Different parts of the dicotyledonous stem are given below:-
A transverse section through a young sunflower stem reveals the following anatomical peculiarities are given below:-
1. Epidermis. This is the outermost layer made of single row of flattened cells fitted closely With each other. The cells may look rectangular in cross-section. Deposition of cutin forms a layer of cuticle over the epidermis. Here and there some multicellular hairs also develop. In young stem stem stomata may be seen.
2. Cortex. It lies below the epidermis and is differentiated into the following:
(a) Hypodermis. It is composed of 3-4 layers of collenchymatous cells. The corners of the cells are thickened due to deposition of pectin and cellulose. The cells are living and may contain a few chloroplasts.
(b) General cortex. It lies below the hypodermis and is composed of living parenchymatous cells having intercellular space between them. The cells are thin walled, large, oval or rounded. Some resin ducts may be present here and there.
1. Vascular bundles in roots are of radial type.
2. Vascular bundles of monocot stem are collateral closed type.
3. Vascular bundles of dicot stems and roots are couateral open type.
4. Vascular bundles having two strips of cambium and two stips of phloem are called bicolateral type.
5. Vascular bundles in monocot stem remain scattered while vascular bundles of dicot stem are arranged in ring.
(c) Endodermis. It is one-celled, a wavy band of layer demarcating the outer cortex from the stele, The cells are barrel shaped, without intercellular space. The cells of endodermis contain starch grains and hence this layer is referred to as starch sheath.
Also read about :
3. Stele. It includes pericycle, vascular bundle, pith and pith rays. All of these elements lie inner to the endodermis.
(a) Pericycle. This lies below the endodermis and is represented by semilunar patches of sclerenchymatous cells intervened by patches of parenchymatous cells. Sclerenchymatous cells which lie above the phloem are called hard bast because they provide strength.
(b) Vascular bundle. These are collateral open and are arranged like a ring. Each bundle is composed of xylem, phloem, cambium, wood parenchyma and bast fibres.
(i) Xylem is composed of vessels, tracheids, wood fibres and wood parenchyme. The vessels are thick-walled cells arranged in radial rows. The protoxylem lies towards the centre and metaxylem towards the periphery. The wood fibres are thick-walled lignified and are intermixed with vessels and tracheids. Wood parenchyma are thin-walled cells found on the inner side of the bundle surrounding the protoxylem.
(ii) Phloem lies externally and is composed of thin, cellulose-walled elements. It consists of sieve-tube, companion cells. phloem parenchyma and bast fibres.
(a) Sieve-tubes are elongated cavities and are provided with sieve plates,
(b) Associated with the sieve tubes are companion cells and
(c) phloem. All phloem elements are living and except the seive-tube all store food materials.
1. Epidermis is the outermost protective layer of stem and leaf.
2. Epiblema is the outermost protective layer of root.
3. Hypodermis is the outermost layer of cortex.
4. Endodermis is the innermost layer of cortex.
5. Pericycle is the cell layer beneath the endodermis. It includes both parenchymatous and sclerenchymatous cells. Secondary roots arise from pericyle.
6. Pith is the ground tissue at the centre of the stem or root surrounded by vascular bundles.
7. Medullary rays are the strips of parenchymatous cells present in between vascular bundles.
(c) Cambium. It is composed of one to many layered, thin-walled, rectangular cells having large nuclei and dense cytoplasm. The cells are meristematic in nature and produce newer phloem elements outside and xylem elements inside during secondary growth. Cambium which remains restricted within the vascular bundle is called fascicular cambium.
(d) Pith or Medula. It is distinct and large, situated in the centre of the stem and composed of thin-walled, oval or polygonal parenchymatous cells with abundant intercellular space.
(e) Pith rays. Pith cells which continue through the space between vascular bundles constitute the pith rays. The cells are fairly big, polygonal or radially elongated.
These are the parts of the dicotyledonous stem