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-gra.s.s or wiregra.s.s, Burmuda gra.s.s, 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 cuc.u.mber 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 pa.s.sage 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 gra.s.s and Bermuda gra.s.s 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 gra.s.s 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 pa.s.sed 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 ma.s.s 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.
CHAPTER XV
FLOWERS
In our study of the parts of plants the flower and fruit have been given the last place because in the growing of most farm plants a knowledge of the functions of the flower is of less importance than that of the roots, leaves and stems. However, a knowledge of these parts is necessary for successful fruit culture and some other horticultural industries.
As with the other parts of the plant our study will not be exhaustive but will be simply an attempt to bring out one or two important truths of value to most farmers.
In the study of flowers the specimens used for study will depend upon the time of the year in which the studies are made and need not necessarily be the ones used here for ill.u.s.tration.
FUNCTION OR USE OF FLOWERS TO PLANTS