HOW PLANTS LIVE AND GROW 29 
from the air. This oxygen it wants, like all other living 
things, to breathe, and without it the protoplasm, for some 
reason we do not understand, would not have the energy 
to carry on its digestive and its building work. 
The roots, meanwhile, have to find all the other nutri- 
ment required, and, above all, they are diligently pump- 
ing up water, which is made up of. the two gases, 
hydrogen and oxygen. Each little root is covered with 
small hairs, which cling to the particles of the soil 
and suck up the traces of moisture that they can find. 
In addition to this, by the help of the acid in the sap, 
they are constantly dissolving various mineral compounds 
of sulphur, potassium, lime, and so forth, which are always 
present in a soil suitable for plants, and pumping up 
certain quantities of these for the making of the necessary 
protoplasm. 
A pretty experiment, showing the acid properties of the 
sap, is to allow the roots of a plant to spread over a 
marble plate just covered with earth. The roots bite 
quite into the marble, slowly dissolving it, and taking 
up the lime that they require. If the plant is pulled 
up afterwards, one finds a complete sunk pattern of the 
roots over the slab. 
The various vessels in the bundles form ready passages 
for the material required, and the wood especially is 
an excellent conductor of water and air all about the 
tree. 
You will remember that in the cells of the green parts 
of the plant, and especially of the leaves, we found that 
part of the protoplasm had collected into green bodies 
called chlorophyll corpuscles. These green bodies are 
never formed unless the roots are able to send up a 
solution of iron, but when once made they have most 
