A stem is the main axis or stalk of a plant and it develops from the plumule of a germinating seed.
Internal structure of a stem in dicot and monocot plants:
The regions of dicot stem tissues in this stem include – epidermis, cortex and stele.
The outermost layer of the stem or epidermis consists of a layer of closely packed cells and is covered with a thin, waterproof layer called the cuticle.
The epidermis also contains stomata and hair-like outgrowths known as trichomes.
The next region of a dicot stem is the cortex. It lies between the epidermis and the pericycle and consists of multiple layers of cells. The main function of this region is storage of food material.
The cortex comprises three sub-zones, namely, the hypodermis, the cortical layers and the endodermis.
The hypodermis is the outermost zone of the cortex which consists of a few layers of collenchymatous cells.
These cells strengthen the young stem of a plant.
Below the hypodermis are cortical layers of round- and thin-walled parenchymatous cells with intercellular spaces.
The third or the innermost zone of the cortex is known as the endodermis. It is a single layer of tightly packed rectangular cells, which are rich in starch grains.Therefore, this layer is also known as the starch sheath.
Stele forms the last region of a dicot stem. This region consists of the pericyle, the vascular bundles and the pith. The pericycle is composed of sclerenchymatous cells and it lies on the inner side of the endodermis above the phloem cells. These cells act as strengthening material. The vascular bundles are located in a ring on the inside of the pericycle. This ring arrangement is a distinctive feature of the dicot stem.
There are layers of radially arranged parenchymatous cells in between the vascular bundles which constitute the medullary ray. Each vascular bundle in a dicot stem is conjoint, open and consists of endarch protoxylem.
The central region of the stem is filled with pith. It is composed of a large number of rounded parenchymatous cells with many intercellular spaces.
The tissues of dicot and monocot stems are quite similar. However, there are some differences as well.
In the monocot stem, the hypodermis is composed of sclerenchymatous cells, while in a dicot stem, it is composed of collenchymatous cells.
The monocot stem has scattered vascular bundles and their arrangement is conjoint and closed. Moreover, water containing-cavities are present within the vascular bundles. On the other hand, in a dicot stem, the vascular bundles are arranged in a ring as conjoint and open.
In a monocot stem, each vascular bundle is surrounded by a sclerenchymatous bundle sheath and parenchymatous ground tissue.
Moreover, the vascular bundles on the periphery are smaller compared to the vascular bundles in the centre.
Therefore, the essential differences between a monocot and dicot stem lies in the arrangement of the epidermis, ground tissue and vascular tissue.