Theory:

Plants to overcome environmental hardships and facilitate growth and reproduction brings out some modifications in their stem. Such modifications are termed as modifications of stems.
There are three major categories of modifications happen in the stem, which include:
  • Aerial 
  • Subaerial
  • Underground
Aerial
  
To overcome evaporation from the leaves, plants in desert or in water scarce areas tend to develop green stems which does the photosysnthesis for the plant, while the leaves become thorns. Such stem modifications are called phylloclade or cladopyll.
 
Subaerial
 
Subaerial here means 'on the ground'.
Plant stems for the sake of stability, growth and reproduction, sometimes tend to grow side wise.
As in the case of Centella (or) vallarai, the stems grow laterally (side wise) and wherever it touches the ground they also develop roots to produce new plants. Such stems are called runners.
 
There is another type of stem modified plants called stolon, in which the tender stem of the plant initially grow upwards to some extent and then start bending to touch the ground. Wherever, it touches the ground it develop roots. For example wild strawberry belongs to this category.
 
Suckers are special type of stems which develop laterally from the main shoot on any direction. As it grows, it develops roots and wherever it develops root that produce new shoots. For example chrysanthemum reproduce by developing suckers.
 
In the case of plants like Eichornia, offshoots develop from the auxiliary buds on the trunk, upon growth these offsets separate from the mother plant and start growing individually developing roots.
 
Underground stem modifications
  
Modification of stems that happen below the ground is called underground modifications. There are four types:
  1. Rhizome
  2. Corm
  3. Tuber
  4. Bulb
The table below shows the different types these underground stems with their characteristics:
 
TypeShapeCharacteristicsExample
RhizomeIrregular Grow horizontally beneath the ground. Have scaly leaves and buds. Buds will develop into a new plant.Ginger and turmeric
Corm RoundHave scaly leaves and buds. Buds can become new plant.Colocasia
TuberSphericalDevelop buds. Buds can develop into a new plant.Potato
BulbDisc-like Upper part of the stem has terminal bud covered with many scaly leaves.Garlic and onion