The following points highlight the top four types of monocot and dicot stems. The types are: 1. Normal Monocot Stems 2. Monocot Stem with Secondary Thickenings 3. Normal Dicotyledonous Stems 4. Anamalous Dicotyledonous Stems.
Monocot and Dicot Stems: Type # 1.
Normal Monocot Stems:
I. Zea mays-Stem:
T.S. of the material shows following tissues from outside within:
It is circular in outline with a well-defined epidermis, hypodermis, ground tissue and many scattered vascular bundles (Fig. 160).
Epidermis:
1. It is the outermost layer of stem.
2. The outer wall of cells is covered by a thick cuticle.
3. The continuity of the layer is broken by few stomata.
4. Epidermal hair are absent.
Hypodermis:
5. It is two to three cells thick, sclerenchymatous and present just below the epidermis.
6. Cells are polygonal is shape.
Ground tissue:
7. It is not differentiated into cortex, endodermis, pericycle and pith.
8. The cells are parenchymatous and extend from below the sclerenchyma up to the centre.
9. The cells are small and compactly arranged below the hypodermis but they are large, round and loosely arranged in the centre.
Vascular Bundles:
10. Vascular bundles are many and scattered in the ground tissue with no definite arrangement.
11. They are small and more in number towards the periphery than the centre of the section.
12. Each vascular bundle is conjoint, collateral, closed and endarch.
13. A well-developed sclerenchymatous sheath surrounds each vascular bundle which is more prominent at its upper and lower faces.
14. Xylem and phloem constitute the vascular bundle.
15. Xylem:
(i) Consists of vessels (protoxylem and metaxylem), tracheids and xylem parenchyma.
(ii) Vessels are in the form of ‘Y’.
(iii) Metaxylem is present at the divergent ends of ‘ Y’ in the form of two big oval vessels.
(iv) Protoxylem is present at the lower arm of’ Y’, consisting of two small vessels.
(v) Protoxylem is surrounded by tracheids and xylem parenchyma.
(vi) Inner protoxylem vessel and parenchyma break down and form a Water-containing cavity called lysigenous cavity.
16. Phloem:
(i) Consists of only sieve tubes and companion cells.
(ii) Phloem fibres and phloem parenchyma are absent.
(iii) The outer parts of the phloem, which is broken and disorganized, is called protophloem.
(iv) Inner phloem contains sieve tubes and companion cells, and called metaphloem.
Identification: 
Special Points:
1. Scattered vascular bundles.
2. ‘Y’-shaped vasels.
3. Presence of protophloem and metaphloem.
II. Canna-Stem:
T.S. reveals the following tissues from outside within:
It is circular in outline with a well-defined epidermis, hypodermis, ground tissue system and many scattered vascular bundles.
Epidermis:
1. It consists of many small, flat and tangentially elongated cells.
2. A thick cuticle covers the outer wall of the cells of epidermis.
3. Epidermal hair are absent.
Ground Tissue System:
4. It consists of cortex, chlorenchyma, patches of sclerenchyma and ground tissue.
5. Just below epidermal layer are present two layers of cortex, consisting of large polygonal cells.
6. Chlorenchyma is present immediately below the cortex in the form of one or two layers.
7. Sclerenchyma patches remain attached with the chlorenchyma.
8. Rest of the portion is filled with may large, thinwalled, parenchymatous cells which form ground tissue.
9. Large intercellular spaces are present in the ground tissue.
Vascular Bundles:
10. Many vascular bundles are scattered in the ground tissue.
11. Vascular bundles are of different size.
12. Each vascular bundle is conjoint, collateral, closed and endarch.
13. Each vascular bundle is covered by incomplete, sclerenchymatous bundle sheath. It is rarely complete.
14. Bundle sheath is present in the form of a large patch on the outer side and a small strip on the inner side of vascular bundle.
15. Each vascular bundle is made up of phloem and xylem
16. Phloem is situated towards the outer side in the vascular bundle and consists of companion cells and sieve tubes.
17. Xylem is situated towards the inner side in the bundle, and consists of few large and small vessels and xylem parenchyma.
Identification:
(a) 1. Presence of vessels in the xylem……………. Angiosperms
(b) 1. Vascular bundles are conjoint, collateral and endarch. Stem
(c) 1.Well-developed ground tissue.
2. Scattered vascular bundles.
3. Vascular bundles are closed.
4. Absence of secondary growth………………………. Monocot
Special Points:
1. Incomplete bundle sheath.
2. Presence of sclerenchymatous patches in the ground tissue.
Monocot and Dicot Stems: Type # 2.
Monocot Stem with Secondary Thickenings:
Dracaena-Stem:
T.S. is circular in outline and reveals the following tissues from outside with-in:
Epidermis:
1. Single-layered epidermis, consisting of rectangular cells, is present in the younger stages but at maturity it gets ruptured due to secondary growth and replaced by cork.
2. Some lenticels are also present.
Periderm:
3. In old stems is present the periderm which consists of cork, cork-cambium and secondary cortex. These are also known as phellem, phellogen and phelloderm, respectively.
4. Cork cambium is one-to few cells deep and consists of barrel-shaped, thin-walled cells. It cuts off cork towards outer side and secondary cortex towards inner side.
