Structure of Stem
The internal structure can be studied if you cut the stem transversely and observe it under a compound microscope.
Internal Structure of Dicot Stem
1. Epidermis: Outermost single layered, covered with cuticle, bears multicellular hairs, protective function.
2. Cortex: Inner to epidermis, there are three regions:
- Hypodermis: 4-6 layers of collenchyma for mechanical support.
- Middle layers: Few layers of parenchyma.
- Endodermis: Innermost layer of cortex, has barrel shaped cells. As cells contain starch grains, it is also called starch sheath.
3. Stele: All the tissues lying internal to endodermis constitute the stele.
(i) Pericycle - Inner to endodermis, multilayered, parenchymatous with patches of sclerenchyma.
(ii) Vascular bundles - Arranged in a ring; each vascular bundle is (a) conjoint (xylem and phloem together in one bundle), (b) collateral (xylem and phloem on the same radius with phloem towards the periphery) and (c) open (cambium present in between xylem and phloem). Xylem is endarch (protoxylem towards centre and metaxylem towards periphery).
(iii) Medullary rays - Narrow regions of parenchymatous cells in between the vascular bundles.
(iv) Pith - The central parenchymatous zone with intercellular spaces.
Internal Structure of Monocot Stem
1. Epidermis - Single layered, covered with cuticle, stem hairs absent.
2. Ground tissue - A mass of parenchymatous tissue. Only a few peripheral layers below epidermis are sclerenchymatous called hypodermis.
1. Vascular bundle - Numerous, scattered in the ground tissue each enclosed by sclerenchymatous bundle sheath. Each bundle is (a) collateral and (b) closed (no cambium strip between xylem and phloem) with (c) endarch xylem. Xylem occurs in the form of letter ‘Y‘ and innermost protoxylem disintegrates to form a water cavity.
