One of the most important foods of green plants is CO2 which they absorb from the air. That, of course, is not all. Every plant must have water as well as minerals. Now, a bare wall is not as bare as it seems to be. In the first place, rain falls upon it, and so the plant gets its water. The water soaked by the wall provides moisture during the period when there is no rain. In some cases, there could be situation where a leaking water pipe keeps the wall wet throughout the year. This is not all. Rain and leaking water itself contain minute quantities of salts which are valuable to the Peepal plant. Besides, water leaches out mineral salts that may be contained in the wall’s mortar. Lime, for instance, is a valuable nutrient, and mortar has plenty of lime. Nothing, of course, is better than a leaking drainage pipe which can supply all essential nutrients for the growth of the Peepal tree. In most cases, however, supplies are very scant, and the Peepal which can grow in this fashion is more or less humble, never reaching a large size.
Sacred fig (Wikipedia)