🌸 Anatomy of Flowering Plants – NEET High Quality Notes | Class 11 Biology

1. Introduction to Tissues

When we look at living organisms, it is easy to notice similarities and differences in their external structures, whether they are plants or animals. Similarly, if we examine the internal structure, we also find many similarities and variations. This chapter focuses on the internal structure and functional organisation of higher plants, which is known as plant anatomy. In plants, the cell is the basic unit, and cells are organized into tissues, which in turn form organs. Each organ in a plant shows distinct internal structures. Among angiosperms, there are noticeable anatomical differences between monocots and dicots. The internal structures of plants are not only different but also show adaptations to various environments, helping the plant survive and function effectively in its habitat.

2. Cellular Tissue Organization in Plants

In plants, tissues are not only classified based on the types of cells, but they also vary depending on their location in the plant body. Their structure and function are closely related to where they are found. Based on their structure and position, plants have three main tissue systems. The first is the epidermal tissue system, which forms the outer protective layer of the plant. The second is the ground or fundamental tissue system, which makes up most of the plant body and is involved in support, storage, and photosynthesis. The third is the vascular or conducting tissue system, which is responsible for the transport of water, minerals, and nutrients throughout the plant. Each of these tissue systems shows special adaptations to perform its specific functions efficiently.

Plant Surface Protective Layer i.e. Epidermal Tissue

The system of epidermal tissue forms the outermost protective covering of the entire plant body and is made up of epidermal cells, stomata, and epidermal appendages like trichomes and hairs. The epidermis is the outer layer of the primary plant body and is usually single-layered, consisting of elongated, compactly arranged parenchymatous cells with a small amount of cytoplasm along the cell wall and a large central vacuole. The outer surface of the epidermis is often covered by a waxy, thick layer called the cuticle, which helps prevent water loss, but this cuticle is absent in roots. On the epidermis of leaves, there are special structures called stomata that control transpiration and gaseous exchange. Each stoma is formed by two bean-shaped guard cells surrounding a stomatal pore; however, in grasses, the guard cells are dumbbell-shaped. The outer walls of guard cells are thin, while the inner walls are thickened, helping them open and close the pore. The guard cells also contain chloroplasts to regulate stomatal movements. Around the guard cells, some specialised epidermal cells are present, called subsidiary cells, and together the stomatal pore, guard cells, and subsidiary cells form the stomatal apparatus. The epidermis also bears hairs: in roots, these are unicellular root hairs that help absorb water and minerals from the soil, while on the stem, the hairs are called trichomes. Trichomes are usually multicellular and may be branched or unbranched, soft or stiff, and sometimes even secretory. They help in reducing water loss by preventing excessive transpiration and also provide protection to the plant.

Internal Support and Storage Tissue in Plants, Ground Tissue

All the tissues in a plant, except the epidermis (outer covering) and vascular bundles (xylem and phloem), together make up the ground tissue. This tissue is mainly formed of simple tissues like parenchyma, collenchyma, and sclerenchyma.

The parenchyma cells are commonly found in several parts of the plant, such as the cortex (outer layer of the stem or root), pericycle (layer just inside the endodermis), pith (central part of the stem), and medullary rays (tissue between vascular bundles). In the primary stems and roots, these cells provide support and storage.

In leaves, the ground tissue is made up of thin-walled cells that contain chloroplasts. This specialized tissue is called the mesophyll, and it is mainly responsible for photosynthesis.

Vascular Tissue System – Transport and Support in Plants

The vascular system of plants is made up of complex tissues called xylem and phloem, which together form the vascular bundles. In dicotyledonous stems, there is a special layer called cambium located between the xylem and phloem. Because of the presence of cambium, these vascular bundles can produce secondary xylem and phloem, which helps the plant grow in thickness. Therefore, they are called open vascular bundles.

In contrast, monocotyledonous stems do not have cambium in their vascular bundles. Since they cannot form secondary tissues, these are called closed vascular bundles.

