STEMS
WHAT ARE STEMS FOR?
Visit the farm or garden and the fields to examine stems and study
their general appearances and habits of growth. Notice that many
plants, like the trees, bushes and many vegetable and flowering
plants, have stems which are very much branched, while others have
apparently single stems with but few or no branches. Examine these
stems carefully and note that there are leaves on some part of all of
them and that just above the point where each leaf is fastened to the
stem there is a bud which may sometime produce a new branch (Fig. 68).
If the stems of trees and other woody plants be examined in the winter
after the leaves have fallen, it will be seen that the buds are still
there, and that just below each bud is a mark or leaf scar left by the
fallen leaf. These buds are the beginnings of new branches for another
year's growth. On some branches will be found also flowers and fruit
or seed vessels.
Buds and leaves or buds and leaf scars distinguish stems from roots.
Some plants have stems under the soil as well as above it. These
underground stems resemble roots but can be distinguished from them by
the rings or joints where will be found buds and small scale-like
leaves (Fig. 69). Quitch-grass or wiregrass, Burmuda grass, white
potato and artichoke are examples of underground stems.
Now study the habit of growth of these stems. Notice that:
Some plants grow erect with strong, stiff stems, for example, corn,
sunflower, maple, pine, elm and other trees. Many of these erect stems
have branches reaching out into the air in all directions. Stand under
a tree close to the stem or trunk and look up into the tree and notice
that the leaves are near the outer ends of the branches while in the
centre of the tree the branches are nearly bare. Why is this? If you
remember the work of leaves and the conditions necessary for their
work you will be able to answer this question. Leaves need light and
air for their work, and these erect, branching stems hold the leaves
up and spread them out in the light and air.
Notice that where several trees grow close together, they are
one-sided, and that the longest and largest branches are on the
outside of the group and that they have more leaves than the inner
branches. Why? Why do the trees in thick woods have most of the living
branches and bear most of their leaves away up in the top of the tree?
Some stems instead of standing up erect climb up on other plants or
objects by means of springlike tendrils which twist about the object
and so hold up the slender stem. On the grape vine these tendrils are
slender branches. On the sweet pea and garden pea they are parts of
the leaves. The trumpet creeper and English ivy climb by means of air
roots. The nasturtium climbs by means of its leaf stems.
Other stems get up into the light and air with their leaves by twining
about upright objects. For example, the morning glory and pole bean.
Some stems will be found that spread their leaves out to the sun by
creeping over the ground. Sweet potato, melon, squash, and cucumber
vines are examples of such plants.
One use of the stems of plants then is to support the leaves, flowers
and fruit, and expose them to the much needed light and air.
=Experiment.=--Get a piece of grape vine and cut it into pieces four
or five inches long; notice that the cut surface appears to be full of
little holes. Cut a piece from between joints, place one end in your
mouth and blow hard. It will be found that air can be blown through
the piece of vine. Now pour about an inch of water in a tumbler or cup
and color it with a few drops of red ink. Then stand some of the
pieces of grape vine in the colored water. In a few hours the colored
water will appear at the upper ends of the sticks. Capillary force has
caused the colored water to rise through the small tubes in the vine.
Repeat this experiment with twigs of several kinds of trees and soft
green plants, as elm, maple, sunflower, corn, etc. It will not be
possible to blow through these twigs, but the red water will rise
through them by osmose, and in a few hours will appear at the upper
ends. If some leaves are left on the stems the colored water will
appear in them. Some white flowers can be colored in this way.
In this manner the stem carries plant food dissolved in water from the
roots to the leaves, and after the leaves have digested it carries it
back to various parts of the plant.
The stem then serves as a conductor or a passage for food and moisture
between roots and leaves.
Visit a strawberry bed or search for wild strawberry plants. Notice
that from the older and larger plants are sent out long, slender,
leafless stems with a bud at the tip. These stems are called runners.
Find some runners that have formed roots at the tip and have developed
a tuft of leaves there, forming new plants. Find some black raspberry
plants and notice that some of the canes have bent over and taken root
at the tips sending up a new shoot and thus forming a new plant. You
know how rapidly wire grass and Bermuda grass will overrun the garden
or farm. One way in which they do this is by sending out underground
stems which take root at the joints and so form new plants.
Another use of the stem then is to produce new plants.
