In some plants the aerial stem is modified to perform a variety of special functions. The aerial stem modifications are as fallows:
1) Stem Tendril: Tendrils develops as a modification of the stem in certain plants. The terminal bud gives rise to a tendril in Cissus quandrangularis and the axillary bud becomes modified into a tendril in Passiflora.
2)Stem Thorn: The stem thorn is a hard, straight and pointed structure. In Duranta, the axillary bud is modified into a thorn. In Carissa the terminal bud is modified into a pair of thorns. The thorn sometimes bears leaves, flowers and fruits as seen in Duranta and Pomogranate. The thorns not only check the rate of respiration but also protect the plants from herbivores.
Thorns are small, modified stems, spines are modified superficial leaves and prickles are sharp, pointed structure without vascular tissue, arising on stem, petioles and inflorescence.
3) Phylloclade: A phylloclade is a flattened stem of several internodes, function in as a leaf. In opuntia the stem is modified into a green flattened structure called Phylloclade. On the surface of the phylloclade clusters of spines are formed. These spines are the modified leaves of the axillary bud. These spines not only check the rate of transpiration but also protect the plant from herbivorous grazing animals. The phylloclade has distinct nodes and internodes. Example: Opuntia
In Muchlenbeckia the stem is a thin, green, flat structure made up of nodes and internodes. Initially small normal leaves are formed but later only scale leaves arise.
4) Cladodes and Cladophylls: A phylloclade of one internode is called as a cladode. There are no suitable examples of cladodes because, Ruscus and Asparagus, which are often considered as cladodes, are in reality, cladophylls. A cladophyll is a flattened leaf like stem arising in the axils of a minute, bract-like, true leaf.
5) Bulbil: Bulbil is a special multicellular body essentially meant for reproduction. In Agave the floral buds are modified into bulbils. These bulbils get detached, come in contact with the soil and develop into new plants. Example-Dioscorea, Agave.
In Dioscorea the axillary bud develops into a bulbil. This bulbil detaches from the mother plant and grows up into a new independent one.