"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.