Study Guides/Biology/Dicot Stem Diagram and Internal Structure
Study Guide · Biology

Internal Structure and Diagram of a Dicot Stem

In Class 11 Plant Anatomy, understanding the Transverse Section (T.S.) of a dicotyledonous (dicot) stem is crucial. A typical example studied in labs is the sunflower stem. If you cut it horizontally and look under a microscope, you will see several distinct concentric layers.

Question (Click to Flip)

Why do dicot stems grow so thick and form wood, but grass does not?

Answer

Because dicot stems have Cambium. Cambium continuously produces new layers of xylem and phloem every year (secondary growth), creating thick, woody tree trunks. Monocots lack cambium.

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Key Facts

In a dicot stem, the Protoxylem (first formed xylem) lies towards the center (pith), while the Metaxylem lies towards the outside. This arrangement is called 'Endarch'.

This structured ring arrangement is completely different from Monocot stems (like grass or maize), where the vascular bundles are scattered randomly everywhere.

1. The Outer Layers

  • Epidermis: The outermost, single protective layer of cells. It is covered by a waxy cuticle to prevent water loss and has multi-cellular stem hairs (trichomes).
  • Cortex: Located just below the epidermis. It consists of multiple layers of cells, providing mechanical support to the young stem and storing food.

2. The Inner Boundaries

  • Endodermis: The innermost layer of the cortex. The cells are rich in starch grains (often called the starch sheath).
  • Pericycle: A layer located right below the endodermis, made of sclerenchyma cells that form semi-lunar (half-moon) patches.

3. The Vascular Bundles (Crucial Feature)

This is the most identifying feature of a dicot stem.

  • The vascular bundles are arranged in a perfect Ring pattern around the center.
  • Each bundle is Conjoint and Open. This means the Xylem (water transport) and Phloem (food transport) are joined together in the same bundle.
  • Because they are 'Open', they contain a strip of Cambium tissue between the xylem and phloem, which allows the tree stem to grow thicker over time (Secondary Growth).

4. The Center

  • Pith (Medulla): The massive central portion of the stem. It is made of loosely arranged, large parenchymatous cells with lots of intercellular spaces. Its main job is to store water and food.

Questions and Answers

Why do dicot stems grow so thick and form wood, but grass does not?+

Because dicot stems have Cambium. Cambium continuously produces new layers of xylem and phloem every year (secondary growth), creating thick, woody tree trunks. Monocots lack cambium.

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