In this article we will discuss about the monocotyledonous and dicotyledonous stem with the help of suitable diagrams.
The stem is the part of the axis, bearing leaves and reproductive bodies and is aerial in nature. The close association of stem and leaf as is indicated by the usage of the word ‘shoot’ combinedly for both structures makes its structure quite complex. Apart from such, the shape of stem is quite variable.
It can be an elongated structure with prominent nodes and internodes, or a condensed structure with discernible nodes and internodes and the leaves crowded in a rosette or bulb, or may be climbing or creeping. Sometimes the stem is underground (rhizome, corm, bulb and tuber). All these variations and the variable arrangement of leaves on the stem make its structure more complicated.
The stem includes three tissue systems, the epidermal, ground and the vascular tissue system. The variations in the primary structure of stem are caused chiefly due to the differences in the relative size of vascular and ground tissue systems.
In dicotyledons, the vascular system forms a hollow cylinder enclosed on both the sides by ground tissues, the cortex and the pith, the vascular bundles are separated from one another by broad gaps of parenchyma, the medullary rays (also ground tissue), or the inter fascicular parenchyma. It interconnects the pith and the cortex.
In monocotyledonous stem the arrangement of vascular tissue is quite complex. In these cases and also is certain herbaceous dicotyledons the bundles are present in more than one ring or scattered throughout the cross-section in the ground tissue. The delimitation of ground tissue in cortex, and pith may not be made out.
The most important anatomical features of stem are as follows:
(1) The protoxylem is always endarch that is, facing towards the centre, and
(2) The vascular bundles are conjoint and collateral.
The stems of the two main groups of Angiosperms show different structures; a few examples of each group, thus, will be taken here separately.
Monocotyledonous Stem:
A. Maize Stem:
It represents a typical monocotyledonous stem and visible following structure:
1. Epidermis:
This is the outermost layer of stem. The cells are compact and strongly cutinized on their outer face. The shoot hairs are lack. Stomata can be present.
2. Hypodermis:
The region below the epidermis is called hypodermis. It is formed of 2-3 layer of sclerenchyma. At the stomatal region it can be interrupted by parenchyma.
3. Ground Tissue:
It extends from below the hypodermis to the centre of the stem and includes thin-walled parenchymatous cells with intercellular spaces.
4. Vascular Bundles:
These are conjoint, collateral and closed. Numerous vascular bundles are irregularly dispersed in the ground tissue. The bundles are smaller in size and more in number towards the periphery and lesser in number and bigger in size towards the centre. Each bundle is surrounded by a sclerenchymatous sheath. In the outer bundles the sheath can be in continuation with the hypodermis.
The xylem in each bundle is arranged in Y-shaped manner. The two arms of Y are occupied by two big vessels (Metaxylem, pitted) and the tail by smaller vessels (Protoxylem-annular and spiral). On the inner side of protoxylem a lysigenous water cavity is present. It is produced by the breaking of protoxylem vessels.
The phloem is present above the xylem. It includes only the sieve tissue and companion cells. Phloem parenchyma is lack. The primary phloem (protophloem) is generally pushed outside and is crushed.
B. Asparagus Stem:
Asparagus forms a typical case. In such stem the epidermis is one layered parenchyma having more or less rounded cells. The cuticle also is well developed. In the epidermis are present certain stomata followed by substomatal cavities. Just below the epidermis there is present some chlorenchyma which is called cortex.
The innermost layer of the cortex is conspicuous and may be known as starch sheath. Inside the cortex is a compact band of 5—6 layers of sclerenchymatous cells, which is followed by ground tissue (compare with normal stems, where the sclerenchyma is present just below the epidermis).
The vascular bundles vary from those of maize in having more of metaxylem and in the lack of water cavity and sheath. The xylem forms a V-shaped structure and the bundles are distributed irregularly in the parenchymatous ground tissue. The stem is hollow. The structure of Asphodelus stem is also quite similar to that of Asparagus stem.
C. Scape (Flowering Stem) of Canna:
In this stem also the epidermis is a single layer of compact parenchyma with well evolved cuticle and is followed by a 2-3 layered parenchymatous zone. This zone can be called hypodermis (compare with maize and other monocots where it is sclerenchyma) and on the inner side is lined by continues layer of chlorophyllous tissue. Below it are present small patches of sclerenchyma.
The vascular bundles are dispersed, conjoint, collateral and closed. The bundle sheath is incomplete on the lateral sides. It is present only on the outer and the inner faces, being more prominent on the outer face. The xylem has one bigger and a few smaller vessels with spiral thickenings. The phloem is on the outer side and includes sieve tubes and companion cells. The water cavity is lack.
