Notes " Anatomy of flowering plants "
Notes " Anatomy of flowering plants "
By :- Nagraj Sir
Tissues - A group of cells having a common origin and usually performing common function are called tissues.
Tissue system :
Based on structure and location, tissue systems are 3 types:
o Epidermal tissue system
o Ground (fundamental) tissue system
o Vascular (conducting) tissue system
1. Epidermal Tissue System
- It forms the outer-most covering of the whole plant body.
- It comprises epidermal cells, stomata and epidermal appendages (trichomes & hairs).
Epidermis
- It is the outermost layer of the primary plant body.
- it is single layered.
- It is elongated, compactly arranged parenchymatous cells
- The outside of the epidermis is often covered with a waxy thick layer (cuticle). It prevents the loss of water. Cuticle is absent in roots.
Stomata
- These present in the epidermis of leaves.
- regulate the transpiration and gaseous exchange.
- A stoma is made of two bean-shaped cells (guard cells).
- In grasses, the guard cells are dumbbell shaped.
The outer walls of guard cells are thin and the inner walls are highly thickened.
- The guard cells possess chloroplasts and regulate the opening and closing of stomata.
- Sometimes, a few epidermal cells near the guard cells become specialized in their shape and size. They are known as subsidiary cells.
Epidermal appendages
- The cells of epidermis bear many hairs.
- Root hairs: Unicellular elongated cells. They help to absorb water and minerals
- Trichomes: epidermal hairs on the stem. They multicellular, branched or unbranched and soft or stiff. Trichomes help to prevent water loss due to transpiration.
2. The Ground Tissue System
- All tissues except epidermis and vascular bundles constitute the ground tissue.
- It consists of simple tissues (parenchyma, collenchyma and sclerenchyma).
- Parenchymatous cells are present in cortex, pericycle, pith and medullary rays, in the primary stems and roots.
- In leaves, the ground tissue consists of chloroplast containing cells and is called mesophyll.
3. The Vascular Tissue System
It consists of complex tissues (xylem and phloem).
Based on the presence or absence of cambium, vascular bundles are 2 types:
o Open type: In this, cambium is present between phloem and xylem. So vascular bundles can form secondary xylem and phloem tissues. E.g. dicotyledonous stems.
o Closed type: In this, cambium is absent. Hence, they do not form secondary tissues. E.g. monocotyledons.
Based on the arrangement of xylem and phloem, vascular bundles are 2 types:
o Radial type: Xylem and phloem are arranged in an alternate manner on different radii. Seen in roots.
o Conjoint type: Xylem and phloem are jointly situated at the same radius of vascular bundles. Seen in stems and leaves. Conjoint vascular bundles usually have phloem located only on the outer side of xylem.
Dicotyledonous (Dicot) Root
Transverse section of the sunflower root shows the following tissue organization:
o Epidermis (epiblema): The outermost layer. Many cells of epiblema protrude in the form of
unicellular root hairs.
o Cortex: It consists of several layers of thin walled parenchyma cells with intercellular spaces.
o Endodermis: Innermost layer of the cortex. It comprises a single layer of barrel-shaped cells without 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.
o Stele: All tissues on the inner side of the endodermis together constitute stele. They include § Pericycle: A few layers of thick-walled parenchyomatous cells next to endodermis. Initiation of lateral roots and vascular cambium during the secondary growth takes place in these cells § Pith: Innermost region of the stele. It is small or inconspicuous.
§ Conjunctive tissue: The parenchymatous cells which lie between the xylem and the phloem.
§ Vascular bundles: 2-4 xylem & phloem patches. Later, a cambium ring develops between the xylem & phloem.
Monocotyledonous (Monocot) Root
- It has epidermis, cortex, endodermis, pericycle, vascular bundles and pith.
- There are usually more than six (polyarch) xylem bundles.
- Pith is large and well developed.
- Monocot roots do not undergo any secondary growth.
o Epidermis: Outermost protective layer. Covered with a thin layer of cuticle, it may bear trichomes & few stomata.
o Cortex: Multiple layers of the cells arranged in between epidermis and pericycle. It consists of 3 sub-zones:
§ Hypodermis: Outer zone. It consists of a few layers of collenchymatous cells just below the epidermis. It provides mechanical strength to the young stem.
§ Cortical layers: Below hypodermis. They consist of rounded thin walled parenchymatous cells with
conspicuous intercellular spaces.
§ Endodermis: Innermost layer. The cells are rich in starch grains. So the layer is also called as the starch sheath. Pericycle is present on the inner side of the endodermis and above the phloem in the form of semilunar patches of sclerenchyma.
o Stele: Consists of pericycle, vascular bundles, medullary rays & pith.
§ Medullary rays: These are few layers of radially placed parenchymatous cells in between vascular bundles.
§ Vascular bundles: Large in number. They are arranged in a ring. Ring arrangement is a characteristic of dicot stem. Each vascular bundle is conjoint, open, and with endarch protoxylem.
§ Pith: Central portion of the stem. It has many rounded, parenchymatous cells with large intercellular spaces.
- It has a sclerenchymatous hypodermis, many scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath, and a large, conspicuous parenchymatous ground tissue.
- Vascular bundles are conjoint & closed. Peripheral vascular bundles are smaller than centrally located ones.
- The phloem parenchyma is absent, and water-containing cavities are present within the vascular bundles.
The vertical section of a dicot leaf through lamina shows 3 main parts: Epidermis, mesophyll & vascular system.
o Epidermis: It covers both the upper surface (adaxial epidermis) and lower surface (abaxial epidermis) of the leaf. It has a conspicuous cuticle. Abaxial epidermis generally bears more stomata than the adaxial epidermis. The latter may even lack stomata.
o Mesophyll: The tissue between the upper and the lower epidermis. It is made up of parenchyma.
They contain chloroplasts for photosynthesis. It has 2 types of cells:
§ Palisade parenchyma: It is adaxially placed. Made up of elongated cells arranged vertically and parallel to each other.
§ Spongy parenchyma: The oval or round and loosely arranged. It is situated below the palisade cells and extends to the lower epidermis. There are numerous large spaces and air cavities between these cells.
o Vascular system: It includes vascular bundles. They can be seen in the veins and midrib. Size of vascular bundles is dependent on the size of the veins. The veins vary in thickness in the reticulate venation of dicot leaves. Vascular bundles are surrounded by a layer of thick walled bundle sheath cells.
Monocotyledonous (Isobilateral) Leaf
The anatomy of monocot leaf is like that of the dicot leaf in many ways. It shows following
differences:
o Stomata are present on both surfaces of the epidermis.
o Mesophyll is not differentiated into palisade and spongy parenchyma.
o In grasses, certain adaxial epidermal cells along the veins modify themselves into large, empty, colourless cells. These are called bulliform cells. When the bulliform cells 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.
o Parallel venation is reflected in the near similar sizes of vascular bundles (except in main veins).
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