4 o 
Garden Work 
arranged along the sides of the cells. Thus the one acts 
as a protection to the other. 
Then there is usually a second layer of cells of the 
same kind, but smaller. These two layers of elongated 
cells are called the palisade layer. Underneath this are 
two or three layers of large, irregularly shaped cells, with 
large air spaces between. These are called the spongy 
Covering Hairs 
parenchyma. If we look again at the leaf, we see at once 
that it has a perfect network of veins. When we have 
obtained a perfect knowledge of the other parts of the 
plant we shall readily understand how necessary these 
veins are. They are like miniature stems. They have 
different kinds of tissue, with different vessels or tubes to 
carry the food substances from the stem and distribute it 
to the chlorophyll-containing tissue of the leaf, where it 
unites with the carbon dioxide of the air and is manu- 
factured into plant substances. It is then collected by 
these veins, and passed along through them into the stem, 
and thence to the parts where it is required. Many leaves 
