Introduction to Anatomy of Dicot and Monocot Leaves

Leaves are extremely significant vegetative organs since they are largely concerned with transpiration and photosynthesis.  Such as stem and roots, leaves also comprise the three tissue systems - ground, dermal, and vascular. The dermal tissue system contains a lower epidermis and an upper epidermis. Stomata take place in both the epidermis but more often in the lower epidermis. The ground tissue system which takes place among the epidermal layers of leaf is termed as mesophyll tissue.  Frequently it is distinguished into spongy parenchyma on the abaxial that lower side and palisade parenchyma on the adaxial that is upper side.

A leaf displaying this separation in mesophyll is entitled as dorsiventral.    It is general in dicot leaves.    If mesophyll is not distinguished such as this in a leaf (that is made up of only spongy / palisade parenchyma) like in monocots, it is termed as isobilateral.  The mesophyll tissue, particularly spongy parenchyma cells enclose lots of air spaces. The existence of air spaces is a unique characteristic of spongy cells.   They make easy the gaseous exchange among the internal photosynthetic tissue (mesophyll) and the external atmosphere by the stomata.

The vascular tissue system is created of vascular bundles.  Vascular bundles are collateral and closed.  The vascular tissue creates the skeleton of the leaf and they are termed as veins. The veins supply minerals and water to the photosynthetic tissue. So the morphological and anatomical features of the leaf assist in the physiological functions of it.

Anatomy of a dicot leaf - Sunflower leaf

Inner structure of dicotyledonous leaves shows epidermis, mesophyll and vascular tissues.

Epidermis

A dicotyledonous leaf is usually dorsiventral.  It has upper epidermis and lower epidermis.  The epidermis is generally formed of a single layer of cells which are packed closely. The cuticle on the upper epidermis is thicker as compared to the lower epidermis.  The minute openings that found on the epidermis are termed as stomata.  Stomata are additional in number on the lower epidermis as compared to the on the upper epidermis.   A stoma is surrounded through a pair of bean shaped cells termed as guard cells.

Diagram: T.S. of sunflower leaf

Every stoma opens into an air chamber. These guard cells consist of chloroplasts, while other epidermal cells do not consist of chloroplasts. The major task of the epidermis is to provide protection to the inner tissue termed as mesophyll.  The cuticle assists to test transpiration.  Stomata are employed for transpiration and gas exchange.

Mesophyll

The entire tissue among the upper and lower epidermis is termed as the mesophyll (Gk meso=in the middle; phyllome=leaf). In the mesophyll there are 2 regions. They are spongy parenchyma and palisade parenchyma. Palisade parenchyma cells are observed below the upper epidermis. It contains vertically elongated cylindrical cells in one or more than one layers. These cells are efficiently arranged with no intercellular spaces. Palisade parenchyma cells comprise more chloroplasts as compared to the spongy parenchyma cells. The task of palisade parenchyma is photosynthesis. Spongy parenchyma takes place below the palisade parenchyma. Spongy cells are unevenly shaped. These cells are arranged very loosely along with several airspaces. As compared to palisade cells, the spongy cells comprise lesser no. of chloroplasts. Spongy cells make easy the exchange of gases along with the help of air spaces. The air space which is found next to the stoma is termed as respiratory cavity / sub-stomatal cavity.

Vascular tissues

These are exists in the veins of leaf.  Vascular bundles are collateral , conjoint, and closed.  Xylem is exists in the direction of the upper epidermis, when the phloem in the direction of the lower epidermis.        Vascular bundles are surrounded through a compact layer of parenchymatous cells termed as bundle sheath / border parenchyma.  Xylem contains protoxylem vessels and metaxylem vessels.  Protoxylem vessels are exists in the direction of the upper epidermis.  Phloem contains sieve tubes, phloem parenchyma and companion cells.  Phloem fibres are not present.  Xylem contains vessels and xylem parenchyma.  Tracheids and xylem fibres are not present.

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