Unit 1
Plant Anatomy
Chapter 1
IntroductionClass 12 Biology Plant Anatomy Notes PDF
Read the original Plant Anatomy notes directly below.
On mobile, swipe inside the PDF to read pages and pinch to zoom.
Introduction
Plant anatomy is the branch of botany that studies the internal structure and organization of plants. In Grade 12 Biology, this unit focuses on plant tissues, their classification, the internal anatomy of dicot and monocot roots, stems and leaves, and the secondary growth of a dicot stem.
1. Concept of Tissue
A tissue is a group of cells that have a common origin and usually work together to perform a particular function. In plants, cells become organized into tissues so that activities such as growth, storage, protection, mechanical support and transport can be carried out efficiently.
Plant tissues are broadly divided into meristematic tissues and permanent tissues. Meristematic tissues remain actively dividing, whereas permanent tissues are formed when meristematic cells mature, differentiate and acquire specialized functions.
Diagram: Classification of Plant Tissues
2. Meristematic Tissue
Meristematic tissue consists of young, living and actively dividing cells. These cells continuously produce new cells and are therefore responsible for plant growth.
Characteristics of Meristematic Cells
- Cells are living and capable of repeated cell division.
- They are usually small and nearly isodiametric.
- Cell walls are thin and mainly made of cellulose.
- The cytoplasm is dense and the nucleus is prominent.
- Vacuoles are absent or very small in actively dividing cells.
- Intercellular spaces are generally absent.
Types of Meristem Based on Position
Apical Meristem
It occurs at the tips of roots and shoots. It brings about primary growth, mainly increasing the length of the plant body.
Intercalary Meristem
It occurs between mature tissues, commonly near nodes or at the base of internodes and leaves in grasses. It helps rapid elongation and regeneration.
Lateral Meristem
It occurs along the sides of stems and roots. Vascular cambium and cork cambium are important examples. It is associated with secondary growth.
Why Meristems Matter
Because plants keep meristematic regions throughout life, they can continue producing new roots, shoots, leaves, flowers and secondary tissues.
Diagram: Position of Meristems in a Plant
3. Permanent Tissue
Permanent tissues are formed from meristematic cells after differentiation. Their cells usually lose the power of active division and become specialized for functions such as photosynthesis, storage, support and conduction.
3.1 Simple Permanent Tissues
| Tissue | Main Features | Major Functions |
|---|---|---|
| Parenchyma | Living, thin-walled cells; usually with intercellular spaces. | Storage, photosynthesis, secretion, wound repair and packing. |
| Collenchyma | Living elongated cells with uneven thickening at the corners. | Flexible mechanical support in young stems, petioles and leaves. |
| Sclerenchyma | Usually dead at maturity; thick, lignified secondary walls. | Strong mechanical support and protection. |
Diagram: Simple Permanent Tissues
3.2 Complex Permanent Tissues
Complex tissues contain more than one type of cell working together. The two major vascular tissues are xylem and phloem.
| Feature | Xylem | Phloem |
|---|---|---|
| Main transport | Water and dissolved minerals | Organic food, especially sugars |
| Main direction | Predominantly upward from roots | Can occur in different directions according to source and sink |
| Important elements | Tracheids, vessels, xylem fibres, xylem parenchyma | Sieve tube elements, companion cells, phloem fibres, phloem parenchyma |
| Extra role | Mechanical support | Food distribution and storage |
Diagram: Xylem and Phloem in a Vascular Bundle
4. Anatomy of Dicot and Monocot Roots
A young root in transverse section generally shows an outer epidermal region, cortex, endodermis, pericycle and the central vascular region or stele. Dicot and monocot roots follow the same basic plan but differ in the number and arrangement of vascular tissues and in the development of pith and secondary growth.
4.1 Dicot Root
- Epiblema/Epidermis: outermost single layer; root hairs may be present.
- Cortex: several layers of parenchymatous cells.
- Endodermis: innermost layer of cortex that regulates movement into the vascular cylinder.
- Pericycle: lies just inside the endodermis and gives rise to lateral roots.
- Vascular tissue: xylem and phloem are arranged on different radii, forming a radial vascular system.
- Pith: small or inconspicuous compared with a monocot root.
4.2 Monocot Root
- It also has epidermis, cortex, endodermis and pericycle.
- The vascular bundles are radial.
- Xylem strands are usually numerous, a condition called polyarch.
- A large and well-developed pith occupies the centre.
- Normal secondary growth is generally absent.
Diagram: Simplified T.S. of Dicot Root and Monocot Root
| Feature | Dicot Root | Monocot Root |
|---|---|---|
| Xylem | Usually fewer xylem arms | Usually many xylem strands (polyarch) |
| Pith | Small or poorly developed | Large and well developed |
| Secondary growth | Can occur in many dicot roots | Usually absent |
5. Anatomy of Dicot and Monocot Stems
5.1 Dicot Stem
In a typical young dicot stem, the epidermis forms the outer protective layer. Below it lies the cortex, followed by vascular bundles arranged in a ring. Each vascular bundle generally contains phloem toward the outside, cambium in the middle and xylem toward the inside. The presence of cambium makes the bundle open.
