Anatomy of Dicotyledonous and Monocotyledonous Plants

Dicotyledonous Root:

T.S.: Dicot root (Primary)

The above figure shows the transverse section of the sunflower root. The internal tissue organisation is as follows:

• Its outermost layer is epidermis. Many of the epidermal cells protrude in the form of unicellular root hairs.

• The cortex consists of several layers of thin-walled parenchyma cells with intercellular spaces and the innermost layer of the cortex is called endodermis.

• It comprises a single layer of barrel-shaped cells without any intercellular spaces. The tangential as well as radial walls of the endodermal cells have a deposition of water impermeable, waxy material-suber in-in the form of casparian strips.

• Next to endodermis lies a few layers of thick-walled parenchyomatous cells referred to as pericycle. 

• Initiation of lateral roots and vascular cambium during the secondary growth takes place in these cells. The pith is small or inconspicuous. The parenchymatous cells which lie between the xylem and the phloem are called conjuctive tissue.

• There are usually two to four xylem and phloem patches. Later, a cambium ring develops between the xylem and phloem. All tissues on the inner side of the endodermis such as pericycle, vascular bundles and pith constitute the stele.

Monocotyledonous Root:

T.S. : Monocot root

• The anatomy of the monocot root is similar to the dicot root in many respects. 

• It has epidermis, cortex, endodermis, pericycle, vascular bundles and pith.

• As compared to the dicot root which have fewer xylem bundles, there are usually more than six (polyarch) xylem bundles in the monocot root.

• Pith is large and well developed.

• Monocotyledonous roots do not undergo any secondary growth.

Dicotyledonous Stem:

T.S. of stem : Dicot

• The dicotyledonous stem has the epidermis is the outermost protective layer of the stem, covered with a thin layer of cuticle, it may bear trichomes and a few stomata. 

• The cells arranged in multiple layers between epidermis and pericycle constitutes the cortex.

• It consists of three sub-zones. The outer hypodermis, consists of a few layers of collenchymatous cells just below the epidermis, which provide mechanical strength to the young stem. Cortical layers below hypodermis consist of rounded thin walled parenchymatous cells with conspicuous intercellular spaces. 

• The innermost layer of the cortex is called the endodermis. The cells of the endodermis are rich in starch grains and the layer is also referred to as the starch sheath. Pericycle is present on the inner side of the endodermis and above the phloem in the form of semi-lunar patches of sclerenchyma.

• In between the vascular bundles there are a few layers of radially placed parenchymatous cells, which constitute medullary rays. A large number of vascular bundles are arranged in a ring which is a characteristic of dicot stem. Each vascular bundle is conjoint, open, and with endarch protoxylem. A large number of rounded, parenchymatous cells with large intercellular spaces occupying the central portion of the stem constitute the pith.

Monocotyledonous Stem:

T.S. of stem: Monocot

• The monocot stem has a sclerenchymatous hypodermis, a large number of scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath, and a large, conspicuous parenchymatous ground tissue.

• Vascular bundles are conjoint and closed.

• Peripheral vascular bundles are generally smaller than the centrally located ones.

• The phloem parenchyma is not available, and water-containing cavities are present within the vascular bundles.

Dorsiventral (Dicotyledonous) Leaf:

T.S. of leaf : Dicot

The dorsiventral leaf through the lamina shows three main parts, namely,

· Epidermis

· Mesophyll

· Vascular system

• Conspicuous cuticle are available in the epidermis which covers both the upper surface (adaxial epidermis) and lower surface (abaxial epidermis) of the leaf. The abaxial epidermis generally bears more stomata and adaxial epidermis, may even lack stomata. The tissue between the upper and the lower epidermis is called the mesophyll.

• Mesophyll, which has chloroplasts, carry out photosynthesis and is made up of parenchyma. It has two types of cells – the palisade parenchyma and the spongy parenchyma. 

• The adaxially placed palisade parenchyma is made up of elongated cells, which are arranged vertically and parallel to each other. The oval or round and loosely arranged spongy parenchyma is located below the palisade cells and spreads to the lower epidermis. Large spaces and air cavities are available between these cells. 

• Vascular system includes vascular bundles, as in the veins and the midrib and the sizes of the vascular bundles are dependent on the size of the veins. The veins vary in thickness in the reticulate venation of the dicot leaves. The vascular bundles are encircled by a layer of thick walled bundle sheath cells.

Isobilateral (Monocotyledonous) Leaf:

T.S. of leaf: Monocot

• In an isobilateral leaf, the stomata are on both the surfaces of the epidermis and the mesophyll is not distinguished into palisade and spongy parenchyma.

• In grasses, certain adaxial epidermal cells along the veins modify themselves into large, empty, colourless cells, called bulliform cells.

• When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface is exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss.

• The parallel venation in monocot leaves is reflected in the near similar sizes of vascular bundles (except in main veins).