Anatomy of Dicotyledonous and Monocotyledonous Plants

Dicotyledonous (dicots) and monocotyledonous (monocots) plants show anatomical differences. Dicots have two cotyledons, reticulate venation, and a taproot system with vascular bundles arranged in a ring and monocots have one cotyledon, parallel venation, and a fibrous root system with scattered vascular bundles. The tissue organisation of roots, stems and leaves are discussed in detail below.

Dicotyledonous Root

The internal tissue organisation of dicotyledonous root is as follows:

• The outermost layer of the root is known as epiblema. The cells of epiblema protrude in the form of unicellular root hairs.
• The cortex consists of several layers of thin-walled parenchyma cells with intercellular spaces.
• Innermost layer of the cortex is called endodermis which characterized by barrel-shaped cells with casparian strips made of water-impermeable suberin. It lacks intercellular spaces.
• Pericycle is present next to the endodermis which consists of a few layers of thick-walled parenchymatous cells. It act as site for initiation of lateral roots and vascular cambium during secondary growth.
• Pith is small or inconspicuous.
• The parenchymatous cells which lie between the xylem and the phloem are called conjuctive tissue.
• Usually there are two to four xylem and phloem patches. and cambium ring develops between xylem and phloem during growth.
• Tissues on the inner side of the endodermis, including pericycle, vascular bundles, and pith, constitute the stele.

Monocotyledonous Root

The internal tissue organisation of monocotyledonous root is as follows:

• The anatomy of the monocot root is similar to the dicot root, both have epidermis, cortex, endodermis, pericycle, vascular bundles and pith.
• In monocot root there are more than six (polyarch) xylem bundles as compared to dicot root which have fewer xylem bundles.
• Pith is large and well developed.
• Monocotyledonous roots do not undergo secondary growth.

Dicotyledonous Stem

The internal tissue organisation of dicotyledonous stem is as follows:

• The epidermis forms the protective outer layer of the stem and is covered with a thin layer of cuticle.
• It consists of trichomes (hair-like structures) and a few stomata for gas exchange.
• Multiple layers between epidermis and pericycle constitute the cortex. It consists of three sub-zones:
  • Outer hypodermis: Consists of few layers of collenchymatous cells just below the epidermis and provides mechanical strength to the young stem.
  • Middle parenchymatous cells: It consists of round thin-walled parenchymatous cells with small intercellular spaces.
  • Endodermis (Starch Sheath): It is the innermost layer of the cortex and cells are rich in starch grains, also called as starch sheath.
• Pericycle is present on the inner side of endodermis and above the phloem in the form of semi-lunar patches of sclerenchyma.
• Medullary Ray is present between the vascular bundles which consists of radially placed parenchymatous cells.
• Vascular bundles arranged in a “ring”, it is the main characteristics of dicot stem. Vascular bundles are conjoint, open, and have endarch protoxylem.
• Pith forms the central portion of the stem and consists of large number of rounded, parenchymatous cells with large intercellular spaces. It provides structural support and storage.

Monocotyledonous Stem

The internal tissue organisation of monocotyledonous stem is as follows:

• Monocot consists of sclerenchymatous hypodermis below the epidermis for mechanical strength.
• There are numerous vascular bundle, scattered throughout the stem. Each vascular bundle is surrounded by a sclerenchymatous bundle sheath.
• Vascular bundles are conjoint and closed. Peripheral vascular bundles are generally smaller than the centrally located ones.
• Phloem parenchyma is absent in the vascular bundles.
• Water-containing cavities are present within the vascular bundles.

Dorsiventral (Dicotyledonous) Leaf

• Epidermis covers both upper (adaxial) and lower (abaxial) surfaces of the leaf and have a small cuticle. Abaxial epidermis has more stomata than the adaxial epidermis.
• Mesophyll is present between the upper and the lower epidermis. It consists of chloroplast-containing parenchyma cells. It contain two type of cells:
  • Palisade parenchyma (adaxially placed): It consists of elongated cells, which are arranged vertically and parallel to each other.
  • Spongy parenchyma ( placed below palisade cells): It consist of oval or round loosely arranged cells that extends to the lower epidermis. Between these cells there are large spaces and air cavities.
• Vascular system consists of vascular bundles which are surrounded by a layer of thick walled bundle sheath cells. It is easily visible in the veins and the midrib of the leaves. The size of the vascular bundles are dependent on the size of the veins.
• The veins shows varying thickness in the reticulate venation of the dicot leaves.

Isobilateral (Monocotyledonous) Leaf

Most of the anatomy of isobilateral leaf is similar to that of the dorsiventral leaf. it has following different features:

• The stomata are present on both the surfaces of the epidermis.
• The mesophyll is not differentiated into palisade and spongy parenchyma.
• Monocot leaves exhibit parallel venation.
• Bulliform Cells in Grasses: Certain adaxial epidermal cells along veins modify into large, empty, colorless cells known as bulliform cells. When turgid (filled with water), bulliform cells expose the leaf surface; when flaccid (due to water stress), they cause leaves to curl inward, minimizing water loss.

This adaptation with bulliform cells and parallel venation is a strategy in grasses to respond to water availability and minimize water loss, important for their survival in various environmental conditions.

Anatomy of flowering plants includes stems for structural support and transport of water and nutrients, roots for anchorage and nutrient absorption, leaves for photosynthesis, and reproductive structures such as flowers containing essential organs like stamens and pistils for sexual reproduction. The plant tissues are broadly classified into meristematic (apical, lateral, and intercalary) and permanent (simple and complex). The main functions of tissues include the assimilation of food and its storage, transportation of water, minerals, and photosynthates, and mechanical support to the plant.