The aerial stem modifications causes massive modification of the stem and they are difficult to be recognized as stems. For this reason they are called metamorphosed stems. In some plants the aerial stem is modified to perform a variety of special functions. The aerial stem modifications are as follows and usually play a definite function:
The thorns is a hard, straight, and pointed structure.The axillary branches stop growing and get modified to straight, hard, pointed structures in such aerial stem modifications. They may bear small leaves; flowers etc. and they have vascular traces.
They act as defensive organs and protect the plant from grazing animals.
Example: Carissa, Duranta, Citrus.
A phylloclade is one of the aerial stem modifications that flattened stem of several internodes functioning as a leaf. Here the stem becomes flattened, fleshy, leaf-like structure. They can store water for future use. The nodes and internodes are not properly differentiated and the nodes are marked with the appearance of flowers and spines.
They help in the preparation of food by photosynthesis. They store food and water and the spines prevent loss of water by transpiration and also protect the plant from grazing animals.
A phylloclade of one or two internodes is called as a cladode. The cladode is a phylloclade with internodes. That is, the internodes in between two distinct nodes become leaf-like. It is photosynthetic, but not as fleshy as phylloclade and is devoid of spine.
The function of this type of aerial stem modifications is photosynthesis and storage of water.
Example: Asparagus (B. Satamuli).
In aerial stem modifications, in which single internodes becomes swollen, fleshy and tuberous in nature is called Pseudo-bulbs. These look like bulbs, but do not have uniformly arranged concentric fleshy leaves. There may be adventitious aerial roots arising from the lower surface.
They store moisture and help the plant to survive during unfavorable condition, i.e., when there is severe shortage of water.
They provide mechanical support to the soft stem of the plant, acting as a climbing organ.
Vine, Passion-flower, Antigonon.
Bulbil is a special multi-cellular body as aerial stem modifications, essentially meant for reproduction. They are specialized, aerial modified axillary buds, both vegetative and floral in nature. They look like bulbs i.e., having a disc-like stem covered by leaves. They are viviparous in nature i.e., give rise to daughter plant, while remaining attached to the stem of the parent plant.
They help in initiating vegetative propagation and also act as storage organs.