184 
CHILDREN'S GARDENS 
VI 
until the bark is reached. One of these “ rings,” 
as they are called, is formed every year, a 
wide one, according as the 
season is dry or moist, and 
so, by counting them, you 
can tell the age of the tree. 
Still more pretty and in¬ 
teresting are the shapes and 
forms assumed by leaves, 
the last of the “nutritive” 
organs to be considered, but 
they too are all busily per¬ 
forming the same functions, 
and help to keep the plant 
alive and well. Not only 
are leaves useful to plants, but the work 
they do is essential for human beings. The 
leaves absorb from the air what is called 
“carbonic acid gas” or “carbon dioxide,” 
which is poisonous for people to breathe, 
and they give off oxygen, which people re¬ 
quire. This is why green trees and growing 
plants are so good and useful in crowded towns, 
as they not only give shade and are pleasant to 
look at, but they are really helping to improve 
the air for human beings and animals. The 
leaves are generally spread out from the stem, 
so as to have every opportunity of getting both 
air and light. Light is necessary to keep 
narrow one or a 
A PIECE OF THE STEM OF 
A LIME TREE, SHOWING 
ANNUAL RINGS. 
