List out two sub-aerial stem modifications with example.

Explanation

Sub aerial stem found in plants with weak stem in which branches lie horizontally on the ground. These are meant for vegetative propagation. They may be sub aerial or partially sub terranean.
Runner: This is a slender, prostrate branch creeping on the ground and rooting at the nodes. eg. Centella (Indian pennywort), Oxalis (wood sorrel), lawn grass (Cynodon dactylon).
Stolon: This is also a slender, lateral branch originating from the base of the stem. But it first grows obliquely above the ground, produces a loop and bends down towards the ground. When touches the ground it produces roots and becomes an independent plantlet. eg. Mentha piperita (peppermint), Fragaria indica (wild strawberry).
Sucker: Sucker develops from a underground stem and grows obliquely upwards and gives rise to a separate plantlet or new plant. eg. Chrysanthemum, Musa, Bambusa
. Offset: Offset is similar to runner but found in aquatic plants especially in rosette leaved forms. A short thick lateral branch arises from the lower axil and grows horizontally leafless for a short distance, then it produces a bunch of rosette leaves and root at nodes. eg. Eichhornia (water hyacinth), Pistia (water lettuce)