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In this article we will discuss about the anatomy of various anomalous dicot stems: 1. Bougainvillea – Stem 2. Salvadora – Stem 3. Achyranthes- Stem 4. Chenopodium – Stem 5. Boerhaavia – Stem 6. Leptadenia – Stem 7. Mirabilis – Stem 8. Amaranthus – Stem 9. Nyctanthes – Stem 10. Bignonia – Stem.
1. Anatomy of Bougainvillea – Stem: A Dicot with Successive Rings of Bundles (Family – Nyctaginaceae):
It is circular in outline and exhibits following tissues from outside with in:
Epidermis:
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1. Single layered epidermis consists of compactly arranged thin walled cells and is covered by thick cuticle.
2. It bears many multicellular hairs when young.
3. In the stem showing secondary growth, cork cambium is present which cuts cork towards outer side and secondary cortex towards inner side.
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Cortex:
4. It consists of collenchyma, parenchyma and endodermis.
5. Collenchyma in young stems is present in patches but in old stems it remains in the form of a continuous ring of few layers, present just below the epidermis.
6. Next to the collenchyma is situated the region of parenchyma consisting of many thin walled, oval to spherical cells with intercellular spaces. Sometimes they develop chlorophyll.
7. Endodermis is the innermost layer of the cortex the cells of which are barrel shaped and contain starch grains. In old stems, however, it is not a conspicuous layer.
Pericycle:
8. It is represented by thick, sclerenchymatous stone cells forming a discontinuous layer.
Vascular System:
9. It consists of phloem and xylem.
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10. In the young stem are present many vascular bundles arranged in ring. Bundles are conjoint, collateral, open and endarch. Many medullary bundles are also present.
11. The old stem shows secondary growth.
12. Just below the pericycle are present the patches of primary phloem.
13. Secondary phloem is present inner to the primary phloem.
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14. Phloem consists of sieve tubes, companion cells and phloem parenchyma. Phloem fibres are absent.
15. Cambium is present in between secondary phloem and secondary xylem.
16. Secondary xylem, which forms the major part of section, consists of tracheids, vessels, fibres and prosenchyma.
17. Primary xylem is present near the pith facing its protoxylem towards the centre of stem.
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Interxylary Phloem:
18. Many groups of secondary phloem are embedded in the region of secondary xylem and called interxylary phloem or included phloem.
Medullary Bundles:
19. Many conjoint, collateral, open and endarch bundles are present in the pith. These are called medullary bundles.
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Pith:
20. It is parenchymatous and the cells are rounded with intercellular spaces.
Anomalous secondary growth is due to the formation of successive rings of collateral vascular bundles. These bundles get embedded in the thick prosenchyma and their phloem appears as included or interxylary phloem. Medullary bundles are innermost secondary bundles.
Identification:
(a) 1. Presence of vessels in the xylem. (Angiosperms)
(b) 1. Vascular bundles are conjoint, collateral, open and endarch. (Stem)
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(c) 1. Multicellular epidermal hairs.
2. Vascular bundles are arranged in ring.
3. Presence of cambium. (Dicot)
Abnormality:
Interxylary phloem and medullary bundles are present.
2. Anatomy of Salvadora – Stem (Family – Salvadorace):
T.S. reveals the following tissues from outside within:
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Epidermis:
1. It is the outermost layer with barrel shaped cells. Cells are covered by a thick cuticle.
Cortex:
2. It consists of parenchymatous hypodermis, few layers of chlorenchyma and an innermost layer of endodermis.
3. Hypodermis is generally thin walled, parenchymatous, but sometimes 2-3 layers of collenchyma are seen.
4. Chlorenchyma is present inner to the hypodermis. It is 3-5 cells deep and cells are filled with chloroplasts.
5. Few layers of parenchyma are also present below the chlorenchyma. The cells contain intercellular spaces.
6. Endodermis is the innermost layer of cortex made up of barrel shaped cells which contain starch grains.
Pericycle:
7. It is a discontinuous layer present in the form of patches consisting of many widely spaced strands of thick walled fibres.
Vascular System:
8. Vascular bundles are conjoint, collateral, open and endarch.
9. Vascular system composed of primary phloem, secondary phloem, cambium, secondary xylem, primary xylem and included phloem.