5.2 Monocot Stem
In a typical monocot stem, vascular bundles are numerous and scattered throughout the ground tissue rather than arranged in a ring. Cambium is generally absent from each vascular bundle, so the bundles are described as closed. Distinct cortex, pith and medullary rays are usually not clearly differentiated.
Diagram: Simplified T.S. of Dicot Stem and Monocot Stem
| Feature | Dicot Stem | Monocot Stem |
|---|---|---|
| Vascular bundles | Arranged in a ring | Scattered in ground tissue |
| Cambium | Usually present; bundles are open | Usually absent; bundles are closed |
| Ground tissue | Cortex and pith are generally distinct | Ground tissue is generally not clearly divided into cortex and pith |
| Secondary growth | Common in many dicot stems | Generally absent in typical monocot stems |
6. Anatomy of Dicot and Monocot Leaves
6.1 Dicot Leaf
A typical dicot leaf is usually dorsiventral, meaning the upper and lower surfaces differ in structure. The mesophyll is commonly differentiated into palisade tissue below the upper epidermis and spongy tissue toward the lower epidermis. Stomata are often more numerous on the lower surface.
6.2 Monocot Leaf
A typical monocot leaf is often isobilateral, with the two surfaces more similar in structure. Mesophyll is usually not clearly differentiated into palisade and spongy regions. In grasses, large bulliform cells may occur in the upper epidermis and help in leaf rolling during water stress.
Diagram: Simplified Internal Structure of a Dicot Leaf
| Feature | Dicot Leaf | Monocot Leaf |
|---|---|---|
| General type | Usually dorsiventral | Usually isobilateral |
| Mesophyll | Often differentiated into palisade and spongy tissue | Usually not clearly differentiated |
| Stomata | Often more abundant on lower epidermis | Often present on both surfaces |
| Bulliform cells | Generally absent | Common in many grasses |
7. Secondary Growth in a Dicot Stem
Secondary growth is the increase in thickness or girth of a stem or root due mainly to the activity of lateral meristems. It is common in woody dicot plants.
Main Steps
- Formation of a cambial ring: fascicular cambium within vascular bundles becomes connected with interfascicular cambium between the bundles.
- Production of secondary vascular tissues: the cambial ring cuts off secondary xylem toward the inside and secondary phloem toward the outside.
- Increase in girth: repeated cambial activity adds more secondary tissues, especially secondary xylem.
- Formation of cork cambium: as the stem expands, a secondary protective tissue system develops.
- Periderm formation: cork cambium produces cork toward the outside and living secondary cortex toward the inside, helping replace the stretched outer epidermal tissues.
Diagram: Secondary Growth in a Dicot Stem
8. Important Exam Points
- Define tissue and plant anatomy.
- Differentiate meristematic and permanent tissues.
- Write characteristics of meristematic cells.
- Differentiate apical, intercalary and lateral meristems.
- Give functions of parenchyma, collenchyma and sclerenchyma.
- Differentiate xylem and phloem.
- Differentiate dicot and monocot root.
- Differentiate dicot and monocot stem.
- Differentiate dicot and monocot leaf.
- Define secondary growth and explain the role of vascular cambium.
One-Minute Revision
- Meristematic tissue = actively dividing tissue.
- Permanent tissue = differentiated tissue with specialized functions.
- Apical meristem = increase in length.
- Lateral meristem = increase in girth.
- Xylem = conduction of water and minerals.
- Phloem = translocation of organic food.
- Dicot stem = vascular bundles usually in a ring and open.
- Monocot stem = vascular bundles scattered and usually closed.
- Dicot leaf = usually dorsiventral.
- Monocot leaf = usually isobilateral.
- Secondary growth = increase in girth due mainly to lateral meristems.
9. Frequently Asked Questions
What is plant anatomy?
Plant anatomy is the study of the internal structural organization of plants, including tissues and the internal structure of organs such as roots, stems and leaves.
What are the two main categories of plant tissues?
Plant tissues are broadly grouped into meristematic tissues and permanent tissues.
Why are vascular bundles of a dicot stem called open?
They are called open because cambium is present between xylem and phloem, allowing formation of secondary vascular tissues.
Why is secondary growth important?
Secondary growth increases the thickness and mechanical strength of stems and roots and produces large amounts of secondary xylem, which forms most of the wood in woody plants.
Study scope: Prepared for NEB Class 12 Biology Unit 1 – Plant Anatomy. The article is written as an original explanatory study resource covering the syllabus areas: concept and types of plant tissues, anatomy of dicot and monocot root, stem and leaf, and secondary growth of a dicot stem.
Discussion
Share a helpful question, idea, or explanation with other students.