To study the typical anatomical structure of a Dicot stem, a Transverse Section (T.S) of a stem of Sunflower to be observed under a microscope. This article gives a detailed account of the various internal tissue arrangement of a dicot stem. The specific characteristic features by which we can identify the dicot stem from monocot stem were also discussed in the article.
Introduction
Anatomy of stem includes the internal organization of various tissues in a specific way. Epidermis, hypodermis, ground tissues (Parenchyma, Collenchyma, Sclerenchyma), vascular tissues (Xylem and Phloem) are tissues we can find in the anatomical structure of a typical dicot stem. To study the anatomical structure, a thin T.S. of a Sunflower stem (Helianthus annuus) or Coat buttons (Tridax procumbens) is taken and observed under the low power of Compound microscope. The following description reveals the specific arrangement of various tissues in general of any dicot stem.
Anatomical structure of typical dicot stem
A T.S. of a young dicot stem of Sunflower (Helianthus annuus) shows the following arrangement of tissues from the periphery towards the center.
Anatomy of Dicot Stem of Sunflower (Helianthus annuus)- Diagram by Author
Epidermis
It is the outermost single-layered structure present around the stem consisting of small flattened living cells that are closely packed. The cells often possess chloroplasts and are covered by a distinct cuticle. Here and there some stomata also can be seen on this layer. Some of the cells in the epidermal layer include multicellular stem hairs. These stem hairs prevent excessive transpiration from the stem.
Cortex
Next to epidermal layer, cortex tissue is present. In the dicot stem, cortex tissue can be divisible into three distinct regions. They are Hypodermis, General Cortex, and Endodermis.
Hypodermis- Just below the epidermal layer, a few layers of small radially compressed oval cells with their corner walls thickened can be seen together known as collenchyma. The cells of this tissue are living and also possess chloroplasts in them
General Cortex- Next to the Hypodermis, several layers of living Parenchyma cells are present as ground tissue. Cells of Parenchyma are large, living, round, thin-walled and are closely arranged but with some intercellular spaces in between them. Here and there oil ducts containing glandular cells can be seen
Endodermis- The last layer of the cortex is known as Endodermis. The cells in this layer are small, closely packed with no intercellular spaces between them. The cells of this layer include starch granules in them and hence this layer is commonly known as Starch sheath or Bundle sheath.
Stele
It is the innermost region to the cortex that consists of Pericycle, Vascular Bundles and Pith.
Pericycle- It is present next to the endodermis layer and composed a few layers of heterogeneous cells. Some of the cells are thin-walled (Parenchyma) and others are thick-walled (Sclerenchyma). The sclerenchymatous portions lie on the top of vascular bundles forming the bundle caps.
Vascular bundles- Next to the pericycle, the wedge-shaped vascular bundles are arranged in the form of a ring. Arrangement of vascular bundles in the form of a ring is an identifying characteristic feature of dicot stem. Each vascular bundle is conjoint, collateral, endarch and open. The vascular bundle described being conjoint if xylem and phloem are present together, collateral, where xylem and phloem are present on the same radius and open means cambium tissue, is present between xylem and phloem. Endarch vascular bundles mean Phloem tissue in the vascular bundle is present towards to periphery and xylem tissue is present towards the center. The phloem tissue located inside the bundle cap is composed of sieve tubes, companion cells, phloem parenchyma, and phloem fibers. A strip of cambium tissue present between xylem and phloem consisting of meristematic cells. These cells by their tangential divisions form secondary xylem and secondary phloem that helps in the growth of the stem. The xylem tissue located in the vascular bundle faces towards the center. Xylem tissue is composed of xylem tracheids, xylem vessels, xylem Parenchyma, and xylem fibers. The protoxylem (newly formed xylem vessels) is facing towards the pith and metaxylem (older xylem vessels) is facing towards cambium
Primary medullary rays- These are long narrow rays present between the vascular bundles and are made of thin-walled Parenchyma cells.
Pith or Medulla - It occupies the central region of the stem next vascular bundle. It is made up of thin-walled Parenchyma cells with large intercellular spaces. Usually, the pith region stores water and food material
Identifying anatomical characteristic features of Dicot stem
The following are the typical identifying features of dicot stem for a practical purpose:-
Multicellular hairs are present over epidermal layer in dicot stem and which cannot be found in monocot stem.
Hypodermis in dicot stem is made up of a few layers of collenchyma and it is sclerenchymatous in monocot stems.
Ground tissue of dicot is differentiated into the cortex, endodermis, pericycle, medullary rays, and pith while in monocot stems it is undifferentiated.
Vascular bundles are wedge-shaped and are arranged in the form of a ring. But in monocot stems the vascular bundles are oval and scattered in the ground tissue.
Vascular bundles are open in dicot stems but closed in monocot stems.
