Sunday, July 26, 2020

Anatomy of flowering plants - Notes (Class 11) Part 2: The Tissue System

6. ANATOMY OF FLOWERING PLANTS

THE 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.

-    Epidermis is usually single layered.

-    It is made up of elongated, compactly arranged parenchymatous cells with small amount of cytoplasm lining the cell wall and a large vacuole.

-    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 are structures present in the epidermis of leaves.

-    Stomata 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 (away from the stomatal pore) are thin and the inner walls (towards the stomatal pore) are highly thickened.

-    The guard cells possess chloroplasts and regulate the opening and closing of stomata.

-    Sometimes, some epidermal cells near the guard cells become specialized in shape and size. They are called subsidiary cells.

-    The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus.


Epidermal appendages


-    Root hairs: Unicellular elongations of the epidermal cells. They help to absorb water and minerals from the soil.

-    Trichomes: They are the epidermal hairs on the stem. They are usually multicellular, branched or unbranched and soft or stiff. They may be secretory. 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 thin-walled chloroplast containing cells and is called mesophyll.


3. The Vascular Tissue System

It consists of complex tissues (xylem and phloem).

Xylem and Phloem together constitute vascular bundles.

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.


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