D. What Stem:
The wheat stem also show some typical feature. In such cost also there is present a well evolved single layered epidermis, but is interrupted here and there by pores, the stomata. The guard cells of stomata are slightly sunken in the hypodermal part. The hypodermis in such case is typical in having alternate patches of chlorenchyma and sclerenchyma.
The stomata are present only above the chlorenchyma in the epidermis. The sclerenchyma continues in the lower region, where it produces a continuous band. Below this sclerenchymatous ring are present numerous vascular bundles scattered in parenchymatous ground tissue. The vascular bundles are closed and collateral and are enclosed in a sclerenchymatous ground sheath. The xylem is typically Y- shaped. The stem in the centre has a hollow cavity.
E. Doob Grass Stem (Cynodon Dactylon):
Doob grass also forms an exceptional case in the monocotyledonous stems.
In this case as general there is present a thick cuticle followed by a single layer of epidermis but the cells walls are lignified. Below the epidermis chlorenchyma is present which is followed by sclerenchyma. This is peculiar feature of such stem and differs from most of the monocotyledonous stem where epidermis is followed by sclerenchyma.
The vascular bundles visible typical characters of a monocotyledonous stem, in being, collateral, closed and dispersed. Sometimes they form 2 rings. They also have a Y-shaped xylem below the phloem, a water cavity and the bundle sheath (sclerenchyma). All these bundles are irregularly distributed in the parencymatous ground tissues.
Dicotyledonous Stems:
A. Cucurbita Stem:
Cucurbita represents a typical kind of the family Cucurbitaceae, whose most of the members visible a similar internal structure. It is a hollow stem having five prominent ridges alternative with equal number of furrows.
The internal details show the following:
1. Epidermis:
It is the outermost layer having several multicellular shoot hairs and is cuticularised. Cells are compactly arranged.
2. Hypodermis:
It is collenchymatous and is more prominent below the ridges having about 6—7 rows of cells. In the furrows it includes one or two layers of cells and bears chloroplasts.
3. Cortex:
This forms a narrow, 2—3 layered bands of parenchyma below the hypodermis. The cells bear chloroplasts and have intercellular spaces.
4. Endodermis:
It is a single, wavy layer of barrel-shaped cells with starch grains. It lies outside the pericycle.
5. Pericycle:
Below the endodermis there is a continuous zone of 4—5 layered sclerenchyma. This is called homogenous pericycle.
6. Vascular Bundles:
There are present 10 vascular bundles arranged in two rings. In each ring there are five bundles. In very few cases the number of bundles can exceed then. The outer bundles are smaller in size and are situated below the ridges. The inner bundles are bigger and are placed below the grooves. Each bundle is conjoint, open and bicollateral.
The sieve tubes are very conspicuous in the phloem. The metaxylem includes two or three large-pitted vessels and the protoxylem has numerous small vessels with annular or spiral thickening. On both the sides of xylem is present a cambium strip. Outside the cambium on both the sides is present a phloem patch.
7. Ground Tissue:
The thin-walled tissue, extending from below the pericycle to the pith cavity, is called ground tissue. The vascular bundles are embedded in it. There is a wide gap of parenchyma between the outer bundles, but between the inner bundles the gap is smaller, so much so that even in certain cases the bundles are almost attached with one another.
B. Sunflower Stem:
The stems of almost all the members of Compositae family show a typical structure in transverse section.
1. Epidermis:
This is the outermost layer. It includes tangentially flattened cells, which are closely united with each other on their radial walls. A cuticle is usually present on the outer surface. Some of the cells bear multicellular hairs and in some cells stomata can be present. The chloroplast is always absent.
2. Cortex:
The tissue beneath the epidermis is known as cortex and can be differentiated into three regions:
(a) Hypodermis:
The outermost part of cortex has a few layers of collenchyma cells. The intercellular spaces are lack due to the thickening of the cells in the corners.
(b) General Cortex:
The tissue below the hypodermis includes a few layers of parenchyma and is known as general cortex or cortex. In this tissue intercellular spaces are well marked.
(c) Endodermis:
This is the innermost layer of cortex. It includes barrel-shaped, closely- fitted cells without intercellular spaces. It includes starch grains, and as such, may also be called as starch sheath.
