IB Biology Topic B3.2.10: Distribution of Tissues in a Transverse Section of the Root of a Dicotyledonous Plant Notes

A Dicot Root Keeps Its Vascular Tissue in the Centre

1. Dicotyledonous plants (dicots) like beans, sunflowers, and oak trees have roots with a distinctive tissue arrangement.
2. In a dicot root the vascular tissues are concentrated at the centre in a single vascular cylinder, called the stele.
3. This central arrangement reinforces the root, helping it push through compacted soil without buckling.

Root Tissue Layers: From Surface to Core

1. Epidermis
   1. A single layer of thin-walled cells without a cuticle.
   2. The lack of a cuticle lets water pass freely into the root by osmosis.
   3. Root hairs are extensions of epidermal cells that greatly increase the surface area for absorbing water and mineral ions.
   4. Each root hair is short-lived but is constantly replaced as the root tip grows through the soil.
2. Cortex
   1. A wide band of loosely packed parenchyma cells with air spaces between them.
   2. These cells store starch and allow gas exchange through the air spaces.
3. Endodermis
   1. A single-cell-thick layer that forms the inner boundary of the cortex, wrapping around the vascular cylinder.
   2. Each endodermal cell has a band of waterproof suberin in its wall, called the Casparian strip.
   3. The Casparian strip forces water and dissolved ions to pass through the cell membrane of the endodermal cells.
   4. This gives the plant selective control over which ions enter the vascular tissue.

The Vascular Cylinder: A Central Xylem Star

1. Pericycle
   1. The outermost layer of the vascular cylinder, just inside the endodermis.
   2. Its cells can still divide, and this is where lateral (branch) roots originate.
2. Xylem
   1. Occupies the centre of the root in a star or X shape, typically with four or five arms in dicots.
   2. It transports water and mineral ions upward to the stem and leaves.
3. Phloem
   1. Found between the arms of the xylem star, not surrounding it.
   2. It transports sugars and organic compounds to wherever the plant needs them.

Plan Diagram: Label These Regions from Outside In

1. A plan diagram shows the distribution of tissues without drawing individual cells.
2. Label these structures, working from outside to inside.
   1. Epidermis, the outermost ring, with root hairs projecting outward.
   2. Cortex, the wide zone of parenchyma between epidermis and vascular cylinder.
   3. Endodermis, the thin ring of cells surrounding the vascular cylinder.
   4. Pericycle, just inside the endodermis.
   5. Xylem, the star shape at the centre.
   6. Phloem, between the arms of the xylem.

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