Definition of Monocot Stem
Monocot stem is a circular-shaped hollow axial part of the plant which gives rise to nodes, internodes, leaves, branches, flowers with roots at the basal end.
- The size of stems varies in different species of monocots, but the size is barely ever as large as dicots.
- Monocot stems are herbaceous as they lack secondary growth due to the absence of cambium in their internal tissue system.
- However, some plants like Palms and Bamboo might have woody stems as a result of anomalous secondary growth.
- Many monocot stems are hollow as the tissues in the stem are not arranged in an orderly fashion.
- Monocot stems arise from the plumule of the embryo and often have a terminal bud at the tip of the shoot. The stem is positively phototropic, unlike the roots.
- The stem gives off leaves and branches at structures called nodes that are present at regular intervals on the stem. In monocots, the leaves develop from the nodes without any petiole, surrounding the stem at the base.
- Monocot stems can exist in different forms depending on the characteristics of the plant species.
- Monocots like coconut and palms have caudex or columnar type of stem which is unbranched, erect, cylindrical, and stout with a crown of leaves at the tip. This type of stem is characterized by scars of fallen leaves throughout the trunk.
- Culm type of stem can be observed in monocot plants like bamboo where the stem is composed of solid nodes and hollow internodes. The nodes are often swollen with tiller branching.
- In some monocots like onions, a scape type of stem is found which is characterized by the absence of aerial stem during the vegetative phase of the plant. A reproductive shoot is observed during later stages which are unbranched and cylindrical with an inflorescence at the tip.
- Monocot stems are essential as they carry most of the parts of the plant from photosynthetic leaves to branches and flowers.
Definition of Dicot Stem
Dicot stem is the solid cylindrical axial part of a plant consisting of nodes and internodes giving rise to leaves, branches, and flowers.
- The most distinguishing feature of dicot stems is the hard and woody trunk as a result of the secondary growth of the plant.
- The diameter of the stem ranges from few millimeters to several centimeters depending on the type of plant species and the age of the plant.
- The stem of dicots is green and somewhat photosynthetic when young, but it becomes hard and woody as the plants continue to grow.
- Dicot stems are exogenous in origin as these originate from the lateral branches of the cortical zones.
- Dicot stems also arise from the plumule of the embryo and are positively phototrophic. Unlike roots, the stems have stomata which enable the exchange of gases.
- The stem consists of nodes which are slightly swollen structures present throughout the stem and the space between two nodes are the internode. The nodes give rise to branches and leaves.
- In the dicot stem, the leaves develop with a petiole, and the number of leaves or branches on the stem differ in different species.
- Most dicot stems remain erect and ascending, but some might lie prostrate on the ground like in sweet potato or strawberries.
- Like in monocots, dicot stems are also modified in different species to give different structure and function to the plant.
- A common form of the stem in dicots is the deliquescent form where the main stem grows for some time and then stops growing. It continues to give out branches resulting in a dome or umbrella-shaped structure of the plant.
- Some dicots might be climbers with soft, flexible stems to enable the plant to grow and move through hard surfaces.
Structure of Monocot and Dicot Stem
The internal structure or anatomy of the dicot and monocot stem consists of the following structures;
- The epidermis is the outermost layer of the stem consisting of a single layer of thin-walled cells arranged compactly without intercellular spaces.
- The cells of the epidermis do not have chlorophyll, but some cells are modified to form multicellular hairs or trichomes. The trichomes are absent in monocot stems.
- Outside the epidermis is a layer of cutin called a cuticle which protects the epidermis and tissues lying underneath.
- The epidermis of the stem is different from that in the roots as it contains stomata for the gaseous exchange.
- The epidermis in the monocot remains the same throughout the growth of the plant as monocots do not undergo secondary growth. The epidermis of dicots might get replaced during secondary growth.
- The most important function of the epidermis is to protect the underlying tissues of the stem against different environmental factors.
- The cortex of monocot and dicot stems differs significantly in the presence of different regions and the number and type of cells.
- In the dicot stem, the cortex is composed of three distinct regions; hypodermis, general cortex, and endodermis.
- The hypodermis is present directly below the epidermis that functions to provide additional support to the epidermis.
- The hypodermis is a thick multicellular layer composed of collenchyma. The cells of this region have chlorophyll and can prepare food for the plant.
- The general cortex region is composed of parenchymatous cells arranged loosely with adequate intercellular spaces. The cells of this region store the food prepared by the cells of the hypodermis.
- The general cortex consists of resin canals or mucilage canals that run throughout the stem.
- The innermost layer of the cortex in dicots is the endodermis which is a thick single-layered tissue. The cells of this region are barrel-shaped and arranged compactly with no spaces.
- The endodermis is also termed the starch sheath as the cells of the endodermis store a large amount of starch.
