By the leaf-mill grinding stones, a gas from the air (carbonic acid gas) is mixed with the plant food soup sent up by the plant’s roots, and starch is formed.
While doing the work of manufacturing starch, the leaf-mill throws off into the air another gas, called oxygen. Oxygen is needed by all animals; carbonic acid gas (or carbon dioxide) is needed by all plants whose leaves make starch.
But even the starch must be changed before the plant can use it to make new growth. It must be made into sugar!
So the leaves act as stomachs, and digest the starch they have made for the plant’s use. In them, in some wonderful way, the starch is changed into sugar, and some mineral matter from the humus soup is mixed with the sugar. This combination forms a perfect food, ready for the plant to make into new growth.
“Isn’t it a wonderful story?” asked Bouncing Bet, as she finished speaking.
“It’s the most surprising garden story I’ve yet heard,” declared Mary Frances.
“I’ll never, never think of leaves again as just ‘for shade,’” declared Eleanor. “But I’m glad they do give shade,” she added.
“Trees give a great deal of shade,” said Bet, “because they expose as large a surface of leaves as possible to the sun. On a large tree, nearly half an acre of leaf surface may be in the sunlight at once.”
“And the sunlight turns the grinding-stones of the leaf-mill,” said Mary Frances softly.
“And they make food for the tree,” Eleanor whispered.