The sub-aerial branches are very easy to so call because they are produced very close to the soil surface from herbaceous plants and act as a common method for vegetative propagation. It helps rapid proliferation of a plant forming a colony. It helps in effective vegetative propagation.
There are four types of sub-aerial stem modifications, which are as follows:
Runner
Structural Characteristics:
Marsilea
The stem is slender, prostrate, creeping in nature. It arises from the axils of the leaves, very close to the ground surface. The creeping habit of the stem is helped by the long inter-node joining the two nodes at regular intervals. The nodes produce the vegetative leaves on the upper surface and adventitious roots on the lower surface. The newly formed node remains joined by the runner to the parent plant and when divided, forms a new individual plant. The runners usually develop in all directions on the soil surrounding the parent plant and thus form a colony.The runner arises from the base of the stem as a lateral branch and runs along the surface of the soil. It develops distinct nodes and internode
Example:
Marsilea(pteridophyte) ; dub-grass.
Offset
Water hyacinth
Structural Characteristics:
The stem is shorter and thicker than the runner, but it is present along the surface of water, instead of soil. The nodes are close to each other because of shorter internodes. Here the tuft of adventitious roots and a cluster of leaves develop in each node. The leaves or petioles are with aerenchyma, helping in floatation.An offset is a short thick runner like branch which produces a new plant at its tip. The offsets grow in all directions from the main stem of the parent plant.
Example:
Water hyacinth, Pistia
Stolon
Mentha
Structural Characteristics:
It is another type of Sub Aerial Stem Modification. Like runners, they are also produced in terrestrial plants, but the branches are longer and slender in nature. Because of greater length, these branches may grow superficially up to a certain height above the soil surface and their tips touch the soil surface at the nodal region. Like the previous cases the nodes produce the adventitious roots on the lower surface and the vegetative leaves above. These structures are called stolon and the increase in their growth increases the arching.The stolons grow horizontally outwards for a varying distance in the soil. Ultimately their end (terminal bud) emerges out of the ground and develops into a new plant.
Example:
Mentha (pippermint plant).
Sucker
Structural Characteristics:
Chrysanthemum
This type of Sub Aerial Stem Modification are arising as a lateral branch from the parent plants and runs underground for a certain distance in the form of an underground runner.A lateral branch arising close to the ground level, traveling underground for some distance, turning up at its end and producing a new plant is a sucker. After growing parallel below the ground surface, it grows obliquely and comes out above the soil and produce a leafy shoot. The newly formed leafy shoot when separated from the parent plant, behave as a new independent plant.
Example:
Chrysanthemum.