The arrangement of xylem and phloem can also vary. When xylem and phloem are placed alternately along different radii, the pattern is called radial arrangement, which is usually seen in roots. On the other hand, in a conjoint vascular bundle, xylem and phloem are together along the same radius, which is commonly found in stems and leaves. In such bundles, the phloem is generally located on the outer side of the xylem.

3. Plant Anatomy – Structure of Dicot and Monocot Plants

To clearly understand how tissues are arranged in different parts of a plant, it is helpful to examine the transverse sections (cross-sections) of roots, stems, and leaves. Studying these sections from the mature zones of these organs makes it easier to see the organization and arrangement of different tissues, such as the epidermis, ground tissue, and vascular tissues. This approach helps in understanding how each tissue contributes to the structure and function of the plant parts.

Dicot Root – Structure and Organization

If we look at a sunflower root in cross-section, the internal tissue organization can be clearly understood layer by layer. The outermost layer is called the epiblema, and many of its cells form unicellular root hairs, which help in absorption of water and minerals. Inside the epiblema is the cortex, made up of several layers of thin-walled parenchyma cells with intercellular spaces that allow gas exchange. The innermost layer of the cortex is called the endodermis, which consists of a single layer of barrel-shaped cells without spaces. The walls of endodermal cells contain suberin, a waxy, water-impermeable material that forms Casparian strips, controlling the movement of water into the vascular system.

Just inside the endodermis, there are a few layers of thick-walled parenchyma cells called the pericycle, which play an important role in the formation of lateral roots and the vascular cambium during secondary growth. The pith at the center is usually small or barely visible. Between the xylem and phloem, the parenchyma cells form the conjunctive tissue. Typically, there are two to four patches of xylem and phloem, and later, a cambium ring develops between them.

All the tissues located inside the endodermis, including the pericycle, vascular bundles, and pith, together form the stele, which is the central part of the root responsible for transport and support.

Monocot Root – Structure and Organization

The monocot root is quite similar to the dicot root in its basic structure. It has all the major tissues such as epidermis, cortex, endodermis, pericycle, vascular bundles, and pith. However, there are some differences. In monocot roots, there are usually more than six xylem bundles (called polyarch) compared to dicot roots, which have fewer xylem bundles. The pith in monocot roots is large and well-developed, unlike in dicots where it is small or inconspicuous. Another important difference is that monocot roots do not show secondary growth, meaning they do not thicken much with age like dicot roots do.

Dicot Stem – Structure and Organization

The transverse section of a young dicotyledonous stem reveals the organization of its tissues in a clear way. The epidermis forms the outermost protective layer of the stem. It is covered by a thin cuticle and may have trichomes (hair-like structures) and a few stomata for gas exchange. Just beneath the epidermis lies the cortex, which is made up of multiple layers and is divided into three sub-zones.

The outer hypodermis consists of a few layers of collenchyma cells that provide mechanical support to the young stem. Below the hypodermis, the cortical layers are made up of rounded, thin-walled parenchyma cells with noticeable intercellular spaces. The innermost layer of the cortex is called the endodermis, which is rich in starch grains and is therefore also called the starch sheath.

On the inner side of the endodermis and just above the phloem lies the pericycle, present as semi-lunar patches of sclerenchyma. Between the vascular bundles, there are a few layers of radially arranged parenchyma cells called medullary rays, which help in radial transport of water and nutrients.

The vascular bundles themselves are arranged in a ring, a typical feature of dicot stems. Each bundle is conjoint, meaning xylem and phloem are together, open (with cambium for secondary growth), and has endarch protoxylem (protoxylem towards the center). The central portion of the stem is occupied by the pith, which is made of a large number of rounded parenchyma cells with big intercellular spaces, providing storage and support.

Monocot Stem – Structure and Organization

The monocot stem has some distinct features compared to the dicot stem. Its hypodermis is made up of sclerenchyma cells, providing mechanical strength. Inside, there is a large amount of parenchymatous ground tissue, which is loosely arranged and occupies most of the stem, giving it storage and support.