10. Primary phloem is crushed and found in the form of patches.
11. Secondary phloem is present just outside the cambium in the form of a ring.
12. Cambium strip consists of rectangular cells arranged in radial rows.
13. Secondary xylem forms a complete cylinder. It is represented by wide vessels and xylem parenchyma.
14. Many medullary rays traverse the secondary xylem.
15. Wide vessels of metaxylem and narrow protoxylem vessels can be observed in the primary xylem present near the pith.
16. Secondary xylem is interrupted by many groups of thin walled phloem representing included phloem, interxylary phloem or phloem islands.
Pith:
17. It is well developed, thin-walled, parenchymatous and present in the centre.
How Does Included Phloem Develop?
Included phloem develops due to the irregular activity of cambium. Some cambium cells which arc normally responsible for the formation of secondary xylem form secondary phloem. Other adjacent cambium ceils keep on normally producing secondary xylem. After sometime the same abnormally behaving cambium cells start to behave normally, and thus many islands of phloem are formed.
Identification:
(a) 1. Presence of vessels in the xylem. (Angiosperms)
(b) 1. Vascular bundles are conjoint, collateral, open and endarch. (Stem)
(c) 1. Vascular bundles in ring.
2. Presence of secondary growth.
3. Well developed pith. (Dicot)
Abnormality:
Formation of included phloem abnormality of Salvadora.
3. Anatomy of Achyranthes- Stem (Family – Amaranthaceae):
T.S. is wavy in outline with ridges and furrows, and reveals the following tissues from outside within:
Epidermis:
1. Single – layered epidermis consists of many tubular cells and covered externally by thick cuticle.
2. From some cells arise multicellular hairs.
Cortex:
3. It is well differentiated into collenchyma, chlorenchyma, parenchyma and endodermis.
4. Collenchyma is present only below the ridges and its breadth and depth varies below different ridges.
5. Chlorenchyma is present below the furrows.
6. Parenchymatous cortex is located below collenchyma and chlorenchyma. It is two to three layers deep and cells contain intercellular spaces.
7. Endodermis is the innermost layer of cortex, consisting of elongated cells which lack casparian thickenings. In the old stem endodermis is not clear.
Pericycle:
8. It is represented by groups of sclerenchymatous cells situated just outside the vascular tissue.
Vascular System:
9. In the young stem, vascular bundles are conjoint, collateral, open and endarch but old stems show secondary growth.
10. In the old stem, the vascular system consists of primary phloem, secondary phloem, cambium, secondary xylem, conjunctive tissue, included phloem and primary xylem
11. Primary phloem is crushed and obliterated.
12. Secondary phloem is present in the form of a complete ring and Consists of sieve tubes, companion cells and phloem parenchyma.
13. A cambial strip is present in between secondary xylem and secondary phloem.
14. Secondary xylem and conjunctive tissue are undistinguishable. Large xylem vessels are very clear in thick walled conjunctive tissue.
15. In the conjunctive tissue (prosenchyma) are present groups of included phloem or interxylary phloem.
16. Primary xylem is present near the pith.
Pith:
17. It is well developed and parenchymatous.
Medullary Bundles:
18. Two medullary bundles are present in pith facing their xylem to each other. They are conjoint, collateral, open and endarch.
Secondary Growth and Medullary Bundles:
In the pericycle region, extrastelar cambium strips develop which produce secondary vascular bundles. Cambium produces the conjunctive tissue. Secondary vascular bundles and conjunctive tissue are present without any sharp limits. So phloem of the secondary vascular bundles appears in the form of patches. This phloem is the included phloem. Medullary bundles are leaf traces.
Identification:
(a) 1. Presence of vessels in the xylem. (Angiosperms)
(b) 1. Vascular bundles are conjoint, collateral, open and endarch (Stem)
(c) 1. Vascular bundles in ring.
2. Presence of secondary growth.
3. Well-developed pith. (Dicot)
Abnormality:
1. Included phloem.
2. Medullary bundles.
4. Anatomy of Chenopodium – Stem (Family-Chenopodiaceae):
T. S. is rounded in outline and reveals the following tissues from outside with -in:
Epidermis:
1. It is composed of a single layer of tangentially elongated cells.
2. A thick cuticle is present on the outer walls.
Cortex:
3. It is well-differentiated and composed of collenchyma and parenchyma. Sometimes a few- layered chlorenchyma is also present in between.
4. Collenchyma is present below epidermis in the form of 3-4 layers.
5. Parenchyma is present inner to collenchyma. It is well developed and contains many intercellular spaces.
6. Endodermis delimits cortex from vascular region. The cells are barrel shaped, filled with starch grains and lack casparian strips.
