Monocot vs. Dicot Stem: Structure, 22 Differences, Examples

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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;

1. Epidermis

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.

2. Cortex

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.

4. Pericycle

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.

7. Pith

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.