PLANTS WITHOUT SEEDS 
must have thick walls or skin to keep the water 
from evaporating, and they need root systems 
to get the moisture from the ground. 
Green scum on the water is made of the living 
members of the family that were the very first 
of all plants that ever lived on the earth. 
The smallest plants are so small that no one 
knew they existed until about one hundred 
years ago. Until the time when microscopes 
were invented, people did not even suspect that 
there were such things. They are called bac¬ 
teria, and in spite of their small size, they are 
very important to the lives of human beings. A 
great many of them are useful. They help us to 
get linen fiber, to tan leather, to turn milk sour, 
to make cheese and butter, to make vinegar, and 
to get rid of dead plant and animal material. A 
few are very harmful; they spoil food which we 
wish to eat, and they cause a great many dis¬ 
eases. We often call the harmful bacteria by 
the name of germs or microbes. 
Let us see how these bacteria work. In order 
to see them at all we must use a powerful micro- 
What are the 
smallest plants? 