Pericycle:
7. Next to the endodermis is present the parenchymatous pericycle.
Vascular System:
8. It consists of well developed conjunctive tissue and secondary vascular bundles, the phloem of which appears like included phloem.
9. Conjunctive tissue is very well developed and consists of thick and lignified cells. It is in very close association-ship with xylem.
10. Vascular bundles are present in a ring and remain embedded in the conjunctive, tissue.
11. These vascular- bundles are formed by extrastelar cambium and thus secondary in origin.
12. Each vascular bundle is conjoint, collateral and endarch.
13. The phloem of these bundles remains embedded in the conjunctive tissue giving the appearance of included phloem or interxylary phloem.
Pith:
14. It is well developed, and consists of thin walled, polygonal parenchymatous cells with large intercellular spaces.
Medullary Bundles:
15. In the pith are embedded many conjoint, collateral, open and endarch vascular bundles.
16. These bundles are primary vascular bundles of the stem and not the leaf-trace bundles.
Secondary Growth:
Secondary growth in Chenopodium is abnormal and takes place by the formation of extrastelar cambium in the pericyclc. The first ring of the cambium is a continuous ring. It first produces a small amount of thin walled ground tissue which pushes the primary vascular bundles towards centre which now appear as medullary bundles. This cambium later on produces secondary vascular bundles and conjunctive tissue which is thick walled.
Such cambial rings are produced in the pericycle successively, and the ring external to the phloem burries the latter in the conjunctive tissue. This phloem gives the appearance of included phloem or interxylary phloem.
Identification:
(a) 1. Presence of vessels in the xylem. (Angiosperms)
(b) 1. Vascular bundles are conjoint, collateral open and endarch.
2. Cortex is well-differentiated. (Stem)
(c) 1. Vascular bundles are arranged in ring.
2. Well-developed secondary growth and well defined pith. (Dicotyledones)
5. Anatomy of Boerhaavia – Stem (Family – Nyctaginaceae):
T.S. is circular in outline and reveals the following tissues from outside within:
Epidermis:
1. Single layered epidermis consists of small, radially elongated cells.
2. Multicellular epidermal hairs arise from some cells.
3. A thick cuticle is present on the epidermis.
4. Some stomata are also present.
Cortex:
5. It is well differentiated and consists of few layered collenchymatous hypodermis followed by chlorenchyma.
6. Collenchyma is 3 to 4 cells deep, but generally near stomata it is only one layered.
7. Chlorenchyma is present inner to collenchyma in the form of 3 to 7 layers.
8. Chlorenchymatous cells are thin walled, oval, full of chloroplasts and enclose many intercellular spaces.
9. Endodermis is clearly developed and made up of many, tubular, thick-walled cells.
Pericycle:
10. Inner to the endodermis is present parenchymatous pericycle but at some places it is represented by isolated patches of sclerenchyma.
Vascular System:
11. Vascular bundles are present in three rings. In the innermost ring are present two large bundles; in the middle ring the number ranges from 6 to 14 while the outermost ring consists of 15 to 20 vascular bundles.
12. Vascular bundles of innermost and middle rings are medullary bundles.
13. Vascular bundles are conjoint, collateral and endarch.
14. Two vascular bundles of the innermost ring arc large, oval and lie opposite to each other with their xylem facing towards centre and phloem outwards.
15. Middle ring consists of 6-14 small vascular bundles.
16. Vascular bundles of inner and middle rings may show a little secondary growth.
17. Phloem consists of sieve tubes, companion cells and phloem parenchyma while the xylem consists of vessels, tracheids and xylem parenchyma.
18. Outermost ring of the vascular bundles contain inter-fascicular cambium which is absent in other two rings.
19. Cambium develops secondarily from the pericycle and becomes active. It cuts secondary phloem towards outer side and secondary xylem towards inner side. Due to these changes the primary phloem becomes crushed and present next to pericycle. Primary xylem is situated near the pith.
20. Interfascicular cambium also soon becomes active and cuts internally the row of cells which become thick walled and lignified and are known as conjunctive tissue.
Pith:
21. It is well developed, parenchymatous and present in the centre.
Identification:
(a) 1. Presence of vessel in the xylem. (Angiosperms)
(b) 1. Cortex is well-differentiated.
