The Food of Plants 49 
carrying on its work. The sugar is made in the cells 
within the leaf. Most of these are loosely arranged ; 
they touch.each other on some sides so that there are 
air spaces between them. Thus part of each cell lies 
against other cells, and part is exposed to the air within 
the leaf. 
On the outside of a leaf there is a very thin covering 
called the epidermis. This is composed of a layer of thin, 
flat cells closely joined together. The outside walls of 
these cells are thickened and usually covered with a 
wax-like material which does not allow water and air to 
pass readily through it. This covering protects the softer 
and more tender cells within. But on the lower side of 
the leaf of a garden plant (and in some plants on the 
upper side of leaves also) the epidermis has many tiny 
openings through which air and water vapor can pass. 
Thus the inner portion of the leaf is in communica- 
tion with the outside air through the openings in the 
epidermis. The oxygen that the cells need for respira- 
tion and the carbon dioxid needed for food-making 
enter the leaf through the little openings in the epidermis, 
and the éxcess of water brought up from the roots passes 
out into the air as vapor. ‘The veins of the leaf are com- 
posed of vessels which connect with those in the stem. 
They bring the water and dissolved minerals from the 
roots to the leaf and carry away to other parts of the 
plant food that the leaves have made. 
How starch is made. Starch is only a changed form 
of sugar, a form that does not dissolve in water and is 
suitable for storage within the plant. In nearly all 
garden plants the sugar is changed to starch before it 
