“That is quite right, Frank,” said Uncle George. “These are the _root-hairs_. Each of these hairs is a long bag or sac, of very thin skin. It is filled with a liquid called _sap_, which is slightly denser, or heavier, than the water in the blotting-paper. Now, do you see how a plant takes food from the soil, when it is growing in the garden or in a field?” “Yes, I think I do,” said Frank. “The water in the soil contains very little plant-food. The water inside the tiny sac contains very much.” “And what have you to say, Tom?” “One of these liquids is denser than the other,” said Tom. “Both are separated by a thin skin. The lighter liquid outside the sac will pass into it quickly, while the heavier liquid will pass out slowly.” “Bravo! Tom!” said Uncle George. “I couldn’t have given a better reply myself. The water from the soil passes in quickly. The sap from the inside of each root-hair passes out slowly. If they were both of the same density, neither would pass through the skin. If the water in the soil were the denser, then the sap would pass out so quickly that the plant would soon be robbed of its water. “Here are two young plants, each growing in a small pot. I want you to water them, Tom. Water one with salt and water (brine), the other with tap-water.” Tom did as he was asked. [Illustration: Plants watered with Fresh and Salt Water.] “See,” said Uncle George, “the one which you watered with brine is drooping. It is bending over the pot. That is because the water outside its root-hairs is denser than that which is inside the plant.” “How does the water get up from the roots to the leaves?” Frank asked. “Just in the same way as the oil travels up the wick of the lamp. Water will always travel up through small spaces.” Uncle George poured some red ink into a saucer, and dipped the corner of a lump of sugar into it. The red ink ran up into the sugar until it was red all over. Next he took a bundle of very small glass tubes, and dipped the ends of them in the ink. The ink ran up the tubes, filling them to the top. “Inside every plant,” Uncle George went on, “there are thousands of long tiny tubes, up which the water travels. In fact the _veins_ of a leaf are just bundles of tubes, something like the bundle I hold in my hand.” Questions and Exercises. 1. Why do branches wither quickly when cut from the tree? How would you keep them from withering quickly? 2. Place two small panes of clean glass close together. Dip the corner of them in coloured water, and watch what happens. 3. Why do we never see the silky _root-hairs_ when we dig or pull a plant out of the ground? III.—THE WEASEL AND THE OTTER. It was Frank who wanted to follow the stream far up towards the hills. He wished to see where it began, for he had heard that its source was in several small streams many miles away.