B3.2.9 Distribution of tissues in a transverse section of the stem of a dicotyledonous plant Notes

Tissue Distribution in a Dicotyledonous Stem

1. A transverse section (T.S.) of a dicotyledonous (dicot) stem reveals the organized arrangement of tissues responsible for support, transport, and protection.
2. The main tissues include:
   1. Epidermis (outer protective layer)
   2. Cortex (storage and support)
   3. Vascular bundles (xylem and phloem)
   4. Pith (central region for storage)

Key Tissues and Their Arrangement in a Dicot Stem

1. Epidermis
   1. Location: Outermost layer of cells, forming the skin of the plant.
   2. Function: Protects the plant from physical damage, prevents water loss, and acts as a barrier to pathogens.
   3. Structure: Single layer of cells, often with a waxy cuticle on the surface to reduce water loss.
2. Cortex
   1. Location: Just beneath the epidermis, surrounding the vascular bundles.
   2. Function: Primarily serves as storage of starch and other nutrients. It also provides some structural support to the stem.
   3. Structure: Composed of loosely packed parenchyma cells, with some collenchyma and sclerenchyma cells for support.
3. Vascular Bundles
   1. Location: Found scattered throughout the stem, arranged in a ring in most dicotyledons. Each bundle contains xylem and phloem.
   2. Function: Vascular bundles transport water, minerals, and nutrients throughout the plant and also support the stem.
   3. Structure: Each bundle is composed of xylem (on the inner side) and phloem (on the outer side), with a cambium layer in between, which is responsible for the plant’s secondary growth.
4. Pith
   1. Location: Central region of the stem.
   2. Function: It stores nutrients and water and provides light structural support.
   3. Structure: It is formed of loosely arranged, thin-walled parenchyma cells.

Plan Diagram of a Dicot Stem (Transverse Section)

When drawing a plan diagram based on a microscopic image, ensure you identify and label the following features clearly:

1. Epidermis: Outer layer of the stem, marking the boundary.
2. Cortex: A layer beneath the epidermis.
3. Vascular Bundles: Usually arranged in a ring and located within the cortex.
4. Xylem: Found toward the center of each vascular bundle.
5. Phloem: Found toward the outside of each vascular bundle.
6. Pith: The central region, often composed of parenchyma cells (not always visible in every section).