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

Morphology of Flowering Plants - The Leaf

The Tissue System

Based on structure and location, tissue systems are of three types:

Forms the outermost covering of the whole plant body.
Comprises epidermal cells, stomata, and epidermal appendages (trichomes & hairs).

The outermost layer of the primary plant body.
Usually single-layered.
Made up of elongated, compactly arranged parenchymatous cells with a 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) to prevent water loss. The cuticle is absent in roots.

Structures present in the epidermis of leaves.
Regulate transpiration and gaseous exchange.
A stoma is made of two bean-shaped cells (guard cells).
In grasses, 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.
Guard cells possess chloroplasts and regulate the opening and closing of stomata.
Some epidermal cells near the guard cells may become specialized in shape and size, called subsidiary cells.
The stomatal aperture, guard cells, and surrounding subsidiary cells together form the stomatal apparatus.

Root hairs: Unicellular elongations of epidermal cells that absorb water and minerals from the soil.
Trichomes: Epidermal hairs on the stem, usually multicellular, branched or unbranched, soft or stiff, and sometimes secretory. They help prevent water loss due to transpiration.

All tissues except epidermis and vascular bundles constitute the ground tissue.
Consists of simple tissues (parenchyma, collenchyma, and sclerenchyma).
Parenchymatous cells are present in cortex, pericycle, pith, and medullary rays in primary stems and roots.
In leaves, the ground tissue consists of thin-walled chloroplast-containing cells, called mesophyll.

Based on the presence or absence of cambium, vascular bundles are of two types:

Open type: Cambium is present between phloem and xylem, allowing the formation of secondary xylem and phloem tissues. Example: dicotyledonous stems.
Closed type: Cambium is absent, so secondary tissues are not formed. Example: monocotyledons.

Based on the arrangement of xylem and phloem, vascular bundles are of two types:

Radial type: Xylem and phloem are arranged alternately on different radii. Seen in roots.
Conjoint type: Xylem and phloem are situated at the same radius. Seen in stems and leaves. Conjoint vascular bundles usually have phloem located only on the outer side of xylem.