Internal structure of dicot and monocot stems
A dicot stem (sunflower) has vascular bundles in a ring with cambium; a monocot stem (maize) has scattered closed bundles in undivided ground tissue.
Dicot stem (sunflower), from outside in
| Layer | Structure and job |
|---|---|
| Epidermis | Single outer layer with a cuticle and multicellular hairs; protects and cuts water loss |
| Hypodermis | A few layers of collenchyma below the epidermis; gives strength and flexibility |
| Middle and inner cortex | A few layers of chlorenchyma (cells with chloroplasts), then a few layers of parenchyma with intercellular spaces that store food |
| Endodermis | Innermost layer of the cortex; a wavy row of barrel-shaped cells full of starch grains, so called the starch sheath |
| Pericycle | Patches of sclerenchyma above each bundle, alternating with parenchyma |
| Vascular bundles | Arranged in a ring; each is conjoint, collateral and open (cambium between xylem and phloem), with endarch xylem |
| Pith and medullary rays | Large central pith of parenchyma stores food; medullary rays of parenchyma run between the bundles |
Monocot stem (maize), from outside in
| Layer | Structure and job |
|---|---|
| Epidermis | Single layer with a thick cuticle; no hairs |
| Hypodermis | A few layers of sclerenchyma; gives strength |
| Ground tissue | A mass of parenchyma, not divided into cortex, endodermis, pericycle and pith |
| Vascular bundles | Scattered in the ground tissue; smaller and crowded near the edge, larger towards the centre; conjoint, collateral, closed (no cambium) and endarch |
| Each bundle | Surrounded by a sclerenchyma bundle sheath; xylem vessels form a Y (or V) shape; has a water-filled protoxylem lacuna |
Dicot stem vs monocot stem
| Feature | Dicot stem | Monocot stem |
|---|---|---|
| Hypodermis | Collenchyma | Sclerenchyma |
| Ground tissue | Divided into cortex, endodermis, pericycle and pith | Not divided |
| Vascular bundles | Few, in a ring | Many, scattered |
| Cambium | Present (open bundles) | Absent (closed bundles) |
| Bundle sheath | Absent | Present, of sclerenchyma |
| Pith and medullary rays | Present | Absent |
| Secondary growth | Present | Absent |
How to draw the T.S. of a dicot stem
- 1Draw a circle. Mark a thin outer line with a few hairs. Label cuticle, epidermis and multicellular hair.
- 2Below it, draw 3 or 4 rows of cells with thick corners: hypodermis (collenchyma). Then draw a few rows of round cells: cortex (parenchyma).
- 3Draw a wavy single row of cells. Label endodermis (starch sheath).
- 4Draw 6 to 8 wedge-shaped vascular bundles in a ring. In each, draw a dark cap (sclerenchyma of pericycle), then phloem, a thin line of cambium, and xylem towards the centre, with the protoxylem innermost.
- 5Fill the centre with large round cells and label pith. Mark the cells between the bundles as medullary rays.
Sugarcane and maize are monocot stems; their scattered bundles are the 'strings' you feel when you chew sugarcane. Monocot stems have no cambium, so a coconut trunk grows tall but does not thicken year after year the way a neem trunk does.
2 marks: “What is a starch sheath?” The endodermis of the dicot stem, whose cells store starch grains.
4 marks: “Give four differences between dicot and monocot stems.” Collenchyma vs sclerenchyma hypodermis; bundles in a ring vs scattered; open vs closed bundles; ground tissue divided vs undivided; pith present vs absent.
7 marks: “Describe the internal structure of a dicot stem with a labelled diagram.” Labelled T.S. of sunflower stem; epidermis with hairs and cuticle, collenchyma hypodermis, cortex, starch sheath, pericycle, ring of conjoint, collateral, open, endarch bundles, pith and medullary rays.
Dicot stem is Disciplined: bundles in a ring, with cambium. Monocot stem is Messy: bundles scattered, no cambium. Dicot stem bundles are CCOE: Conjoint, Collateral, Open, Endarch.