3. Pericycle:
The tissue between the vascular bundles and the endodermis is known as pericycle. It includes the sclerenchyma (above the V. Bs.) alternating with parenchyma and can be known as hetergenous pericycle. The sclerenchyma associated with phloem is known as hard bast.
4. Vascular Bundles:
The vascular bundles are conjoint, collateral and open. These are arranged in a ring and include typical xylem, phloem and cambium.
5. Pith:
This is the central part of the stem. It includes numerous rounded or polygonal, thin-walled living cells with conspicuous intercellular spaces.
C. Peristrophe Stem:
In Peristrophe stem there is present a single layered epidermis. A cuticle is also well evolved. The hypodermis, made up of collenchyma is present only in the ridges while at rest of the places collenchyma is lack. The cortex has large parenchyma cells arranged in 2—3 layers and is chlorenchymatous.
The endodermis forms a watertight compartment between vascular tissue and cortex. It is typical single layer of barrel shaped cells. The pericycle is composed of few layers of parenchyma and mixes with the phloem. Vascular bundles are collateral and arranged in a ring. Pith is well evolved and also there are present the medullary rays (Fig 8.13.).
D. Ricinus Communis Stem:
In such case, there is a single layered epidermis which has a uniform moderately thick cuticle. Below the epidermis the hypodermis is well marked having 5—8 layers of collenchyma. The cortex is very small in size being about 2 to few layers in thickness. Oil cavities are present within the cortex. Endodermis is well represented with typical layer of cylindrical, barrel shaped cells.
The pericycle is heterogenous having smaller patches of sclerenchyma. Vascular bundles are arranged in a ring and are collateral, open and conjoint. In the centre there is a pith of well evolved parenchyma. The parenchymatous medullary rays are also well marked.
On the basis of their general structures, the stems are divided into 4 major types:
i. Woody Dicotyledonous:
In these stems interfascicular regions are narrow and generally can be seen only in young stage. The primary xylem after secondary growth is insignificant and secondary xylem forms the main part. The rays generally are narrow but dialate in the phloem region viz., Tilia and Michelia etc.
ii. Herbaceous Dicotyledonous:
They resemble young woody stems in most of the cases. The interfascicular region in these stems is wall marked. The primary xylem generally remains distinct even after the secondary growth has taken place.
iii. Dicotyledonous Vines:
In these stems the primary vascular bundles are separated by wide inter fascicular regions. The vascular bundles are generally both large and small sized. The pericycle in some cases is sclerenchymatous e.g., Cucurbita and Aristolochia, while in others like Thunbergia it has dispersed patches of sclerenchyma. The secondary growth in good number of cases is confined in the fascicular region.
iv. Herbaceous Monocotyledons:
These stem generally lack secondary growth, and increase in thickness only by primary growth. They have a uniform ground tissue with dispersed vascular bundles. The bundles have Y-shaped xylem and bundle sheath.
Nodal Anatomy:
The vascular system of the leaf is in continuation with the vasculature of stem. This connection is observed in the nodal region of stems. In this part the strand from the stem bundles move towards the leaf. Such bunble extending from the base of leaf to the point of its junction with strand in the stem is called leaf trace.
The leaf trace can thus be described as the cauline part of the vascular supply of the leaf. At the point in the node from where the leaf trace begins, a region of parenchyma occurs in the vascular cylinder of stem and is called leaf gap. Thus, the leaf gap is formed due to the origin of leaf trace.
The number of leaf traces and gaps vary even within the same plant. These variations are of phylogenetic importance. Special terms are utilized to designate nodal anatomy. The term lacuna is used in place of gap and the nodes are known unilacunar, trilacunar and multilacunar depending upon whether one leaf is associated with one, three or several lacunae at the node. In case more than one leaf is attached to a node, the lacunar condition is described with respect to one leaf. To be more clear, if each leaf is connected with one gap, the node is known as unilacunar, no matter if actually there are two or more gaps.
In certain, plants viz., Clerodendron, a special condition is met with. Two leaf traces are associated with a single gap. It is a primitive condition and called two-trace unilacunar condition.
The evolutionary sequence can be described as either:
(i) Two-trace unilacunar, trilacunar, and multilacunar, of
(ii) Two-trace unilacunar, one-trace unilacunar, trilacunar and multilacunar.
The one-trace unilacunar could have been derived from trilacunar situation.
Leaf gaps also vary in extent, both laterally and vertically in stem having many axially elongate inter-fascicular regions or with scattered bundles, it is difficult to designate a leaf gap and hence, in these cases the concept of leaf gap is applied only theoretically. It may be easily recognized in stems with seconder growth.
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