Monocot and Dicot
Internal Structure of Dicot Stem
Epidermis: it is the outermost layer of the stem. It is made up of single layer of compactly arranged parenchymatous cells. In T.S they look rectangular. They consist of opening of the stomata and multicellular root hair at various places. Its function is protection and gaseous exchange.
Hypodermis: it lies beneath the epidermis and consists of a few layer of collenchymatous tissue. Its cells are thickened at the tangential wall (sunflower) or at the angles (caster) by the deposition of the pectin. The cells are living and may contain chloroplast. It provides mechanical support and helps in the storage of food as it is a living tissue. Cortex: it consists of several layers of thin walled, oval or rounded angular parenchymatous cells. Its major function is the storage of food.
Endodermis: it is the single layer of barrel celled wavey structure that separates the cortex from the pericycle. They usually consist of the starch grains as reserve food. Thus endodermis is also known as starch sheath. Pericycle: it consists of few layers of cells. It lies between the endodermis and the vascular bundle. Pericycle is either sclerenchymatous completely or sclerenchymatous in opposite to the vascular bundle and parenchymatous opposite to the medullary rays. It provides mechanical support to the stem. Pith: It occupies the central position of the stem. It is made of thin walled parenchymatous cells which enclose small intercellular space. The pith cells stores food. In some, after maturity the pith cells disintegrate and a cavity is formed.
Secondary Growth in Dicot: The normal process of growth that occurs in every plant body is known as primary growth. It is the result of the activity of primary meristem. The process of primary growth results in the formation of primary permanent tissues such as primary xylem, primary phloem and primary cortex. However in the dicot plants, there is a process of growth that begins after a known period of primary growth. Such a growth is known as secondary growth. It is the result of the activity of secondary meristem. It results in the formation of secondary permanent tissues such as secondary xylem, secondary phloem and secondary cortex. As a result, secondary growth brings about an increase in the girth of the plant body. In a dicot stem, secondary growth occurs both in the stele and cortex. The process occurs simultaneously but is caused by separate strips of secondary meristem. In the stele, secondary growth is initiated by vascular cambium, while in the cortex, it is initiated by cork cambium.
Internal Structure of monocot stem
Epidermis: This is single outermost layer with thick cuticle on outer surface. In epidermis few stomata may be seen.
Hypodermis: (SCLERENCHYMA) This forms narrow zone of sclerenchyma two or three layers thick below the epidermis. Ground Tissue: It is continuous mass of thin walled parenchyma arising below sclerenchyam to the centre. It is not differentiated into cortex, endodermis, pericycle etc as in Dicot stem. The cells of ground tissue enclose many intercellular spaces.
VASCULAR BUNDLES: These are collateral and closed and lie scattered in the ground tissue. They are more numerous and lie close together near periphery than the centre. Peripheral ones are also seen to be smaller in size than central ones. Each vascular bundle is oval and surrounded by a sheath of sclerenchyma developed on two sides upper and lower. The bundle consists of (a) Xylem (b) Phloem, but cambium is absent.
Xylem: It consists of four distinct vessels arranged in the form of Y and a small number of tracheids arranged irregularly. Two smaller vessels (annular and spiral) lying radially towards the centre constitute protoxylem and two bigger vessels (pitted) laying laterally together with the small pitted tracheids lying in between them form metaxylem.
Phloem: It consists of sieve tubes and companion cells. No phloem parenchyma is present in Monocot stem. Outermost portion of phloem which is a broken mass is the protophloem and the inner portion is metaxylem. The former soon gets disorganized and the latter shows distinct sieve tubes and companion cells.