"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.