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Theory:

dicot stem final.jpg
Dicot stem
 
Are you familiar with tree rings? The tree rings, also known as annular rings, are used to determine the tree's age. The cross-section of a woody dicot stem contains these rings. The patterns generated on the vascular bundles due to cambium on the dicot stem are known as annual rings. Due to the presence of cambium, these vascular bundles are open and can undergo secondary growth. In this topic, we are going to study the internal structure of a dicot stem.
 
dicotstem.png
Transverse section of dicot stem
 
The above picture is showing a sector of the transverse section of a dicot stem-sunflower.
 
The following structures are seen in the section. 
i. Epidermis:
It is the outermost layer present in the dicot stem. It is made up of parenchyma cells. Cuticle, a protective layer, covers the outer wall of this layer. The epidermal layer consists of multicellular hair structures called trichomes. It aids in the prevention of water evaporation due to transpiration.
ii. Cortex:
Inner to the epidermis is the region of the cortex. This region is divided into three layers.
  1. Hypodermis
  2. Middle cortex
  3. Inner cortex
1. Hypodermis:
 
 This region is made up of collenchyma cells. It consists of \(3\) - \(6\) layers. It gives mechanical support to the plant.
 
2. Middle cortex:
 
This layer is made up of a special type of parenchyma cells called chlorenchyma; due to the presence of chloroplasts. These cells are involved in photosynthesis.
 
3. Inner cortex:
 
This region is made up of few layers of parenchyma cells. These cells help to store food materials and also helps in gaseous exchange.
iii. Endodermis:
The innermost layer of the cortex is called as endodermis. This single layer is closely packed with barrel-shaped cells contains starch grains. So it is called a starch sheath.
iv. Stele:
The central part of the stem inner to endodermis is known as the stele. It consists of a pericycle, vascular bundle and pith.
 
a. Pericycle:
 
It is a multilayered region that is present between the vascular bundle and endodermis. It is made up of parenchymatous cells with alternating patches of sclerenchymatous cells. So it is called heterogeneous.
 
Bundle Cap:
 
It is a region where there is a patch of hard sclerenchyma tissue present outside the phloem of the vascular bundle.
 
b. Vascular bundle:
 
Vascular bundles are arranged in the form of a ring around the pith and are a unique characteristic feature of the dicot stem. Vascular bundles are conjoint, collateral, endarch and open.
 
c. Pith:
 
The large central portion of the stem surrounded by vascular bundles is called pith. This portion is made up of parenchymatous cells and helps in the storage of food materials.
 
Video explaining the internal structure of a dicot stem