Modification of Stems

  Our Objective

To study modifications of stem 

  The Theory

The stem is the ascending part of the axis bearing branches, leaves, flowers and fruits. It develops from the plumule of the embryo of a germinating seed. The stem bears nodes and internodes. The region of the stem where leaves are born are called nodes while internodes are the portions between two nodes. 

The main function of the stem is spreading out branches bearing leaves, flowers and fruits. It conducts water, minerals and photosynthates. Some stems serve as a source of food storage, support, protection, and vegetative propagation. 

Let's study how stems are modified to serve different functions.

For storage of food 

  • The underground stems of potato, ginger, turmeric, Colocasia are modified to store food in them. 

For support    

  • Tendrils are the modifications of the stem that provide support to climbing plants. These stem tendrils which develop from axillary buds, are slender and spirally coiled and help plants to climb. 

    Examples: Passion fruit, Cucumber, Pumpkins, Watermelon etc.  

For protection 

  • Axillary buds of stems may also get modified into woody, straight and pointed thorns. They protect plants from browsing animals. These thorns are found in many plants.  

    Examples :  Citrus, Bougainvillea.  

 

For photosynthesis 

  • Some plants of arid regions, modify their stems into flattened or fleshy cylindrical structures. They contain chlorophyll and carry out photosynthesis.   

    Examples :  Opuntia and Euphorbia 

 

For vegetative propagation 

  • Subaerial Stem Modifications 

In these kinds of plants, the stem is present on the ground or partially underground. It grows horizontally under the soil and then comes above the ground and forms a new plant. They are used for faster propagation.  These stems are modified into: 

1. Runner – Runners are slender prostrate branches arising from axillary buds. It originates from the elongation of the basal internode and runs horizontally over the ground and gives rise to a new plant. 

Examples: Grasses, Oxalis etc. 

2.Stolon – A short and weak aerial branch arises from the main axis and after growing aerially for some time it bends downwards to touch the ground. A new plant develops from there. 

Examples: Mint, Jasmine, Strawberry 

 

3.Sucker – Sucker arises from the underground portion of the stem; it grows horizontally under the soil and then comes above the ground. It develops adventitious roots, shoots with leaves, and forms a new plant. 

Examples: Chrysanthemum, Banana, Pineapple. 

4.Offsets – They are found mostly in aquatic plants. Offset having a single long horizontal internode growing up to some distance producing a tuft of leaves above and a cluster of roots below at the apex 

Examples: Pistia, Eichornia. 

Learning outcomes

  1. Students will understand the concept of modifications in plan.
  2. Students will learn about different types of stems and their modified regions. 
  3. Students will understand the functions of the modified stem.