Question

whydo lichens not occur in Delhi where as the commonly grow in hills

Answer 1

This is because lichens grow in areas where sulphur dioxide pollution is very less, e.g., in Manali and Darjeeling. Delhi is highly polluted place where lichens cannot grow and survive.

Answer 2

Lichens need moist and pollution free environment for their growth.

Moreover they are highly sensitive to Sulphur dioxide.

Delhi has lakhs of automobiles which a meeting huge amount of Sulphur dioxide in atmosphere.

In such a highly polluted environment legends cannot grow while in Manali and Darjeeling the environment remains moist throughout the year and is relatively free from Sulphur Dioxide only two lesson number of automobiles.

This provides favourable conditions for the growth of Lichens.