On the farm we make use of this habit of stems when we wish to
produce new white potato plants. We cut an old potato in pieces and
plant them. The buds in the eyes grow and form new plants. One way of
getting new grape plants is to take a ripened vine in the fall and cut
it in pieces with two or three buds and plant them so that one or both
of the buds are covered with soil. The pieces will take root and in
the spring will send up new shoots and thus form new plants.
You can obtain new plants from geranium, verbena, nasturtium and many
other flowering plants, by cutting and planting slips or parts of the
stems from them.
In parts of the South new sweet potato plants are obtained by cutting
parts of the stems from growing plants and planting them.
Florists produce large numbers of new plants by taking advantage of
this function of stems.
=Experiment.=--Take a white potato which is a thickened stem and place
it in a warm, dark place. It will soon begin to sprout or send out new
stems, and as these new stems grow the potato shrinks and shrivels up.
Why is this? It is because the starch and other material stored in the
potato are being used to feed the new branches. When we plant potatoes
in the garden and field the new plants produced from the eyes of the
potato are fed by the stored material until they strike root and are
able to take care of themselves.
All stems store food for the future use of the plant.
Annual plants, or those which live but one year, store food in their
stems and leaves during the early part of their growth. During the
fruiting or seed forming season this food material is transferred to
the seeds and there stored, and the stems become woody. This is a fact
to bear in mind in connection with the harvesting of hay or other
fodder crops. If we let the grass stand until the seeds form in the
head, the stem and leaves send their nourishment to the seeds and
become woody and of less value than if cut before the seeds are fully
formed.
In plants of more than one year's growth the stored food is used to
give the plant a start the following season, or for seed production.
The rapid growth of leaf and twig on trees and shrubs in spring is
made from the food stored in the stem the season before.
Sago is a form of starch stored in the stem of the sago palm for the
future use of the plant.
Maple sugar is made from the food material stored in the trunk of the
maple tree for the rapid growth of twig and leaf in the spring.
Cane sugar is the food stored in the sugar cane to produce new plants
the next season.
If we examine the stem of a tree that has been cut down we find that
it is woody, that the wood is arranged in rings or layers and that the
outer part of the stem is covered with bark. We will notice also that
the wood near the centre of the tree is darker than the outer part.
This inner part is called the heart wood of the tree. The lighter
wood is called the sap wood. It is through the outer or sap wood that
the water taken in by the root is passed up to the leaves where the
food which it carries is digested and then sent back to the plant. The
returning digested food is sent back largely through the bark. Between
the bark and the wood is a very thin layer which is called cambium.
This is the active growing tissue of the stem. In the spring it is
very soft and slippery and causes the bark to peel off easily. This
cambium builds a new ring of wood outside of the old wood and a new
ring of bark on the inside of the bark. In this way the tree grows in
diameter.
Now if the bark is injured, or any part of the stem, all parts below
the wound are cut off from the return supply of digested food and
their growth is checked. When such a wound does occur, or if a wound
is made by cutting off a branch, the cambium sets to work to repair
the damage by pushing out a new growth which tends to cover the wound.
We can help this by covering the wound and keeping the air from it to
prevent its drying and to keep disease from attacking it before it is
healed.
HOW THE WORK OF THE STEM MAY BE INTERFERED WITH
If there are any peach trees near by, examine the trunks close to the
ground, even pulling away the soil for a few inches. You will very
likely find a mass of gummy substance oozing from the tree. Pull this
away and in it and in the wood under it will be found one or more
yellowish white worms. These are tree borers. They will be found in
almost all peach trees. They interfere with the work of the stem and
in many cases kill the trees. These worms may be kept somewhat in
check by keeping papers wrapped about the lower part of the tree. But
the surest way to keep them in check is to dig them out, spring and
fall, with a knife and wire.
Borers attack the other fruit trees and also ornamental trees and
shrubs.
Rabbits sometimes gnaw the bark from trees during severe winters.
Careless workmen sometimes injure the bark of trees by allowing plows
and mowing machines or other tools which they are using among them to
come in contact with the trees and injure the bark.
Young trees purchased from the nursery generally have a label fastened
to them with a piece of wire. Unless this wire is removed or is
carefully watched and enlarged from time to time it will cut into the
bark as the stem grows and interfere with its work and often kill the
top of the tree or injure a main branch.
These are a few ways in which the work of the stem is sometimes
checked and the plant injured thereby.