, high density.
Heartwood
Central or innermost region of stem which is hard, durable and resistant to attack of Microorganisms and insects & Not involved in conduction of water, gives mechanical support to stem.
Sapwood
Peripheral region stem, light in colour Involved in conduction of water and mineral.
Anatomy of Leaf
| Dorsiventral (Dicot) Leaf | Isobilateral (monocot) Leaf |
| Stomata are absent. | Stomata present on both sides. |
| Mesophyll is differentiated into two parts. | Mesophyll is undifferentiated. |
| Bundle sheath is single layered. | Bundle sheath is double layered |
| Hypodermis of the mid-rib region, is collenchymatous. | Hypodermis of the mid-rib region is sclerenchymatous. |
| Stomata have kidney shaped guard cells. | Stomata have kidney shaped dumb cells. |
Cork Cambium
- Meristematic tissue which develops in the cortex region is called cork cambium or phellogen.
- The phellogen cuts off cells on both sides. The outer cells differentiate to form cork or phellem while the inner cells differentiate into secondary cortex or phelloderm.
- Phellogen, phellem and phelloderm are collectively called periderm.
- Due to activity of the cork cambium, pressure builds up on the remaining layers peripheral
Lenticels
- At certain regions, 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 called lenticels.
- Lenticels permit the exchange of gases between the outer atmosphere and the internal tissue of the stem.
Secondary Growth in Roots
- The vascular cambium of the dicot root originates from the tissue located just below the phloem bundles. A portion of pericycle tissue present above the protoxylem forms a continuous wavy ring. It gradually becomes circular. Rest of the steps are similar as in dicot stem.
- Secondary growth takes place in stems and roots of gymnosperms. No secondary growth occurs in monocots.