The vascular bundles in monocot stems are scattered throughout the ground tissue instead of being arranged in a ring. Each vascular bundle is conjoint (xylem and phloem together) and closed, meaning it does not have cambium and therefore cannot form secondary tissues. The peripheral bundles near the edge are usually smaller, while the central bundles are larger. Unlike dicot stems, phloem parenchyma is absent, and some water-containing cavities are present inside the vascular bundles, helping in water storage and transport.

Dicot Leaf (Dorsiventral) – Structure and Organization

The vertical section of a dorsiventral (dicot) leaf through its lamina shows three main parts: the epidermis, mesophyll, and vascular system. The epidermis covers both the upper surface (adaxial epidermis) and the lower surface (abaxial epidermis) of the leaf and has a noticeable cuticle for protection against water loss. The abaxial epidermis usually has more stomata than the adaxial side, and sometimes the adaxial epidermis may lack stomata entirely.

The tissue between the upper and lower epidermis is called the mesophyll, which contains chloroplasts and carries out photosynthesis. The mesophyll is made of parenchyma cells and has two types: palisade parenchyma and spongy parenchyma. The palisade parenchyma is placed near the upper side (adaxial) of the leaf and consists of elongated cells arranged vertically and parallel to each other, which help in maximum light absorption. Below the palisade layer is the spongy parenchyma, which is oval or round, loosely arranged, and extends to the lower epidermis. There are many air spaces and cavities between these cells, aiding in gas exchange.

The vascular system includes vascular bundles, which are seen in the veins and the midrib. The size of the vascular bundles depends on the size of the veins, and in dicot leaves with reticulate venation, the veins vary in thickness. Each vascular bundle is surrounded by a layer of thick-walled bundle sheath cells for protection and support. In the vascular bundle, the xylem is generally located towards the upper side, while the phloem is found towards the lower side of the bundle.

Monocot Leaf (Isobilateral) – Structure and Organization

The isobilateral (monocot) leaf has an anatomy that is similar to the dorsiventral (dicot) leaf, but there are some important differences. In an isobilateral leaf, stomata are present on both upper (adaxial) and lower (abaxial) surfaces of the epidermis, unlike dorsiventral leaves where stomata are mostly on the lower surface. The mesophyll is not divided into palisade and spongy parenchyma; instead, it consists of uniform parenchyma cells throughout.

In grasses, certain adaxial epidermal cells along the veins transform into large, empty, colourless cells called bulliform cells. These cells help in water conservation: when turgid (full of water), the leaf remains open, and when flaccid (water-stressed), the leaf curls inward to reduce water loss.

The parallel venation seen in monocot leaves is reflected in the vascular bundles, which are generally of similar size throughout the leaf, except for the main veins where bundles may be larger. This arrangement helps in efficient transport of water and nutrients across the leaf.

3. Chapter Overview – Key Points and Concepts

An anatomical view of a plant shows that it is made up of different types of tissues, each performing specific functions. Broadly, plant tissues are classified into meristematic tissues and permanent tissues. Meristematic tissues include apical, lateral, and intercalary meristems, which are responsible for growth. Permanent tissues can be simple (like parenchyma, collenchyma, sclerenchyma) or complex (like xylem and phloem) and perform functions such as food assimilation and storage, transport of water, minerals, and photosynthates, and mechanical support.

Plant tissues are organized into three main tissue systems: epidermal, ground, and vascular. The epidermal tissue system consists of epidermal cells, stomata, and epidermal appendages, forming the outer protective layer of the plant. The ground tissue system makes up the bulk of the plant and includes three zones: cortex, pericycle, and pith, which provide storage, support, and metabolic functions. The vascular tissue system is formed by xylem and phloem, which act as conducting tissues, transporting water, minerals, and food.

Vascular bundles vary in their structure and arrangement depending on the presence of cambium and the position of xylem and phloem. This variation is particularly seen when comparing monocotyledonous and dicotyledonous plants. These plants differ in the type, number, and location of vascular bundles. Additionally, secondary growth, which increases the thickness of roots and stems, occurs in most dicotyledonous roots and stems, but is generally absent in monocotyledons.