2. Vascular bundles are conjoint, collateral, open and endarch. (Stem)
(c) 1. Vascular bundles are present in ring.
2. Well-developed secondary growth.
3. Well-defined pith. (Dicotyledones)
6. Anatomy of Leptadenia – Stem (Family – Asclepiadaceae):
It is circular in outline and reveals the following tissues from outside with-in:
Epidermis:
1. Outermost, single-layered epidermis consists of many barrel-shaped cells arranged compactly.
2. The cells are covered externally by thick cuticle.
Cortex:
3. It consists of hypodermis, chlorenchyma and endodermis.
4. Hypodermis follows epidermis and consists of thin walled, parenchymatous cells arranged in one to three layers.
5. Chlorenchymatous layers (3 to 6 or more) are present inner to the hypodermis. The cells are filled with chloroplasts and show many intercellular spaces.
6. Endodermis is the innermost layer of cortex consisting of barrel shaped cells, filled with starch grains. The cells lack characteristic casparian strips.
Pericycle:
7. A big zone of pericycle is present below the endodermis consisting of parenchymatous cells.
8. It is interrupted by the sclerenchymatous patches at certain intervals.
Vascular System:
9. It consists of primary phloem, secondary phloem, cambium, secondary xylem, interxylary phloem, primary xylem and intraxylary phloem.
10. A ring of vascular bundles is present in the primary state which are conjoint, bicollateral, open and endarch.
11. Primary phloem is present in patches at certain places.
12. Secondary phloem ring is present inner to pericycle and consists of sieve tubes, companion cells and phloem parenchyma with no phloem fibre.
13. Cambium consists of thin-walled, brick shaped, actively dividing cells arranged in one layer but later on new cells are cut off and it becomes multilayered.
14. Secondary xylem zone consists of xylem vessels, tracheids and xylem parenchyma. The zone is traversed by many secondary medullary rays.
15. Many patches of interxylary phloem or included phloem are present in the secondary xylem cylinder. Their development is centripetal.
16. Primary xylem consists of protoxylem and metaxylem. The protoxylem is endarch and present near the pith.
17. Intraxylary phloem is present in the form of patches at the periphery of the pith.
Pith:
18. It is thin walled and parenchymatous.
Abnormal Secondary Growth:
Due to the irregular activity of the cambium at certain places, the secondary phloem is formed towards inner side instead of secondary xylem. Other adjacent cambium cells are normally producing secondary xylem towards inner side.
After some time the cambium resumes its normal activity and thus forms many patches of secondary phloem in the secondary xylem. These are called interxylary phloem or included phloem patches Internal or intraxylary phloem is the primary structure of the primary bicollateral bundles.
Identification:
(a) 1. Presence of vessels in the xylem.
2. Vessels have perforated end walls with scalariform and regularly arranged holes. (Angiosperms)
(b) 1. Conjoint, collateral, open and endarch vascular bundles. (Stem)
(c) 1. Vascular bundles in a ring.
2. Presence of cambium. (Dicotyledones)
7. Anatomy of Mirabilis – Stem (Family – Nyctaginaceae):
It is rectangular in outline and reveals the following tissues from outside within:
Epidermis:
1. Single-layered epidermis consists of compact, rectangular cells, the outer face of which is covered by a thin cuticle.
2. From some cells arise multicellular hairs.
Cortex:
3. A broad cortical region is present below the epidermis consisting of collenchyma, parenchyma and endodermis.
4. Collenchymatous cells are thickened at their corners and oval or polygonal in shape. The region is 3-5 cells deep.
5. Parenchyma is present below the collenchyma, the cells of which are spherical or polygonal in shape with many intercellular spaces.
6. Parenchymatous region is three to many cells deep and cells are generally filled with many chloroplasts.
7. Endodermis is the innermost layer of the cortex consisting of barrel shaped cells.
8. Endodeimal cells lack casparian strips and filled with starch grains.
Pericycte:
9. It is represented by few parenchymatous layers below the endodermis.
Vascular System:
10. It is composed of primary phloem, secondary phloem, cambium, secondary xylem, conjunctive tissue and primary xylem. Many medullary bundles are also present
11. Vascular bundles are conjoint, collateral, open and endarch.
12. Primary phloem is crushed and occurs in patches.
13. A complete ring of few cells thick secondary phloem occurs just outside the cambium.
14. Cambium is present in the form of a strip in between the xylem and phloem.
15. Secondary xylem is present inner to the cambium, and consists of tracheids, vessels, fibres and xylem parenchyma
16. A continuous cylinder of conjunctive tissue (prosenchyma) is present which is composed of thick and lignified cells.
17. Near the pith are situated groups of primary xylem just opposite to the location of primary phloem.
Medullary Bundles:
18. Many medullary bundles are scattered in the pith which are ‘leaf traces’.
19. Medullary bundles in the pith, present towards its periphery, are smaller while that of the central position are larger in size.
