When the sweet-peas in the garden were nearly full grown, Uncle George
sowed some sweet-peas in a pot. In a fortnight those sown in the pot
were about four inches in height, and those in the garden were in
flower.
“Let us go into the garden, boys,” he said, “and see if we can learn
something from the sweet-pea.”
In the garden the sweet-peas were really lovely. They looked, as Frank
said, like so many beautiful butterflies on the wing; and they filled
the air with delicious perfume.
“I think,” said Frank, “that our row of sweet-peas is by far the best
thing in the garden.”
“That is quite true, Frank,” said his uncle, “but it is not so much the
flowers we are going to study at present. The sweet-pea is certainly one
of our finest flowering plants. It is also one of the most interesting.
Can you tell me why we put stakes up for our sweet-pea plants to cling
to?”
“Because they have long, slender stems—too slender and weak to grow up
by themselves,” said Frank.
“Quite right, Frank. If the sweet-pea were a wild plant, where would it
grow?”
“In the hedges,” said Tom.
“Right again,” said Uncle George. “If we grow the sweet-pea in the
garden, we must imitate its surroundings in the wild state—we must give
it a hedge of some kind to cling to, otherwise it would trail along the
ground.”
“Then it would get choked among the other plants,” said Frank.
[Illustration]
Rose Leaf.
Vetch Leaf.
Ash Leaf.
“What do you mean by ‘_choked_,’ Frank?”
“Well, it would die for want of air”——
“And light,” his uncle added. “No green plant can live without air and
sunlight; and, in order to get these, our weak-stemmed sweet-pea has to
climb. It clings to its stronger brethren just as the wild peas do in
the hedgerow. Now notice how it climbs. The end part of each leaf
consists of little gripping organs called tendrils. These twine tightly
round the smallest twigs near them.
[Illustration]
1. Ox Eye.
2. Daisy.
3. Corn Marigold.
“Here is a rose leaf. It is composed of several leaflets, and at its
base there are two small, brownish-green things called _stipules_. If we
compare this leaf with the leaf of the sweet-pea, we find that in some
points they are similar, but in others very different.
[Illustration]
Sweet-Pea.
Edible Pea.
“The rose leaf is called a compound leaf, because it is made up of many
small leaflets. The sweet-pea leaf is also a compound leaf, but it has
only two leaflets, and these are of enormous size. The rose leaf has two
small stipules. The stipules of the sweet-pea leaf are large and green.
“If I place the rose leaf beside a sweet-pea leaf, you will notice that
the latter has _no upper leaflets_.”
“It has tendrils ‘instead,’” said Frank.
“Exactly!” said Uncle George. “These tendrils correspond to leaflets.
Now, if the plant has been forced to turn all its leaflets, except two,
into gripping organs, how does it manage to make up for this loss of
green leaves?
[Illustration: Young Edible Pea.]
“We have already learned that plants take in food from the air by means
of the green substance in their leaves. The more of this green substance
a plant possesses, the more food it can take in from the air. If it
sacrifices leaves in order to climb up to the light and air, its amount
of the green material must be lessened. But the sweet-pea makes up for
this loss of leaves. Can you see how it does so?”
“The stipules are very large,” said Frank.
“And the lower part of the leaf-stalk is very flat, broad, and green,”
added Tom.
“Very good,” said Uncle George, “but look at the long, slender stem. It
has flat, green, ribbon-like outgrowths on each side throughout its
whole length. So, you see, what the plant loses in leaves, it makes up
in another way.
“Let us now look at the ordinary pea plant. Its leaf has four pairs of
leaflets and three pairs of tendrils besides the terminal tendril—seven
tendrils altogether. The stipules here are also very large—much larger
than those of the sweet-pea. But the stem is round and smooth. There are
no green outgrowths. Here is a wild pea, which I found to-day, which has
no leaves at all. All its leaflets have been turned into tendrils or
gripping organs. But look at the enormous stipules it has! They are much
bigger than ordinary pea leaves.
[Illustration]
A Wild Pea with no Leaves.
Young Sweet-Peas have no Tendrils.
“Let us now look at the tendrils of the sweet-pea, and see how they grip
their supports. Notice that they begin to twist long before they reach
the supporting twig. Then, when they have reached it, they twist round
it _in the opposite way_—just like a piece of string, if you twist it at
one end, it untwists at the other.”
“Can you see any reason for this coiling before it reaches the twig?” As
he spoke, Uncle George pulled a sweet-pea plant out from its supports,
and the boys saw that the tendril was really a spiral spring. The plant
could be pulled out a considerable length without breaking the tendril.
“You see,” Uncle George continued, “if it were not for that first
coiling of the tendril—that is to say, if the tendril grew out straight
and only began to twist when it reached the twig—the first breeze of
wind would snap the plant from its supports and it would fall to the
ground.
“Look now, at the young sweet-peas which we grew in the pot. They are
without tendrils of any kind. This shows us, I think, that the sweet-pea
was at one time a small plant, like its relative the clover. It grew in
open places and did not have to struggle for light and air. But, by and
by, when strong growing plants took up the soil, it became forced to
produce climbing organs or become choked out. And, having turned its
leaves into climbing or gripping organs, it made up for the loss of
leaves by producing large stipules and green out-growths to stem and
leaf.”
Exercises on Lesson V.
1. Draw a leaf of the wild rose and one of the sweet-pea side by side.
2. Make a list of all the climbing plants you know, and state how each
climbs.
3. Look for stipules on the apple, pansy, and bean leaves—make
sketches.
4. Those plants of the pea family which have many leaflets have smooth
round stems. Those which have few leaflets have “winged”
stems. Can you explain this?