Structures of Mustard (Brassica Campestris) Stem

Give an illustrated account of the external and internal structures of mustard (Brassica Campestris) stem

EXTERNAL STRUCTURE OF STEM:

 A stem develops from the plumule of a seed away from the soil upwards. The stem is hemoaceous and branched. It bears leaves and flowers. The point on the stem or a branch from where leaf arises is called a node. The part of the stem between two adjacent nodes is called internode. The stem provides support to the leaves in such a way that they may receive maximum sunlight tor preparing me food during photosynthesis. The stems bear flowers which are the reproductive organs of plants and take part in reproduction.

Internal Structure of Mustard Stem: 

A transverse section of a mustard stem under a microscope shows the following parts.

(1) Epidermis: It is the outermost single layer of stems. There are no intercellular spaces in the epidermal cells. The epidermis of stem has a thick and waxy cuticle which is a special chemical substance called cutin. Stomata and lenticel pores are present. 

Function: 

• It prevents water loss by waterproofing stem. 
• It protects the inner tissues. 
• Exchange of gases and water vapour take place during transpiration through the epidermis.

(2) Cortex: Inside the epidermis lies the cortex which is packed with parenchyma. Sometimes chloroplasts are present in the outer region forming the collenchyma tissues. 

Function: 

• Transport water. salts and gases across the stem. 
• Photosynthesis takes place in green stems. 
• Sometimes they store food as in rhizomes.

(3) Vascular Bundles: (several in a ring or scattered). Vascular bundles are present in a ring. Each vascular bundle consists of an outside phloem of sieve tubes and an inner xylem of vessels. A few layers of cells called cambium is present between the xylem and phloem.

Function: 

• Translocation of organic food materials up and down from the leaves to all parts of the plant. 
• Transport of water and salts upwards from the roots to leaves. 
• Formation of new phloem and xylem tissues. 

(4) Endodermis: The innermost layer of the cortex is called the endodermis. It is not prominent in stem. 

Function: 

• This layer controls the transport of water from xylem to cortex. 

(5) Pith: The central part of the stem is called pith. It is made up of living, rounded, thin walled parenchyma cells. 

Function: 

• It stores food. 
• It gives limited support.