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Read this essay to learn about the anatomy of some monocotyledonous stems: 1. Maize 2. Wheat 3. Stem of an Aquatic Monocotyledon 4. Floral Axis of Canna (Scape) 5. Asparagus Stem.
Essay # 1. Anatomy of Maize (Monocotyledonous Stem):
Zea mays belongs to the family Poaceae and its stem in t.s. shows three tissue systems — epidermis, ground tissue and the vascular bundles. No stele is formed (Fig. 5.94 & 95).
(i) Epidermis:
It is uniseriate layer of cells consisting of small compactly arranged cells. The outer walls are cuticularised and stem hairs are absent.
(ii) Ground tissue:
It is not well-differentiated. Next to epidermis a few layers of sclerenchyma cells form the hypodermis. It is usually continuous but parenchyma may intervene here and there. Rest of the ground tissue internal to the hypodermis is composed of thin-walled parenchyma cells with profuse intercellular spaces. The vascular bundles remain scattered throughout the ground tissue without forming any stele.
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(iii) Stele:
The vascular bundles are not surrounded by any endodermis or starch sheath to form the central cylindrical core of tissues or stele. Still this type of arrangement of vascular bundles throughout the ground tissue is called atactostele. The peripheral vascular bundles are comparatively smaller in size.
The vascular bundles are conjoint, collateral and closed (Fig. 5.96). Xylem components are aligned in the form of the letter Y. The two wider metaxylem vessels with pitted thickening remain at the two arms of the Y and one or two narrower protoxylem vessels with spiral or annular thickening remain at the base of the Y.
At the vicinity of the protoxylem a few tracheids remain. With the advancement of the development the xylem elements become more and more lignified and the lowermost protoxylem disintegrates to form a lacuna or cavity known as protoxylem cavity.
The small phloem tissue consisting of only sieve tubes and companion cells remains in- between the two arms of the Y. Phloem parenchyma is absent. The individual vascular bundles remain surrounded by sclerenchymatous bundle sheath.
The bundle sheaths of small peripheral bundles may coalesce with the sclerenchymatous hypodermis. As a result, those peripheral bundles appear to be embedded in the sclerenchymatous sheath.
Essay # 2. Anatomy of Wheat (Monocotyledonous Stem):
In wheat (Triticum vulgare) the internode is hollow and a T.S. through this portion shows following anatomical features (Fig. 5.97 & 98):
(i) Epidermis:
The single-layered epidermis is composed of compactly set tubular cells with cuticularised outer walls interrupted by stomata here and there.
(ii) Ground tissues:
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Just beneath the epidermis a few cell layers of sclerenchyma occur as discontinuous patches intermingled with chloroplast-containing parenchyma cells where stomata are present on the epidermis. The rest of the ground tissue is parenchymatous with conspicuous intercellular spaces among the cells and hollow pith at the centre.
(iii) Vascular bundles:
Vascular bundles are arranged in two rings. The outer ring contains comparatively smaller bundles associated with subepidermal sclerenchyma patches. The bundles of the inner ring are larger in size. Individual bundles are collateral and closed and usually remain surrounded by sclerenchymatous bundle sheath. Naturally, the sheaths of the bundles of the outer ring fuse with the subepidermal sclerenchyma patches.
Essay # 3. Anatomy of the Stem of an Aquatic Monocotyledon:
Leptochloa chinensis of family Poaceae is an aquatic grass.
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Transverse section of it shows the following arrangement of the tissues (Fig. 5.99):
(i) Epidermis:
The uniseriate epidermis is composed of a row of compactly arranged cells with a thin layer of cuticle outside.
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(ii) Ground tissues:
Just beneath the epidermis two or three layers of sclerenchyma cells form the hypodermis next to which parenchymatous ground tissue with lots of intercellular spaces occur. Another band of 1-2 cell layer of sclerenchyma is present again in the ground tissue. The central portion of the stem is hollow.
(iii) Vascular bundles:
The vascular bundles are scattered throughout the ground tissue. The peripheral bundles are smaller in size. Individual bundles are conjoint, collateral and closed with typical monocotyledonous features.
The xylem contains two metaxylem vessels with pitted thickenings and one or two protoxylem vessels with spiral or annular thickenings. Protoxylem cavity is present. The phloem is comparatively smaller than xylem and composed of sieve tubes and companion cells but devoid of phloem parenchyma. Individual bundles remain surrounded by sclerenchymatous sheaths.
Essay # 4. Anatomy of the Floral Axis of Canna (Scape):
Canna indica of subfamily Cannaceae shows the following monocotyledonous anatomical features in transverse section (Fig. 5.100):
(i) Epidermis:
The epidermis is uniseriate consisting of compactly arranged tubular cells with a layer of cuticle outside.
(ii) Ground tissues:
Next to epidermis a few parenchymatous cell layers form a small cortex, internal to which is a band of chloroplast- containing cells referred to as chlorophyllous tissue. Sclerenchyma patches remain here and there attached to the chlorophyllous tissue. The remaining portion is the parenchymatous ground tissue bearing the scattered vascular bundles.
(iii) Vascular bundles:
The bundles are conjoint, collateral and closed each containing small amount of xylem and phloem. The phloem contains only sieve tubes and companion cells. Two patches of sclerenchyma are present on the outer and inner sides of the bundles. Of them the outer patches are larger forming bundle caps.
Essay # 5. Anatomy of Asparagus Stem (Monocotyledonous Stem):
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A transverse section through the stem of Asparagus racemosus of the family Liliaceae shows the following arrangement of tissues (Fig. 5.101, 102):
(i) Epidermis:
The uniseriate epidermis is composed of roundish cells with cuticularised outer walls.
(ii) Ground tissue:
Next to epidermis a few layers of parenchyma cells laden with chloroplasts form a green band which may be called the cortex. Internal to this green band a starch sheath is present.
Beneath the starch sheath a band of sclerenchyma of few cells thick remains encircling the ground tissue with scattered vascular bundles. The center is occupied by thin- walled comparatively larger parenchyma cells with distinct intercellular spaces.
(iii) Vascular bundles:
Of the scattered vascular bundles the central ones are larger. The peripheral bundles may touch the sclerenchymatous band. The vascular bundles are conjoint, collateral and closed.
The xylem occurs in the form of the letter V, the metaxylem vessels form the arms and protoxylem forms the base of the letter V. Phloem remains within the arms of V. It is composed of sieve tubes and companion cells and phloem parenchyma is absent.
