In this essay we will discuss about the anatomy of some dicotyledonous stems:- 1. Sunflower Stem 2. Cucurbita Stem 3. Leonurus Stem 4. Calotropis Stem 5. Enhydra Stem.
Essay # 1. Anatomy of Sunflower Stem:
Transverse sections through the internode of a young sunflower stem (Helianthus annuus) exhibits the following tissue arrangements from the periphery to the centre (Fig. 5.86 & 87).
It is the outermost uniseriate zone, consisting of tubular cells joined end to end without intercellular spaces but interrupted by stomata. The cells are living, achlorophylous and vacuolate. The outer walls of the epidermal cells have cuticle to check water loss. Numerous multicellular hairs develop on the epidermis.
It forms the extrastelar ground tissues beneath the epidermis.
Cortex is differentiated into three zones:
1. Peripheral collenchymatous hypodermis:
Cells of this zone are usually angular collenchyma and form a continuous band. It gives mechanical support to the growing stem.
2. Parenchymatous zone:
Next to hypodermis a few layers of thin-walled parenchyma cells with conspicuous intercellular spaces occur. A few glands with hollow cavity surrounded by small epithelial cells are present here and there.
3. Starch sheath:
This is the innermost wavy layer of cortex made of compactly set barrel-shaped parenchyma cells containing abundant starch grains. This layer is homologous to the endodermis in root.
The central core of tissues encircled by the starch sheath constitutes the stele. It is a dissected siphonostele consisting of vascular bundles arranged in a ring and the intrastelar ground tissue.
(iv) Vascular bundles:
The vascular bundles are conjoint, collateral and open with internal xylem and external phloem on the same radius. A strip of lateral cambium is present in-between xylem and phloem.
The xylem is endarch showing centrifugal mode of differentiation from the procambium. The protoxylem elements like narrower tracheids and tracheae with annular and spiral thickenings occur towards the centre. Metaxylem with pitted and other types of thickenings remains on the periphery. Xylem parenchyma cells are smaller than other parenchyma cells of the stele.
The phloem is composed of sieve elements (SE), companion cells (CC) and phloem parenchyma. The sieve tubes and companion cells collectively from the SE-CC complex.
The cambium of the vascular bundle known as fascicular cambium appears to consist of two or three layers of fusiform cells which appear rectangular in cross-section. This tissue is responsible for growth in thickness.
(v) Intrastelar ground tissues:
There exists a patch of sclerenchyma on the top of every vascular bundle which is called bundle cap. This bundle cap is termed hard bast as it is considered to belong to phloem.
A full transverse section of the stem shows that the bundles remain in a ring towards the periphery of the stele and the large central portion remains occupied by parenchyma with large intercellular spaces. This part is known as pith or medulla. Radiating from the central pith parenchymatous strips of ground tissue remain in between the vascular bundles (interfascicular regions) called the medullary rays.
Essay # 2. Anatomy of Cucurbita Stem:
Cucurbita maxima is a climber. Transverse section of its stem shows certain characteristic anatomical features (Fig. 5.88 & 89). The section shows distinct ridges and furrows giving a wavy outline. The vascular bundles occur in two concentric rings.
Of them the five bundles of the outer ring are smaller present beneath the ridges representing the leaf trace bundles. Five larger vascular bundles of the inner ring occur beneath the furrows. The central part of the stem is hollow due to disintegration of pith.
The t.s. of Cucurbita stem shows the following tissue distribution from periphery to the center:
It is the uniseriate outer layer made of compactly-set tubular cells with cuticular outer covering. The cells of this layer are living with vacuolate protoplast. Numerous multicellular hairs are present on the epidermis. Stomata may be present in the young stem.
It is differentiated into three zones:
1. Peripheral collenchymatous hypodermis:
The thickness of the hypodermis is not uniform. It is most reduced beneath the furrows and even interrupted by some parenchyma cells containing abundant chloroplasts just beneath the epidermis where stomata are present. Conspicuous collenchymatous patches are formed at the ridges. Chloroplasts may also be present in the collenchyma cells.
2. Internal to the hypodermis there are a few layers of parenchyma cells which are larger in size having intercellular spaces among them. These cells contain lesser number of chloroplasts.
3. The innermost layer of cortex is the starch sheath the cells of which are barrel-shaped and larger than other cells of cortex. These cells contain abundant starch grains.
It is the central cylinder core containing the vascular bundles and intrastelar ground tissues encircled by the starch sheath. Immediately beneath the starch sheath a few layers of sclerenchyma form a continuous band.
Little parenchyma cells are present between this band and the vascular bundles. These tissues together form the pericycle. Pith disintegrates early in Cucurbita stem. The cells occurring in between the vascular bundles are referred to as internal parenchyma.
(iv) Vascular bundles:
The vascular bundles are conjoint, bicollateral and open having two patches of phloem and two strips of cambium on either side of xylem. Therefore, the sequence of tissues in the vascular bundle from the periphery is outer phloem, outer cambium, xylem, inner cambium and inner phloem.
The phloem is composed of sieve tubes, companion cells and phloem parenchyma. Sieve tubes are quite conspicuous with numerous sieve plates. The outer phloem is more massive than the inner one.
Of the xylem elements metaxylem vessels are very larger in size with pitted thickenings characteristic of most climbers. The protoxylem vessels occur towards the centre having usually spiral and annular thickenings. Xylem parenchyma cells are abundant but tracheids and fibres are few or lacking.
