How can a plant grow on a bare wall?
One of the most important parts of the food of green plants is carbon, and the plant finds it — or rather carbon dioxide, which contains carbon — in the air. But that is not all. The plant must also have water and salts.
When we begin to look at a bare wall closely, we find that it is not as bare as it seems.
First, we may say that rain falls on it, and so the plant on the wall gets its water. Rain also contains small quantities of salts. As the rain goes down the wall, it gathers more salts.
So we can see that plants can get, even on a bare wall, those things which are necessary for their life. But, of course, a plant which grows on a bare wall cannot get much food. So a plant on a bare wall can never reach a large size.
Why does a tree stop growing?
We may ask the same question about men, or horses, or birds.
The growth of living things is not like the growth of a snowball. A snowball grows as long as it gets more and more snow. There is nothing inside it that limits its growth.
But every living thing has something inside that limits its growth. We may say that every living thing grows to the size which suits it. After that, it stops growing and lives as long as it can.
Did you know that a growing plant can push very hard against anything that gets in its way? If soft, little ferns begin to grow underneath a sidewalk, they can even crack the cement.