- The radial and transverse walls of the cells of the endodermis are thick due to the deposition of lignin resulting in Casparian strips.
- In the case of monocot stems, the cortex region is only composed of the hypodermis. The hypodermis is 2-3 layers thick and is composed of sclerenchymatous cells.
- The hypodermis of the monocot stem is more rigid, whereas that of the dicot stem is more elastic.
Image Source: Byjus.
3. Ground Tissue
- In monocot stem, the ground tissue is not differentiated into units like pericycle and medullary rays.
- The ground tissue is a mass of parenchymatous cells present underneath the hypodermis and extending up to the center.
- The cells are thin-walled and round and are arranged loosely with intercellular spaces. The ground tissue serves as the matrix for the vascular bundles lodged in this region.
- In the case of the dicot stem, the ground tissue is differentiated into pericycle, medullary rays, and pith.
- The pericycle is the tissue present between the endodermis and the vascular bundles. In the dicot stem, the pericycle is multilayered and functions to protect the underlying tissues.
- The cells of the pericycle are sclerenchymatous, and thus, it is often termed as hardbast. The sclerenchymatous cells of the pericycle are present in the form of semilunar patches above the vascular bundles.
5. Medullary rays
- Medullary rays are the part of the pith that surrounds the vascular bundles and are involved in the radial conduction of food and water between the bundles.
6. Vascular bundles
- The arrangement, number, and components of vascular bundles are quite different in monocot and dicot stems.
- The vascular bundles of the dicot stem are arranged in the form of a ring around the central pith present underneath the pericycle.
- The vascular bundles are wedge-shaped, conjoint, collateral, and open. The stele of the dicot stem is of the eustele type.
- The vascular bundles are limited in number and have a uniform size. Each bundle is composed of a patch of xylem present towards the center separated by a patch of phloem present towards the periphery by a strip of cambium.
- The xylem is present towards the pith of the stele and is composed of tracheids, vessels, xylem parenchyma, and xylem fibers.
- The tracheids and vessels are composed of smaller protoxylem and larger metaxylem units. The components of the xylem are arranged in rows.
- The phloem is present below the sclerenchymatous patch of the pericycle and occurs in the form of undifferentiated mass.
- Phloem bundles in the dicot stem are larger in size and consist of elements like sieve tubes, companion cells, and phloem parenchyma.
- The vascular bundles in the monocot stem are numerous and remain scattered in the ground tissue or the cortex. The stele of the monocot stem is of the atactostele type.
- The bundles are oval, conjoint, collateral, closed, and endarch. The bundles are of different sizes depending on their position in the stem. Larger bundles are present towards the center whereas the bundles at the periphery are of smaller size.
- The bundles consist of xylem present towards the center and phloem present towards the periphery without any cambium.
- Fewer xylems are present in monocot stem consisting of metaxylem with two large vessels and protoxylem with one smaller vessel. The vessels are arranged in the shape of the letter Y or V.
- Few tracheids are present between the metaxylem vessels. Underneath the protoxylem vessel, the schizolysigenous water cavity is present as a result of the disintegration of the xylem elements and the parenchyma below.
- Phloem is present outside the xylem with some of it present around the metaxylem vessels.
- The phloem in the monocot stem consists of sieve elements and companion cells. In mature phloem bundles, the protophloem gets crushed below the vascular sheath, so just the meta-phloem region remains.
- Pith is the central mass of cell which is well developed in dicot plants but is reduced and undifferentiated in monocot stem.
- The cells are parenchymatous and are present around the vascular bundles in the form of medullary rays.
- The cells are rounded or polygonal that are arranged with or without intercellular spaces. The cells in the pith region store food and help in the conduction of food and water between the bundles.
Functions of Monocot and Dicot Stem
The following are some of the functions of monocot and dicot stems;
- The stem is the axis of s plant that supports different parts and appendages like leaves, branches, flowers, and fruits.
- Stem acts as the mode of transportation of food from leaves to different parts of the plants, and water from roots to different parts of the plant.
- Young stems in some plants are photosynthetic and can prepare food for the plant.
- The cells of the stem store a large number of food particles like starch and other nutrients.
- In some plants, stems are specialized for definite functions like climbing and storage of food.
- The meristem tissue of the stem grows continuously to generate new living tissue annually to ensure the growth of the plant.
- The stomata in the stem are involved in the process of transpiration and help in the loss of excess water.
- In plants with a modified stem-like cactus, the stem stores a large amount of water and food to prevent water loss.
Image Source: Byjus.
