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 excess 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 