Pith:
20. It is parenchymatous. The cells are oval with many intercellular spaces.
Secondary Growth:
Anomalous secondary growth occurs in the form of succession of rings of vascular bundles. De Bary (1884) was of the opinion that a de novo, extrastelar cambium ring arises in the pericycle but according to Mahcahwari (1930) separate strips of interfascicular cambium develop in the medullary rays between the outer rings of normal bundles. It forms a complete cambium ring by joining with the strips of fascicular cambium.
Identification:
(a) 1. Presence of vessels in the xylem. (Angiosperms)
(b) 1. Presence of well developed cambium and multicellular hairs.
2. Vascular bundles are conjoint, collateral, open and endarch. (Stem)
(c) 1. Vascular bundles in ring.
2. Presence of secondary growth. (Dicotyledones)
8. Anatomy of Amaranthus – Stem (Family – Amaranthaceae):
Outline of T. S. shows many ridges and furrows, and reveals the following tissues from out side with in:
Epidermis:
1. It consists of single layer of barrel shaped cells covered externally by thick cuticle.
2. Lateral and inner walls are thin.
Cortex:
3. It is well differentiated into collenchyma and chlorenchyma.
4. Collenchyma is present just below the epidermis. It is more prominent below ridges. Comers of the cells are thick and the cells are-oval or polygonal in shape.
5. Chlorenchyma is present inner to collenchyma. Thin walled cells are spherical to oval in shape, filled with chloroplasts and contain many intercellular spaces.
6. Endodermis is poorly developed and sometimes absent. The cells are elongated and lack casparian strips.
Pericycle:
7. It consists of few layers of thin walled, compactly arranged cells. It becomes sclerenchymatous in older stems.
Vascular System:
8. Vascular system consists of primary phloem, secondary phloem, cambium, secondary xylem and primary xylem. Many medullary bundles are present in pith.
9. Vascular bundles are conjoint, collateral, open and endarch.
10. Primary phloem is crushed and present in patches.
11. Secondary phloem is present in the form of a complete ring which consists of sieve tubes, companion cells and phloem parenchyma.
12. Cambium is distinct and present in one to many layers located in between phloem and xylem.
13. Secondary xylem remains embedded in conjunctive tissue and consists of proto-and metaxylem vessels and abundant parenchyma.
14. Conjunctive tissue is present in abundance and consists of thick walled and lignified cells.
15. Primary xyslem is present near the pith facing its protoxylem towards centre.
Medullary Bundles:
16. Many scattered medullary bundles are present in the pith.
17. Each medullary bundle is conjoint, collateral and endarch, with the cambium either feebly developed and functionless or absent.
Pith:
18. It is parenchymatous and cells show some intercellular spaces.
Secondary Growth:
In the pericycle region the outer primary bundles become meristematic and develop few layered cambium. This cambium cuts collateral vascular bundles towards inner side consisting of secondary phloem and secondary xylem. Cambium also cuts many layered parenchymatous conjunctive tissue which becomes lignified and thick walled. This vascular bundle lies completely embedded in conjunctive tissue.
Identification:
(a) 1. Presence of vessels in the xylem. (Angiosperms)
(b) 1. Cortex is well differentiated.
2. Vascular bundles are conjoint, collateral, open and endarch. (Stem)
(c) 1. Vascular bundles are present in a ring.
2. Well developed secondary growth. (Dicotyledones)
9. Anatomy of Nyctanthes – Stem (Family – Oleaceae):
The outline of T.S. appears quadrangular and reveals the following tissues from outside with-in:
Epidermis:
1. Single-layered epidermis consists of rectangular cells.
2. A thick uninterrupted cuticle is present on the epidermis.
3. Many multicellular hairs are present.
Cortex:
4. It is differentiated into collenchyma and parenchyma.
5. Collenchyma is several cells deep below the four protruded comers while only few layers deep at the other places just beneath the epidermis.
6. Parenchyma is present below the collenchyma. Many intercellular spaces are present. The region extends upto the vascular tissue.