The outer cambium is many-layered containing rectangular cells in t.s. The inner cambium strip is thin and usually curved.
Due to the activity of the outer cambium the primary phloem is pressed towards the pericycle whereas the secondary phloem remains adjacent to the cambium. The primary and secondary phloems are not distinguishable as they have the same type of elements. Secondary xylem is formed internal to outer cambium and the primary xylem is pressed towards the center. The vessel elements in secondary xylem are much larger.
Essay # 3. Anatomy of Leonurus Stem:
Young stem of Leonurus is almost square in outline and shows the following tissues in transverse section (Fig. 5.90 & 91):
It is single-layered with tubular cells without intercellular spaces but externally covered by a layer of cuticle. Multicellular hairs and stomata are present in the epidermis.
The cortex in this stem is comparatively shallow but differentiated into three zones – the hypodermis, parenchymatous middle zone and the starch sheath. The collenchymatous hypodermis is densely aggregated at the corners of the stem. These diagonally placed collenchymatous patches form the I-girders to give mechanical support to the stem.
These patches extend from the corners to some extent but do not form the continuous band of hypodermis. The innermost layer of the cortex is the starch sheath consisting of barrel-shaped compactly arranged cells with abundant starch grains. In between the hypodermis and starch sheath a few layers of thin-walled parenchyma cells remain. The cells of this layer contain abundant chloroplasts and appear as a green belt under the microscope.
The central core of tissues made of vascular strands and intrastelar ground tissues encircled by the starch sheath is the stele. Internal to the starch sheath a few layers of sclerenchyma form a continuous band. It may be called pericycle or perivascular tissue. Large parenchymatous pith present at the center presses the vascular bundles towards the periphery. The primary medullary rays become indistinct due to secondary growth in thickness.
The vascular bundles are conjoint, collateral and open and remain arranged more or less in a ring. The phloem remains towards the periphery having sieve tubes, companion cells, and phloem parenchyma.
Internal to the phloem there is a strip of cambium, made of a few layers of fusiform cells appearing more or less rectangular in outline. Internal to the layer of cambium xylem occurs with usual tracheary elements—tracheids and tracheae, parenchyma and fibres. Xylem is endarc, i.e., protoxylem remains towards centre and metaxylem towards circumference.
Essay # 4. Anatomy of Calotropis Stem:
A transverse section through the internode a young stem of Calotropis procera of family Asclepiadaceae shows the following arrangement of tissues (Fig. 5.92):
Epidermis is uniseriate composed of tubular cells with cuticularized outer walls. There may be further deposition of wax external to the outer wall of the epidermis. Stomata are present here and there.
It is differentiated into hypodermis, parenchymatous zone and the starch sheath. The hypodermis is composed of two or three layers of lacunate collenchyma. The intermediate zone consists of large parenchyma cells with intercellular spaces and the starch sheath is made of barrel-shaped compactly arranged cells with starch grains with starch grain accumulation inside.
The central core of tissues consists of compactly arranged vascular bundles with intervening ray cells and intrastelar ground tissues. The pericycle is made of a few cell layers of parenchyma with patches of cellulosic fibres here and there. The central portion of the stele is occupied by large parenchymatous pith.
The vascular bundles are made of internal xylem with endarch arrangement, external phloem and cambium ring in-between the two. Presence of patches of intraxylary phloem is a distinctive characteristic.
Essay # 5. Anatomy of Enhydra Stem:
Enhydra fluctuant of the family Compositae is an aquatic plant. So naturally its stem is soft with abundant air chambers and rudimentary mechanical tissues.
A t.s. through the internode of the stem shows the following tissue arrangements (Fig. 5.93):
It is single-layered composed of comparatively small tubular thin-walled cells with very feeble cuticularisation on the outside wall.
Enhydra stem has less massive cortex in comparison to other terrestrial dicotyledonous stems. It is differentiated into three zones:
1. A few layers of parenchymatous hypodermis with comparatively thicker walls forming a continuous band. The absence of peripheral collenchyma is thus a notable feature.
2. Internal to the hypodermis there are a few layers of large parenchymatous cells having abundant spaces and conspicuous air chambers. These air spaces and air chambers give buoyancy to the whole plant apart from normal aeration. The parenchyma cells contain chloroplastids in them.
3. The innermost layer of cortex is as usual the starch sheath, the barrel shaped compactly arranged cells of which store starch grains inside.
The central cylindrical core of tissues constitutes the stele consisting of the vascular bundles arranged in a ring and intrastelar ground tissues. There is a bundle cap or hard bast against every vascular bundle as a patch of sclerenchyma similar to sunflower stem but much smaller in size.
These sclerenchyma patches together with intervening parenchyma form the pericycle, though doubt remains regarding the origin of the fibres. The pith is hollow as it disintegrates leaving a large cavity at the central region. Some parenchyma cells occur in the interfascicular regions.
The vascular bundles remain arranged in a ring. They are conjoint, collateral and open. The amount of phloem tissue is rather small in comparison to xylem with the usual components like sieve tubes, companion cells and phloem parenchyma. A strip of cambium occurs in between the xylem and phloem. Xylem is composed of a few tracheary elements including protoxylem and metaxylem with endarch orientation and a few xylem parenchyma cells.