Stem vs Root
In the primary structure of the dicot stem, the outermost layer is the epidermis. This is normally a single layer. Above the epidermis is the cuticle. Multicellular epidermal hairs and stomata are present in the epidermis. Below the epidermis is the cortex. It is relatively narrow and is differentiated into collenchymas, chlorenchyma and parenchyma. Endodermis is less marked due to the absence of casparian strips.
The pericycle may be a single layer of parenchyma cells or many layered sclerenchymatous tissue. When there is sclerenchyma the pericycle, which is also called phloem fibres, may not be continuous. These sclerenchymatous cells appear as caps above the vascular bundles. There are a large number of vascular bundles regularly arranged in the form of a ring around the pith. They are collateral and open. The xylem is endarch. Prominent pith is present and which is relatively large compared to the cortex.
In the primary structure of the dicot root, the outermost layer is the epidermis. It is a layer of living cells with unicellular root hairs. There is no cuticle, no stomata and no chloroplast. Inner to this is the cortex which is relatively broad and not differentiated. Several layers of parenchyma are present. The last layer of the cortex is the endodermis. It is a single layer of closely packed living cells with casparian strips. Casparian strip is formed by the deposition of suberin and is found encircling the cell. The walls facing the epidermis and the xylem do not have the casparian strip. Sometimes, when the root grows old, there is extra suberin deposited completely blocking the walls. In these areas, there are passage cells infront of protoxylem cells that allow passage of water. Inner to the endodermis is the pericycle. It is a single layer of parenchyma cells. Pericycle is important in forming lateral roots and also in the formation of vascular and cork cambium. Inner to the pericycle is vascular bundles. In dicot roots, there are less vascular bundles. They are radial bundles in the primary structure. There is no cambium, and the xylem is exarch; that is, the protoxylem grows facing outwards. Innermost is the pith which is very much reduced or absent.
What is the difference between Stem and Root?
• Unicellular root hairs are present in the primary dicot root, whereas root hairs are absent in the primary dicot stem.
• No cuticle or stomata can be found in the primary dicot root, whereas cuticle and stomata are present in the primary dicot stem.
• In the primary dicot root, the cortex is not differentiated and is made only of parenchyma but, in the primary dicot stem, the cortex is differentiated into collenchymas, chlorenchyma and parenchyma.
• Cortex is relatively broad in the primary dicot root, and the cortex is relatively narrow in the primary dicot stem.
• In the primary dicot root, the endodermis is well marked with casparian strips, whereas, in the primary dicot stem, endodermis is less marked without casparian strips.
• Pith is absent or reduced in the primary dicot root, whereas a large pith is present in the primary dicot stem.