In this article we will discuss about the anatomy of different normal dicot stems: 1. Helianthus Annuus – Stem 2. Xanthium – Stem 3. Luffa – Stem.
1. Anatomy of Helianthus Annuus – Stem (Family-Compositae):
T.S. shows the following details:
It is circular in outline and remains differentiated into epidermis, cortex, vascular bundles and pith.
Epidermis:
1. It is the outermost layer of the stem.
2. Externally it is surrounded by a well-defined cuticle.
3. It is single-layered with tangentially flattened cells.
4. From some of the cells develop multicellular epidermal hairs.
5. The continuity of the epidermal layer is broken by certain stomata.
Cortex:
6. It consists of outer few layers of collenchyma, some layers of parenchyma and an innermost layer of endodermis.
7. Collenchyma region is 3 to 6 layered, and the cells are thickened at the corners due to the deposition of pectin and cellulose.
8. Parenchyma is present inner to the collenchyma in the form of few layers.
9. The cells are thin walled and the parenchymatous region contains many intercellular spaces.
10. Endodermis is the innermost layer of cortex and consists of barrel shaped cells.
11. Cells of the endodermis contain casparian strips and many starch grains.
Pericycle:
12. It is present in the form of semi-lunar patches of sclerenchyma outside the phloem of each vascular bundle.
13. Sclerenchymatous patches of pericycle are interrupted by parenchyma.
Vascular Bundles:
14. These are conjoint, collateral, open and endarch.
15. These are arranged in a ring, and each consists of phloem, cambium and xylem.
16. Phloem consists, of sieve tubes, companion cells and phloem parenchyma.
17. Sieve tubes are narrow and associated with the companion cells.
18. Cambium is present in between xylem and phloem of the vascular bundles and consists of thin walled, rectangular cells arranged in radial rows.
19. Xylem:
It consists of vessels, tracheids, wood fibres and woody parenchyma:
(i) Big vessels represent metaxylem while the smaller vessels represent the protoxylem.
(ii) Metaxylem consists of reticulata vessels while the protoxylem consists of spiral or scalariform vessels.
(iii) Trachcids are thick walled cells and surround the metaxylem.
(iv) Wood fibres are thick walled and lignificd.
(v) Wood parenchyma is present in the form of thin walled cells.
Pith:
20. A well developed pith is present in the centre consisting of thin walled, rounded or polygonal, parenchymatous cells.
Identification:
(a) 1. Presence of vessels in xylem. (Angiosperms)
(b) 1. Vascular bundles are conjoint, collateral, open and endarch. (Stem)
(c) 1. Presence of cambium.
2. Vascular bundles are arranged in a ring.
3. Well differentiated cortex.
4. Well developed pith. (Dicotyledones)
Special Point:
Pericycle is present in the form of semilunar patches of sclerenchyma outside the phloem.
2. Anatomy of Xanthium – Stem (Family – Compositae):
T.S. exhibits following details:
It is circular in outline and remains differentiated into epidermis, cortex, vascular bundles and pith.
Epidermis:
1. It is the outermost layer of the stem and remains surrounded by a well developer cuticle.
2. Single layered epidermis consists of tangentially elongated compact cells.
3. From some of the epidermal cells arise multicellular hairs.
4. Continuity of the epidermal layer is broken by certain stomata.
Cortex:
5. It is clearly differentiated into collenchyma, parenchyma and a layer of endodermis.
6. Collenchyma is 3-5 cells thick, having depositions of cellulose at their comers. It devoids any intercellular space.
7. Parenchyma is present below collenchyma and consists of rounded or oval thin walled cells with many intercellular spaces.
8. Endodermis is the innermost and wavy layer of cortex consisting of compact barrel shaped cells filled with starch grains.
Pericycle:
9. It is present in the form of semi-circular bands of sclerenchyma associated with the vascular bundles. It is parenchymatous in the region of medullary rays.
Vascular Bundles:
10. These (6-12) are conjoint, collateral, open and endarch, and remain arranged in a ring.
(a) In Young Stem:
11. Each vascular bundle consists of phloem, cambium and xylem.
12. Phloem is present outside in the vascular bundles and consists of sieve tubes, companion cells and phloem parenchyma.