Cortical bundles:
7. Four vascular bundles are present in the cortex, situated one each in each protruded bulge.
8. Each conical bundle faces its pointed xylem end towards outer side, i.e., epidermis, and is conjoint, collateral, open and exarch.
9. These bundles may show secondary growth at maturity.
Endodermis:
10. Not well-developed.
Pericycle:
11. It is in the form of sclerenchymatous patches.
Vascular System:
12. It consists of primary phloem, secondary phloem, cambium, secondary xylem and primary xylem.
13. Primary phloem is crushed and irregularly present in patches below pericycle.
14. Secondary phloem is present in the form of a continuous ring and consists of sieve tubes, companion cells and phloem parenchyma
15. Cambium is one to three cells thick continuous layer present in between phloem and xylem.
16. Secondary xylem is present just inner to the cambial ring and consists mainly of thick walled wood parenchyma and fibres. Tracheids and vessels are also present
17. Primary xylem is situated just near the pith facing its protoxylem towards the centre.
Pith:
18. It is thin walled and parenchymatous.
Abnormality:
Abnormality in Nyctanthes ii the presence of cortical bundles, which are inversely oriented, 4 in number and never directly connected with the main axial ring of the vascular cylinder. These are leaf trace bundles.
Cortical bundles have also been reported in some other families such as Casuarinaceae (Casuarina), Umbelliferae (Eryngium), Papilionaccae (Latkyrus marytimus). Mclastomaccac, Rutaccae, etc.
Identification:
(a) 1. Presence of vessels in the xylem. (Angiosperms)
1. Presence of multicellular hairs.
2. Vascular bundles are conjoint, collateral, open and endarch. (Stem)
(c) 1. Vascular bundles in ring.
2. Presence of cambium. (Dicotyledones)
10. Anatomy of Bignonia – Stem (Family – Bignoniaceae):
T.S. shows many ridges and furrows and reveals the following tissues from outside with-in:
Epidermis:
1. Single-layered epidermis consists of rectangular cells.
2. A thick cuticle is present.
3. A few multicellidar hairs are also arising from some cells.
Cortex:
4. Ii is well-differentiated into collenchyma and parenchyma.
5. Collenchyma is present below the epidermis in the ridges in young stem but at maturity there develops sclerenchyma.
6. Parenchyma is present below the sclerenchyma or collenchyma in the ridges and directly below the epidermis in the grooves.
7. In old stem cortex consists of cork, cork cambium and cortex.
8. Endodermis is undistinguishable from cortical cells. The cells lack casparian strips.
Pericycle:
9. It is in sclerenchymatous patches.
Vascular system:
10. It phloem, secondary phloem, xylem and primary xylem.
11. Four longitudinal furrows of secondary phloem arc present which are wedged in between the secondary xylem cylinder.
12. Vascular bundles are conjoint, collateral, open and endarch.
13. Primary phloem is crushed and present in small patches.
14. Secondary phloem is in the form of a ring which remains intruded into the secondary xylem at four places.
15. Intruded furrows (four) of secondary phloem are arranged in the form of a cross.
16. In Bignonia unguis-catae, bars of sclerenchyma are present in the furrows of secondary phloem.
ADVERTISEMENTS:
17. Cambium is single layered, present in between xylem and phloem and bent towards inner side along the furrows of secondary phloem.
18. Secondary xylem consists of vessels, tracheids, fibres and xylem parenchyma.
19. Due to the intrusion of the phloem at four places, secondary xylem is ridged and furrowed at four places.
20. Primary xylem is present close to the pith facing its protoxylem towards the centre. Its location is just opposite to the patches of primary phloem.
Pith:
21. It is thin walled and parenchymatous.
Secondary Growth:
Formation of four furrows of secondary phloem in the secondary xylem is due to the abnormal functioning of cambium which was behaving normally sometimes earlier.
At four or more places cambium produces less amount of secondary xylem towards inter side and large amount of secondary phloem towards outer side. Thus four wedges of secondary phloem are formed. They intrude into the secondary xylem and so xylem cylinder appears ridged and furrowed.
Identification:
(a) 1. Presence of vessels in the xylem. (Angiosperms)
(b) 1. Multicellular epidermal hairs.
2. Conjoint, collateral, open and endarch vascular bundles. (Stem)
(c) 1. Vascular bundles in a ring.
2. Presence of cambium. (Dicotyledones)
