“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.
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.”