Question

define aerial roots and describe their function

Answer 1

http://en.wikipedia.org/wiki/Aerial_root
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Answer 2

These are modifications of Tap root for the purpose of breathing. In this type of modification, some of the branches of tap root grow vertically upward and comes on the surface of the soil. These are generally found in marshy areas where scarcity of oxygen is found. Rhizophora, Mangrove plants etc. are some of the examples

Answer 3

Aerial roots are also found in Epiphytic plants.
epiphytes are the plants that grow on other plants for basic need like shelter, food etc.
Epiphytic habit is also called as commensalism where only the epiphytes get benefitted and they do not harm nor benefit the host on which they live.

Epiphytes produce aerial roots that can absorb water from the moisture and they also have tap root that is used to penetrate into the host tissues without harming them.

The best example of epiphyte is VANDA - orchid.