Plant Anatomy

ANATOMY OF DICOTYLEDONOUS AND MONOCOTYLEDONOUS PLANTS

Dicotyledonous Root

Epidermis: The epidermis forms the outermost layer. Unicellular root hairs are present. The cortex is composed of many layers of thin-walled parenchyma; with intercellular spaces. The innermost layer of the cortex is called endodermis.


Endodermis: This is composed of a single layer of barrel-shaped cells. Intercellular space is absent. The tangential as well as radial walls of endodermis have a deposition of water-impermeable, waxy material; called suberin; in the form of casparian strips. A few layers of thick-walled parenchymatous cells lie next to the endodermis. This is called pericycle. The cells of the pericycle initiate lateral roots and vascular cambium during secondary growth. The pith is small and inconspicuous.

transverse section of dicotyledonous root

Vascular Bundle: There are usually two to four patches of xylem and phloem. A cambium ring develops between the xylem and phloem at a later stage. All tissues on the inner side of the endodermis constitute the stele.

Monocotyledonous Root

transverse section of moncotyledonous root

The anatomy of monocot root is similar to dicot root in many aspects. Epidermis, cortex, endodermis, pericycle, vascular bundles and pith are present. There are usually more than six xylem bundles in the monocot root. Pith is large and well developed. Secondary growth does not happen in monocot roots.


Dicotyledonous Stem

transverse section of dicotyledonous stem

Epidermis: The epidermis is covered with a thin layer of cuticle. Trichomes and stomata may be present.

Cortex: The cortex is made up of the multiple layers of cells between epidermis and pericycle. There are three sub-zones in the cortex. The outer sub-zone is called hypodermis. The hypodermis is composed of a few layers of collenchyma. The middle layer is composed of thin-walled parenchyma with distinct intercellular spaces. The innermost layer is called endodermis.

Endodermis: The cells are rich in starch grains and hence this layer is also called the starch sheath. Pericycle is present on the inner side of endodermis and above the phloem. The pericycle is in the form of semi-lunar patches of sclerenchyma.

Medullary Rays: Layers of radially placed parenchyma between the vascular bundles are called medullary rays.

Vascular Bundle: A large number of vascular bundles are arranged in a ring. It is important to remember that the ring-like arrangement of vascular bundles is the characteristic of dicot stem. Each vascular bundle is conjoint, open. Protoxylem is endarch. The pith is composed of rounded parenchymatous cells; with large intercellular spaces.

Monocotyledonous Stem

transverse section of moncotyledonous stem

The hypodermis in a monocot stem is made up of sclerenchyma. A large number of vascular bundles are scattered. Each vascular bundle is surrounded by a sclerenchymatous bundle sheath. Ground tissue is distinct and is made up of parenchyma. Vascular bundles are conjoint and closed. Peripheral vascular bundles are usually smaller than the centrally located vascular bundles. Phloem parenchyma is absent. Water-containing cavities are present within the vascular bundles.


Dorsiventral (Dicotyledonous) Leaf

transverse section of dicotyledonous leaf

There are three main parts in the leaf lamina of a dorsiventral leaf, viz. epidermis, mesophyll and vascular system.

Epidermis: The epidermis covers both the upper and lower surfaces. The upper epidermis is called adaxial epidermis, while the lower one is called abaxial epidermis. Cuticle is distinct. A higher number of stomata are present on the abaxial epidermis than on the adaxial epidermis. Stomata may be absent also in the adaxial epidermis.

Mesophyll: The tissue between the two epidermises is called mesophyll. The mesophyll is composed of parenchyma and contains chlorophyll. There are two types of cells in the mesophyll, viz. palisade parenchyma and spongy parenchyma. The palisade parenchyma is placed adaxially. It is made up of elongated cells; which are arranged vertically and parallel to each other. The spongy parenchyma is situated below the palisade parenchyma and extends to the lower epidermis. There are numerous large spaces and air cavities between the cells of spongy parenchyma.

Vascular Bundle: The vascular bundles can be seen in the veins and the midrib. Vascular bundles are surrounded by a layer of thick-walled bundle sheath cells. Vascular bundles are of different sizes because of reticulate venation.

Isobilateral (Monocotyledonous) Leaf

transverse section of monocotyledonous leaf

The anatomy of isobilateral leaf is similar to that of dorsiventral leaf in many aspects. Stomata are present on both the surfaces of an isobilateral leaf. The mesophyll is not differentiated into palisade and spongy parenchyma.

Certain adaxial epidermal cells; along the veins in grasses; are modified into large, empty, colourless cells. These are called bulliform cells. When the bulliform cells absorb water and become turgid, the leaf surface is exposed. When the bulliform cells become flaccid, the leaves curl inwards to minimize water loss.

Vascular bundles are of similar size, because of parallel venation. However, the vascular bundle of the main vein is somewhat bigger.



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