four layers in thickness. These cells are capable of forming new cells during secondary growth . Xylem Xylem consists of xylem fibres, xylem parrenchyma vessels and tracheids. Vessels are thick walled and arranged in a few rows.
Xylem conducts water and minerals from the root to the other parts of the plant body. Pith or medulla The large central portion of the stem is called pith . It is composed of parenchyma cells Pith The central portion is occupied by a large pith. It consists of thin-walled parenchyma cells with intercellular spaces.
These cells are filled with abundant starch grains. Primary Structure of Dicot Stem (Sunflower stem) The transverse section of the dicot stem (Sunflower) shows the following plan of arrangement of tissues from the periphery to the centre. Epidermis It is protective in function and forms the outermost layer of the stem. It is a single layer of parenchymatous rectangular cells.
The cells are compactly arranged without intercellular spaces. The outer walls of epidermal cells have a layer called cuticle. The cuticle checks the transpiration. The cuticle is made up of waxy substance known as cutin.
Stomata may be present here and there. A large number of multicellular hairs occur on the epidermis. Cortex Cortex lies below the epidermis. The cortex is differentiated into three zones.
Below the epidermis, there are few layers of collenchyma cells. This zone is called hypodermis. It gives mechanical strength of the Stem. These cells are living and thickened at the corners.
Inner to the hypodermis, a few layers of chlorenchyma cells are present with conspicuous intercellular spaces. This region performs photosynthesis. Some resin ducts also occur here. The third zone is made up of parenchyma cells.
These cells store food materials. The innermost layer of the cortex is called endodermis . The cells of this layer are barrel shaped and arrange compactly without intercellular spaces. Since starch grains are abundant in these cells, this layer is also known a starch sheath .
This layer is morphologically homologous to the endodermis found in the root. In most of the dicot stems, endodermis with casparian