"What time of the year do the trees grow the most?" asked Uncle Robert. "In the spring I should think," said Frank. "That's when the sap begins to run." "What is sap?" "It must be the water that the trees take up from the ground," said Frank. "We've tapped some maple trees for sap," said Donald. "And we could see it run right out of the tree," said Susie. "I've told the children how we used to make maple sugar in New England," said Mrs. Leonard. "Do you remember, Robert, what a quantity of sap it took to make just a little sugar?" "Yes, and I also remember how long I thought it took to boil it down into the wax I was so fond of." "About thirty gallons of sap can be taken from one tree each year," said Mr. Leonard. "But I should think that would hurt the tree," said Frank. "No," replied Uncle Robert, "for the hole they make is only about an inch across. If they were to cut all around the tree, you see, it would stop the running of the sap and kill the tree." "That is called girdling," said Mr. Leonard. "They used to clear off hundreds of acres of land in that way when this country was first settled. Instead of cutting down the trees, they girdled them near the ground. In a very short time they died, because they could get no food from the earth. The dead trees lost their strength, and a strong wind would blow them over. Then they were piled up and burned." "How do you know when a tree is dying?" asked Uncle Robert. "The leaves turn yellow," said Donald. "But the leaves turn yellow in the fall," said Frank, "and the trees do not die." "The leaves of my spruce don't turn yellow in the fall," said Donald. "They stay green all winter." "What makes the leaves green?" asked Uncle Robert. No one answered. "What is the color of the potato sprouts in the cellar?" "Yellow," said Susie. "When you take up a board that has lain on the grass, what is the color of the grass?" "Yellow," said Donald. "Why?" asked Uncle Robert. "Because they don't get any light," said Frank. "You know why we put our plants in the south window in winter?" said Mrs. Leonard. "Oh, yes," said Susie, "because the sun shines in at that window." "Warmth and water and air help trees and plants to grow," said Uncle Robert, "but without sunlight their leaves would be yellow and their stems and branches weak. The greatest forests on earth are where it is very hot and moist. The sun is a wonderful artist, and every leaf it paints makes the tree stronger." "But what makes the leaves turn yellow and red just before they fall off?" asked Frank. "Does the sun paint them then?" "That is a question that no one has been able to answer," replied his uncle. "But how can the sap flow up the tree?" said Donald. "I should think it would run down." "It would unless there was something to draw it up," said Uncle Robert. "I suppose the sun does that, too," said Frank. "Where does it go after it reaches the leaves?" asked Uncle Robert. "Why, back again," said Susie.