Monocot Stem vs Dicot Stem (22 Key Differences)
|Characteristics||Monocot Stem||Dicot Stem|
|Definition||Monocot stem is a circular-shaped hollow axial part of the plant which gives rise to nodes, internodes, leaves, branches, flowers with roots at the basal end.||Dicot stem is the solid cylindrical axial part of a plant consisting of nodes and internodes giving rise to leaves, branches, and flowers.|
|Internodes||The internodes of the monocot stem are hollow.||The internodes of the dicot stem are solid.|
|Transverse section||The monocot stem is differentiated into the epidermis, hypodermis, and ground substance in the transverse section.||The dicot stem is differentiated into the epidermis, cortex, and stele.|
|The internal tissues in the monocot stem are arranged randomly in no order.||The internal tissues of the dicot stem are arranged in concentric layers or rings.|
|Epidermal hairs||Epidermal hairs are present in the dicot stem.||Epidermal hairs are absent in the monocot stem.|
|Hypodermis||The hypodermis is sclerenchymatous and mostly non-green.||The hypodermis is mostly collenchymatous and mostly green.|
|Cortex||The cortex of the monocot stem is less developed and is represented by the hypodermis.||The cortex of the dicot stem is well developed and differentiated into hypodermis, endodermis, and general cortex.|
|The general cortex is the parenchymatous.||The general cortex is reduced or absent.|
|Endodermis||Endodermis is absent in monocot stem.||The endodermis is present in the dicot stem and contains cells with starch granules.|
|Stele||The stele of the monocot stem is larger and of an advanced type.||The stele in the dicot stem is larger than the cortex and moderately developed.|
|The stele consists of ground tissues and vascular bundles.||The stele is differentiated into pericycle, vascular medulla, and medurally rays.|
|Pericycle||Pericycle in monocot stem is reduced or completely absent.||Pericycle in dicot stem is present in either completely or partially sclerenchymatous.|
|Vascular bundles||The vascular bundles in the monocot stem are scattered irregularly throughout the ground tissue.||The vascular bundles in the dicot stem are arranged in the form of a broken ring.|
|The bundles are numerous and of different sizes.||The bundles are fewer and mostly uniform in size.|
|The vascular bundle is oval, conjoint, collateral, closed, and endarch.||The vascular bundle is wedge-shaped, conjoint, collateral, open, and endarch.|
|The vascular bundles are covered with a sclerenchymatous sheath.||The bundle sheath is absent.|
|The vascular bundles remain the same throughout the life of the plant.||Older vascular bundles are replaced by newer ones many times throughout the life of the plant.|
|The xylem bundles are composed of vessels that are arranged in the form of the letter Y.||The xylem bundles contain fewer vessels that are arranged in rows or columns.|
|The xylem contains protoxylem lacunae.||The xylem doesn’t have defined protoxylem lacunae.|
|The phloem units are smaller in size and lack phloem parenchyma.||The phloem units are larger in size and have phloem parenchyma.|
|Secondary growth||Secondary growth is not observed in the case of monocots.||Dicot stems exhibit secondary growth due to the presence of secondary vascular tissues and periderm formation.|
|Medullary rays||Medulla and medullary rays are absent in monocot stems.||Medulla and medullary rays are present in dicot stems.|
Examples of Monocot Stem
Palm tree stem
- The size, dimension, and appearance of palm stems or trunk vary among different species, but in general, the stem is cylindrical to slightly tapered and sometimes bulging in appearance.
- The surface of the stem can be smooth to rough and knobby with sharp spines. Some stems might have leaf scars indicating the points where the leaves were attached to the stem.
- The internal structure of the stem corresponds with the general internal structure of most monocots.
- The stems consist of a single apical meristem which is responsible for the formation of new cells and the growth of the plant.
- The oldest parts of the palm stem are strengthened by the deposition of lignin and cellulose on the cells by the xylem and phloem parenchyma.
- The xylem and phloem of the stem remain alive throughout the life of the plant which in some species can be hundreds of years.
- The tissue of the hypodermis and epidermis of the stem is present outside the central cylinder of the stem, which is collectively called pseudobark.
Examples of Dicot Stem
- The cactus stem is a modified dicot stem consisting of fleshy tissue for the storage of large quantities of water.
- The epidermis is composed of thin-walled cells arranged in close contact with one another. Outside the epidermis is the hydrophobic cuticle which also helps in the prevention of water loss.
- The epidermis does have small stomatal openings that are controlled by guard cells. Underneath the epidermis are the cortex, vascular bundles, and pith.
- The cortex is the most prominent region of the stem, which is composed of long-lived thin-walled parenchymal cells. Some of the cells of the cortex are specialized as mucilage cells and laticifers.
- The inner region of the cortex consists of chlorophyll-containing cells that can carry out photosynthesis. The content of chlorophyll progressively becomes lower in the inner layers of the region.
- The vascular bundles have more xylem bundles which are involved in the conduction of water throughout the plant body.
References and Sources
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- Hesse, Linnea et al. “Biomechanics and functional morphology of a climbing monocot.” AoB PLANTS vol. 8 plw005. 27 Jan. 2016, doi:10.1093/aobpla/plw005
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