Anatomy of Stem: The stem is the ascending part of the plant which bears branches, leaves, flowers, and fruits. The stem generally moves in the direction of light (positively phototropic) and moves opposite to gravity (negatively geotropic). Generally, stems develop from the plumule of the developing embryo. The stem is generally green in colour in the initial young stages, but later, it becomes woody and dark brown. Anatomically, stems have epidermis, cortex, pericycle, vascular bundle, and pith. Stems have a conjoint vascular bundle with an endarch arrangement of xylem.

Stem

The stem is the ascending axis of the plant and develops from the plumule of the embryo. In flowering plants, the stem is differentiated into nodes and internodes. The lateral organs of the stem ( leaves and branches) are exogenous in origin. The young stems have trichomes (stem hairs) which is an epidermal outgrowth; it can be unicellular (e.g., Urtica, Cannabis, and Gossypium) or multicellular (e.g., Tridax and Withania) and cuticle covering to prevent water loss. 

Let us now understand the tissue organization of stems of dicot and monocot plants.

Dicotyledonous Stem 

The transverse section of a typical stem shows the following zones:

a. Epidermis: It is the outermost part of the plant, and a thin cuticle covers it. It may bear epidermal appendages in the form of trichomes and a few stomata (in young stems).
b. Cortex: Next to the epidermis is the multilayered cortex which extends to the pericycle. It has three subzones in a dicotyledonous stem:
i. Hypodermis: It is the outer part of the cortex which is present just below the epidermis. It is collenchymatous in nature with no intercellular spaces. Their cell walls are thickened at the corner due to the deposition of cellulose, hemicellulose, and pectin. The collenchymatous cells in the hypodermis provide mechanical strength to the young stem. 
ii. General Cortex: Below hypodermis and above endodermis, the general cortex is present, which is parenchymatous, thin-walled with intercellular spaces.
iii. Endodermis: It is the innermost layer of the cortex that stores the starch grains. Therefore it is also called the starch sheath.
c. Pericycle: It is present below the endodermis and above the phloem present in the form of patches. These patches are sclerenchymatous and are semilunar (half-moon) in shape.
d. Vascular Bundle: Dicot stems have a large number of vascular bundles. Each vascular bundle is conjoint, collateral, open, and wedge shape.
i. Xylem is located towards the inner side of each vascular bundle and is endarch, i.e., protoxylem lies towards the inner side (pith) and metaxylem towards the periphery.
ii. The cambium consists of meristematic cells. During secondary growth, meristematic cells add to the phloem towards the periphery and the xylem towards the centre.
iii. A characteristic ring arrangement of vascular bundles is seen in the dicot stems. The bundles are arranged in a ring-like manner around the central pith.
e. Pith: The central portion of the stem is made up of parenchymatous cells called pith. These cells are rounded in shape and have large intercellular spaces. In some higher plants, the pith is covered by thick-walled cells called a medullary sheath, which protects the younger pith cell.
f. Medullary Rays: The cells of pith have radially arranged parenchymatous cells called primary medullary rays or primary pith rays. It helps in the radial conduction of food. 

Monocotyledonous Stem

The transverse section of a monocotyledonous stem shows the following layers if we move from outer to inner side:

a. Epidermis: It is the outermost layer covered with a cuticle made up of a single layer of cells.
b. Hypodermis: It is present below the epidermis. It is constituted by the sclerenchymatous cells. Due to the presence of sclerenchyma tissue, hypodermis provides mechanical support to the stems.
c. Ground Tissue: The monocot stem is occupied by the parenchymatous ground tissue. Unlike dicot stems, the ground tissue in monocot stems is not well differentiated into different layers.
d. Vascular bundles: The vascular bundle in the monocot stem is scattered rather than present in a ring-like manner in the dicot stem. The vascular bundles are smaller in size and densely arranged towards the periphery. The larger vascular bundles are loosely arranged towards the centre of the stem.
i. Vascular bundles are oval in shape, having xylem and phloem without any cambium present in between. So, the vascular bundles are conjoint and closed. Hence, monocot stems cannot undergo secondary growth.
ii. Endarch xylem is present as the protoxylem lies towards the centre and the metaxylem lies towards the periphery.
iii. Phloem consists of sieve tubes, companion cells, and phloem fibres. Phloem parenchyma is absent in the monocot stem.

Comparison between Dicot and Monocot Stem Anatomy

Character	Dicot Stem	Monocot Stem
Epidermal Hairs	Epidermal hairs or trichomes are present	Epidermal hairs or trichomes are absent in monocot stem
Ground Tissue	It is differentiated into cortex and pith.	No differentiation of ground tissue.
Hypodermis	Collenchymatous in nature.	Sclerenchymatous in nature.
Endodermis	Single layered	Absent
Vascular Bundle	1. Uniform in size 2. The vascular bundle is a wedge shape 3. Arranged in a ring-like manner 4. Conjoint and open 5. Phloem parenchyma present	1. Large towards the center and smaller towards the periphery 2. The vascular bundle is oval in shape 3. Scattered 4. Conjoint and closed 5. Phloem parenchyma absent
Pith	Made of parenchymatous cell	Absent
Pith ray	Present between vascular bundles	Absent
Stele	Eustele	Atactostele
Secondary Growth	Present due to presence of vascular bundle and periderm	Absent
Medullary Rays	Present	Absent

Summary

Stems are the aerial part of the part; they always move in the direction of light and move in the opposite direction of gravity. The stem bears leaves, flowers, and fruits. Stems generally have a single-layered epidermis with the presence of trichomes in the dicot stem only. Cortex can be differentiated in dicot stem or non differentiated in monocot stem. The hypodermis is collenchymatous in the dicot stem and sclerenchymatous in the monocot stem. Endodermis and pericycle are well developed in the dicot stem, whereas it is absent in the monocot stem. The vascular bundle is conjoint with the xylem and phloem present together. Cambium rings can be present in the dicot stem showing secondary growth, whereas, in the monocot stem, the cambium is absent, showing no secondary growth.

Frequently Asked Questions (FAQs) on Anatomy of Stem

Q.1. What kind of vascular bundle is present in the dicot stem?
Ans: The stem has a conjoint, closed vascular bundle with xylem and phloem present together with a cambium strip present in between them.

Q.2. What are the three types of the stem?
Ans: The three types of stem are underground stem, aerial stem, and subaerial stem.

Q.3. What is a trichome?
Ans: Trichomes are also known as stem hairs. It is a unicellular epidermal appendage that is covered with a cuticle.

Q.4. What is the function of medullary rays?
Ans: Medullary rays or Pith rays help in the radial conduction of food.

Q.5. Does monocot stem has phloem parenchyma?
Ans: Phloem parenchyma is absent in the monocot stem.

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