Internal structure of dicot and monocot roots
Root layers run epiblema, cortex, endodermis, pericycle, then vascular tissue. Dicot roots have few xylem groups (bean: four); monocot roots are polyarch with a large pith.
Layers of a root, from outside in
| Layer | Structure | Job |
|---|---|---|
| Epiblema (piliferous layer) | Single outer layer of thin-walled cells; no cuticle; bears unicellular root hairs | Absorbs water and minerals; protects |
| Cortex | Many layers of thin-walled parenchyma with intercellular spaces | Stores food; passes water inwards |
| Endodermis | Innermost layer of the cortex; barrel-shaped cells with Casparian strips; passage cells opposite the protoxylem have no strips | Controls the flow of water into the vascular tissue |
| Pericycle | A single layer of thin-walled cells just inside the endodermis | Gives rise to lateral roots |
| Vascular tissue | Xylem and phloem on different radii (radial bundles); xylem exarch; parenchyma between them is conjunctive tissue | Xylem carries water up; phloem carries food |
| Pith | Central parenchyma (small or absent in dicot roots, large in monocot roots) | Stores food |
Dicot root vs monocot root
Both have an epiblema with root hairs, a cortex, an endodermis with Casparian strips, a pericycle, and radial, exarch vascular bundles.
Words examiners look for
- Radial bundle: xylem and phloem lie on different radii, taking turns around a ring, not together in one bundle.
- Exarch: protoxylem (first-formed xylem) faces the outside; metaxylem (later xylem) lies towards the centre.
- Casparian strip: a band of suberin and lignin in the walls of endodermis cells that stops water leaking between the cells.
- Passage cells: endodermis cells without Casparian strips, opposite the protoxylem, that let water into the xylem.
- Conjunctive tissue: parenchyma lying between the xylem and phloem groups.
How to draw the T.S. of a dicot root
- 1Draw a large circle. Just inside it, draw one row of small cells with a few root hairs sticking out. Label epiblema.
- 2Fill a wide band with loosely packed round cells. Label cortex.
- 3Draw a ring of barrel-shaped cells with dark dots on their side walls. Label endodermis and Casparian strip, and mark one passage cell.
- 4Just inside, draw one row of cells. Label pericycle.
- 5Draw four star-like xylem arms pointing outwards with phloem patches between them. Label protoxylem, metaxylem, phloem and conjunctive tissue.
- 6For a monocot root, draw many (more than six) xylem groups around a large central pith.
The root hairs of paddy and groundnut seedlings absorb water from the soil, so farmers try not to tear them while transplanting. The Casparian strip works like a security gate that checks what enters the plant's water pipes.
2 marks: “What are Casparian strips? What is their role?” Band-like thickenings on the radial and inner walls of endodermis cells. They stop water seeping along the cell walls, so water passes through the endodermis cells into the xylem.
4 marks: “Give four differences between dicot and monocot roots.” Tetrarch vs polyarch xylem; pith absent or small vs large; cambium formed vs absent; secondary growth present vs absent; examples bean and maize.
7 marks: “Describe the internal structure of a dicot root with a labelled diagram.” Labelled T.S. of bean root; epiblema with root hairs, cortex, endodermis (Casparian strips, passage cells), pericycle, radial exarch tetrarch xylem, phloem, conjunctive tissue, small or no pith.
Outside in: 'Every Cow Eats Purple Vegetables Politely': Epiblema, Cortex, Endodermis, Pericycle, Vascular tissue, Pith.