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Internal Structure of Dicot and Monocot Plants

After learning about the major types of tissues, their main functions and their location in plants, let us now consider the organisation of various tissues in various structures such as stem, leaf and root, of flowering plants. The study of anatomical details helps in understanding the various physiological functions of organ systems. Besides, an understanding of plant anatomy helps in identifying adulterations in spices, coffee, tea, vegetable dyes, tobacco, saffron, Asafoetida, plant drugs and woods used for construction, furniture, ship-building etc. Pharmacognosy is the science that deals with the sources, characteristics and possible uses of medicinal substances in their natural or unprepared state. This, again, heavily depends on a detailed knowledge of plant anatomy and physiology. Studies on the anatomical details of plants and their functional significance are becoming increasingly relevant in forensic medicine.

Primary Structure of Dicot Stem

A typical dicot stem shows the following histological details: The outer layer of cells form a skin or the epidermis; the inner cells make up the cortex and pith. Between the cortex and pith are a number of vascular bundles containing specialised cells which carry food and water.

 T.S. of a Dicot Stem


The outer epidermis which is composed of a single layer of closely arranged cells is effective in holding the inner cells in shape, preventing loss of water, affording protection from damage and preventing entry of fungi, bacteria and dust. This layer is relatively impermeable to liquids and gases. Oxygen can enter and carbon dioxide can escape only through stomata in young stems and through lenticels in older stems. The lenticels are small gaps in the bark, usually circular or oval and slightly raised on the bark surface. In lenticels the cells of the bark fit loosely, leaving air gaps which communicate with the air spaces in the cortex.

Hypodermis is the outer part of the cortex. It is formed by three zones of different tissues.

The first zone consists of unevenly thickened collenchymatous cells.

The second zone is the presence of parenchymatous cell with intercellular spaces with resin ducts, oil cavities etc.

Endodermis is the innermost layer of cortex and is made up of a single row of barrel shaped cells compactly arranged without intercellular spaces. The endodermis is parenchymatous and their walls are thickened by suberin. It was first recognised by Casparay and therefore is known as Casparian band/strip. In roots the endodermal cells do not possess thickening and hence are called as passage cells. They help in the entry of water from the cortex to the xylem.


  1. Endodermis acts a barrier between cortex and stele.
  2. In dicot stems it stores starch.


  1. Pericycle
    It is the outermost layer of the stele and is found beneath the endodermis. In roots it is composed of thin walled rectangular parenchymatous cells. In dicot stems it is thick walled and sclerenchymatous.

Vascular Bundles

Sometimes these are called veins and these are made up of vessels and sieve tubes with fibrous and packing tissue between and around them. Vascular bundles are arranged like a ring. Vessels consist of long tubes formed from columns of cells whose walls have become impregnated with a woody substance and whose protoplasm has died. In these vessels water is carried from the roots, through the stem and to the veins in the leaves. Sieve tubes consist of columns of living cells the horizontal walls of which are perforated. These perforations allow dissolved substances to flow from one cell to the other, carrying food made in the leaves to other parts of the plant.

Fig: Magnified View of the T.S. of a Vascular Bundle of a Dicot Stem

Vessels and sieve tubes are surrounded by cells that space them out and support them. The tissue consisting of vessels and the long fibre-like cells among them is called xylem. The sieve tubes and their packing are called phloem.

Cambium is a layer of narrow, thin-walled cells between the xylem and phloem. Once cells differentiate after formation from meristematic tissues they are no longer capable of dividing to make new cells. The cells in the cambium, however, do not lose their ability and make new cells.

Secondary Growth

Cambium is, at first, restricted to the vascular bundles. Later it forms a continuous cylinder within the stem between the cortex and pits. Its cells divide in such a way as to make new xylem cells on the inside and new phloem cells on the outside. In dicotyledonous trees and shrubs, this continues throughout their lifetime. The continuous division of cambium cells adds new cells to the stem and it increases in thickness (secondary thickening). In such woody stems the epidermis is often replaced by a dead corky layer, bark, which itself is made by a separate layer of cork cambium, just beneath the epidermis. The phloem becomes a thin layer of living cells between the bark and the woody core of xylem.

Primary Structure of Monocot Stem

The internal structure of monocot stems differs in some respects from that of dicots. The most important feature of monocot stem is the occurrence of scattered vascular bundles and absence of distinction between cortex and pith. A transverse section of a monocot stem will show only two distinct regions namely epidermis and ground tissue (Fig below).

Fig: T. S. of a Monocot Stem

Epidermis is the outermost one-cell thick layer of the stem. The outer walls of the cells are generally covered with a thick cuticle. A few epidermal cells get modified to form stomatal apparatus. A few layers of cells internal to the epidermis constitute the hypodermis. The hypodermal cells are generally thick-walled, lignified and sclerenchymatous. The hypodermis gives mechanical strength to the stem.

Ground tissue is the entire tissue internal to hypodermis. Majority of ground tissue is parenchymatous with cells rich in food reserves such as starch. The cells immediately beneath the hypodermis are smaller, polygonal and compactly arranged. In the interior, the cells are larger, polygonal and loosely arranged. The vascular bundles are scattered and embedded in the ground tissue. Each vascular bundle is surrounded by a two or three layered sheath of sclerenchymatous fibres known as the bundle sheath. Since the vascular bundles lack cambium, they are called closed. The xylem and phloem in each bundle lie on the same radius and protoxylem elements are towards the center and metaxylem towards the center and metaxylem towards the periphery. Such vascular bundles are described as conjoint, collateral and endarch. The phloem is exclusively composed of sieve tubes and companion cells and there are no fibres and parenchyma. The major differences between dicot and monocot stems are detailed in Table.


Table: Comparison of the structures in dicot and monocot stems 


Dicot stem (Helianthus)

Monocot stem (Zea)


a. Usually single layer of rectangular cells without chlorophyll.

b. Protective in nature.

c. Some of the cells are extended into multicellular hairs.

d. Externally the epidermis is covered with the cuticle.

Same as in dicots







a. A vast tissue situated between the epidermis and vascular tissue.

b. Differentiated into two regions: outer hypodermis and inner general cortex which contain many layer of thin walled, parenchymatous cells with prominent intercellular spaces.

a. It is referred to as ground tissue in which are scattered vascular bundles.

b. Sometimes hypodermis is present below the epidermis, otherwise the ground tissue is undifferentiated.


a. Innermost layer of the cortex surrounding the vascular tissue.

b. Its cells contain starch and do not possess thickenings (Casparian bands).







a. In the form of sclerenchyma patches intermixed with some parenchyma, situated between the endodermis and the vascular bundle ring.




a. A ring of Vascular bundles inner to pericycle.

b. Xylem and phloem present on the same radii xylem inner and phloem on the outer side. Conjoint collateral bundles.

c. Xylem with smaller vessels (protoxylem towards the centre and with larger cavities (metaxylem) towards the periphery-endarch condition.

d. Two or three layers of rectangular meristematic cells present between the xylem and phloem (Cambium).

a. Vascular bundles scattered in the ground tissues.

b. Same as in dicot stems.

c. Xylem occurs in the form of the letter Y metaxylem occupying the arms and protoxylem at the base. The phloem is located between the arms of the xylem.

d. Each vascular bundle is surrounded by a sclerenchymatous sheath, called the bundle sheath. Cambium is absent.




a. Central tissue

b. Consisting of parenchymatous cells larger than cortical cells having intercellular spaces.





These are the columns of cells, between the vascular bundles, thin walled, radiating from the pith to the cortex.




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