13. Cambium is present in between phloem and xylem, and consists of few layers of thin walled, rectangular and radially arranged cells.
14. Xylem is situated internally facing its protoxylem towards centre.
(i) Protoxylem consists of narrow, annular, spiral or scalariform vessels.
(ii) Metaxylem consists of big vessels, and remains surrounded by certain tracheids.
(iii) Xylem parenchyma is also present.
15. In between the vascular bundles are present thin walled, parenchymatous cells of medullary rays.
In Old Stem:
16. Secondary growth is present.
17. Just outside the cambium is present secondary phloem. Cambium cuts secondary xylem towards inner side.
18. Xylem elements are situated in the conjunctive tissue.
19. Primary phloem is situated outside the secondary phloem while the primary xylem is present inner to the secondary xylem.
Pith:
20. Thin walled, parenchymatous pith occupies the large central portion of the stem.
Identification:
(a) 1. Presence of vessels in xylem. (Angiosperms)
(b) 1. Vascular bundles are conjoint, collateral, open and endarch.
2. Presence of multicellular short hairs. (Stem)
(c) 1. Vascular bundles are arranged in a ring.
2. Presence of cambium.
3. Well-differentiated cortex and well-developed pith. (Dicotyledones)
Special Point:
Pericycle is in the form of sclerenchymatous patches.
3. Anatomy of Luffa – Stem (Family – Cucurbitaceae):
T.S. exhibits following details:
It is wavy in outline, usually with five ridges and five furrows, and ten vascular bundles remain arranged in two rings of five each.
Epidermis:
1. Single layered epidermis consists of many barrel shaped cells covered with cuticle.
2. Some of the epidermal cells protrude out as multicellular shoot hairs.
Cortex:
3. It consists of collenchymatous hypodermis, chlorenchyma and an innermost layer of endodermis..
4. Collenchyma is present just below the epidermis in the form of six to ten or more layers in the ridges and only a few layers or none in the furrows.
5. Chlorenchyma is present in the form of two to three layers in between the collenchyma and endodermis. The cells are filled with chloroplasts.
6. Endodermis is the innermost layer of cortex. It is wavy in outline. The cells are filled with starch grains and lack casparian strips.
Pericycle:
7. It consists of four to five layers of thick walled, lignified sclerenchymatous zone present just below the endodermis.
Ground Tissue:
8. The space between sclerenchyma and the central pith cavity is filled with many thin walled, parenchymatous cells of ground tissue, in which are embedded the vascular bundles.
Vascular Bundles:
9. Ten vascular bundles are arranged in two rows of five each.
10. Five vascular bundles of outer ring are present opposite the ridges whereas rest five of the inner ring face the furrows.
11. Vascular bundles are conjoint, bicolloterol, open and endarch.
12. Each vascular bundle consists of centrally located xylem, surrounded on its outer and inner faces by strips of outer and inner cambia. Outside the outer cambium is present a patch of outer phloem, and inner to the inner cambium is present the inner phloem, thus representing the open and bicollateral condition of vascular bundles.
13. Xylem consists of wide vessels present on outer side representing the metaxylem and narrow vessels present towards inner side representing the protoxylem. Xylem also contains certain tracheids, wood fibres and xylem parenchyma.
14. Cambium is present in the form of strips on both the sides of the xylem. It consists of thin walled, rectangular cells arranged in radial rows.
(i) Outer cambium is flat and many layered.
(ii) Inner cambium is curved and only few layered.
15. Phloem is situated in the form of patches of outer and inner phloem. It consists of companion cells, thin walled cells of phloem parenchyma, and well developed sieve tubes.
Pith:
16. Thin walled parenchymatous cells of ground tissue form the pith.
Identification:
(a) 1. Presence of vessels in xylem. (Angiosperms)
(b) 1. Vascular bundles are conjoint, bicollateral, open and endarch.
2. Multicellular epidermal hairs. (Stem)
(c) 1. Vascular bundles are arranged in rings.
2. Presence of cambium.
3. Well- differentiated cortex and well-developed pith. (Dicot)
Special Point:
Presence of bicollateral open, vascular bundles.