5. Cork consists of rectangular and dead cells.
6. Secondary cortex is either parenchymatous or chlorenchymatous. It is many cells deep.
Vascular System:
7. Primary structure consists of numerous, scattered vascular bundles which are closed and collateral.
8. In the later stages the stem shows secondary growth.
9. At the time of secondary growth many-layered cambium (meristematic tissue) develops outside the primary bundles in the parenchyma.
10. Cambium or meristematic tissue cuts many secondary vascular bundles only towards inner side.
11. A small amount of thin-walled parenchyma is cut off on the outer side by the cambium (meristematic zone).
12. Primary bundles are central in position, scattered and large.
13. Secondary bundles are peripheral in position, small in size, oval in transection and amphivasal, i.e., phloem is surrounded by xylem.
14. Phloem consists of sieve tubes, companion cells and phloem parenchyma.
15. Xylem consists of tracheids and xylem parenchyma.
Ground Tissue:
16. It is well-developed, thin-walled and parenchymatous.
Secondary Growth:
The meristematic tissue, instead of cutting separate vascular tissues, cuts individual vascular bundles. Primary bundles are large and collateral whereas these secondary bundles are small and amphivasal. This meristematic zone stops functioning after sometime. It originates near leaf primordia.
Identification:
(a) 1. Presence of vessels in the xylem.
2. Vessels have perforated end walls with scalariform and regularly arranged holes ……….Angiosperms
(b) 1. Vascular bundles are conjoint and collateral.
2. Well-developed ground tissue……………………. Stem
(c) 1. Vascular bundles are closed.
2. Scattered vascular bundles…………………… Monocot
Monocot and Dicot Stems: Type # 3.
Normal Dicotyledonous Stems:
Luff a-Stem:
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 hair.
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. Its 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 thinwalled, parenchymatous cells of ground tissue, in which the vascular bundles remain embedded.
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 the remaining five of the inner ring face the furrows.
11. Vascular bundles are conjoint, bicollateral, 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 the 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 combium is curved and only few-layered.
15. Phloem is situated in the form of patches of outer phloem 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 hair ………..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.
Monocot and Dicot Stems: Type # 4.
Anamalous Dicotyledonous Stems:
I. Bougainvillea-Stem:
It is circular in outline and exhibits following tissues from outside with-in:
Epidermis:
1. Single-layered epidermis consists of compactly arranged thin-walled cells and is covered by thick cuticle.
2. It bears many multicellular hair when young.
3. In the mature stems showing secondary growth, cork cambium is present which cuts cork towards outer side and secondary cortex towards inner side.
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.
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.
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 the section, consists of tracheids, vessels, fibres and prosenchyma.
17. Primary xylem is present near the pith facing its protoxylem towards the centre of stem.
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.
Pith:
20. It is parenchymatous and its 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
(c) 1. Multicellular epidermal hair.
2. Vascular bundles are arranged in ring.
3. Presence of cambium…………………………… Dicot
Abnormality:
Interxylary phloem and medullary bundles are present.
II. Achyranthes-Stem:
T.S. is wavy in outline (Fig. 165) with ridges and furrows, and reveals the following tissues from outside with-in:
Epidermis:
1. Single-layered epidermis consists of many tubular cells and covered externally by thick cuticle.
2. From some cells arise multicellular hair.
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 its 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 include^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. They face 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 tissues 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.
III. Boerhaavia-Stem:
T.S. is circular in outline and reveals the following tissues from outside with-in:
Epidermis:
1. Single-layered epidermis consists of small, radially elongated cells.
2. Multicellular epidermal hair 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 it is only one-layered near stomata.
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 are 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 becomes 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 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 ring.
2. Well-developed secondary growth.
3. Well-defined pith……………………… Dicotyledons
IV. Leptadenia-Stem:
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…………………… Dicotyledons
V. Nyctanthes-Stem:
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 hair are present.
Cortex:
4. It is differentiated into collenchyma and parenchyma.
5. Collenchyma is several cells deep below the four protruded corners while only a 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 tissues.
Cortical Bundles:
7. Four vascular bundles are present in the cortex, situated one each in each protruded bulge.
8. Each cortical bundle faces its pointed xylem end towards outer side, i.e. epidermis, and it 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 of mainly 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 is 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.
Coritical bundles have also been reported in some other families such as Casuarinaceae (Casuarina), Umbelliferae (Eryngium), Papilionaceae (Lathyrus marytimus), Melastomaceae, Rutaceae, etc.
Identification:
(a) 1. Presence of vessels in the xylem……….. Angiosperms
(b) 1. Presence of multicellular hair
2. Vascular bundles are conjoint, collateral, open and endarch………………. Stem
(c) 1. Vascular bundles in rings.
2. Presence of cambium…………………… Dicotyledons
VI. Bignonia-Stem:
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 multicellular hair are also arising from some cells.
Cortex:
4. It 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 the form of sclerenchymatous patches.
Vascular system:
10. It consists of primary phloem, secondary phloem, cambium, secondary xylem and primary xylem.
11. Four longitudinal furrows of secondary phloem are 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.
17. Cambium is single-layered, present in between xylem and phloem and remains 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 sometime earlier. At four or more places cambium produces less amount of secondary xylem towards inner 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 the 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………………………….. Dicotyledons
