A whole week went by before Uncle George was ready for the next lesson.
At last he called the children and said to them—“Tom, will you please
fetch me the seeds which we put on the wet blotting-paper under the
tumbler? Frank, bring me two leafy branches from a rose-bush in the
garden; and, Dolly, please fetch two glasses from the kitchen.”
Now there was nothing the children liked better than to help their Uncle
George, and all three rushed off at once to do his bidding.
While they were away Uncle George himself went into the garden, dug up
two young plants, and brought them to the children in the study.
[Illustration: Sun-flower Plant in Water.]
“Now, children,” said Uncle George, “we are ready to begin our lesson.
Fill one of the glasses with water, Frank, and put one of your leafy
rose branches in each glass—one branch in water and the other in a dry
glass. Can you tell me what will happen to the branches?”
The children had many times seen what had happened to flowers when the
maid had forgotten to put water in the vases, so that Tom readily said,
“Yes, the one in the dry glass will wither, while the one in the water
will keep fresh for some time.”
“How do you know that?”
“Because it always happens, Uncle,” said Frank. “Why, sometimes the
flowers we gather in the wood are faded before we get home. They often
come back to life when put in water.”
“You are quite right, Frank,” Uncle George replied. “See, here are two
young sun-flower plants, which I brought in from the garden. I placed
one in water. The other I left lying on the table. The one on the table
is dead. What does this show us?”
[Illustration: The Sun-flower, faded.]
“That plants require water,” said Tom. “That whenever you cut off their
water supply they die.”
“Quite right, Tom. We shall see by and by that plants are always giving
off a great deal of water to the air from their leaves.[1] Where do they
get this water from?”
“It must be from the soil,” said Frank.
“It must be,” said Uncle George. “If we keep a plant in a pot without
watering it, it soon dies. All the water it contains will by and by
travel up to the leaves. It passes out through tiny pores in the leaves
into the air. If no more water comes up from the soil, the plant withers
and dies. The roots of plants search the soil in all directions for
water. And in this water there is but little plant-food. We saw this
when we boiled away the clear water which covered the soil in the glass.
“If we boil away some water from the tap, we shall find some solid
matter left behind in the dish. Thus we see that in order to get a
_small_ quantity of food, plants take up a _great_ quantity of water
from the soil. Most of the water is sent into the air from the leaves.
But the food stuff remains in the plant, just as it did in the dish.
“Now I am going to show you how this water gets into the plant. Look at
these little seeds on the damp blotting-paper!
“Each seed has a small plant with a long root, and small, stout green
leaves. Look at the roots and tell me what you see?”
“They are covered with silky stuff,” said Frank.
“Yes. Now take this glass, which will make things look bigger than they
really are, and look at the roots once more.”
“The silky stuff is a number of fine hairs,” said Frank.
[Illustration: THE OTTER]
[Illustration: FIELD-MICE]
“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?