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VIII. BUDS AND STEMS
Structural Differences between the Stem and the Root. Wash carefully the earth from the roots of a pea seedling that you have grown in the laboratory. Try to make out the following points: (1) note the differences in color between the stem and roots; (2) compare the manner in which the root and stem give off lateral branches. Notice that the leaves and lateral branches from a stem are more or less regular in position, while the small roots have no regular method of leaving the primary root. The places on the stem where leaves are given off are known as nodes, the part of the stem between being called an internode.
In a growing stem of any woody plant, notice the end of the stem; the bud at the termination is a future stem. It is well for us to remember that a stem is a developed bud, as we shall see when we take up the work on the bud more in detail.
One difference which is very noticeable between the stem and the root is the color, the young root being whitish or gray, and
the young stem being green. The following experiment will serve to explain this difference:
A pocket garden which has been kept in complete darkness for several weeks. Notice the condition of stems and leaves.
Effect of Absence of Light on Young Plants. Plant some peas in sawdust within a box, or wide-mouthed bottle which has been previously blackened so that no light is admitted to the interior. Grow some of the same seed in a box alongside the covered material, giving each the same amount of heat and moisture. After the young plants have grown, take one out, compare note the difference in color. color of the root?
The seedlings which have been grown in darkness show some other interesting conditions. The stems are long and more or less reclining on the sawdust. The leaves are hardly worthy of the name, being reduced to little scales. We know that they are
modified leaves because they come out on the stem in the position taken by leaves. The stem of the plant grown in the darkness is much longer and thinner than the one grown in the light. Can you think of any reasons for these differences?
Effect of Light on Plants. We can explain the changed condition of the seedling grown in the dark only by assuming that light has some effect on the protoplasm of the seedling and induces the growth of the green part of the plant. Numerous instances could be given in which plants grown in sunlight are healthier and better developed as to their green parts than those in the shady parts of a garden or field. On the other hand, some plants thrive in the shade. Such plants are the mosses and ferns. Still other plants, minute organisms hardly visible to the eye, do not thrive in the light, and may be killed by its influence. Such are molds, mildews, and some bacteria. It is to be noticed that such plants as do not need light are not green. As a matter of fact, the stem, which has but little chlorophyll, develops somewhat more rapidly under conditions where it receives no light. Heliotropism.- We saw
that the stems of the plants kept in the darkness did not always lift themselves erect as in the case of stems in the light. If your seedlings have been growing on a window sill, or where the light comes in from one side, you have doubtless noticed that the stem and leaves of the seedlings incline in the direction from which the light comes. The tendency of young stems and leaves to grow toward sunlight is called positive heliotropism.
The experiment pictured on the following page shows this effect of light very plainly. A hole was cut in one end of a cigar box and barriers erected in the interior of the box so that the seeds planted in the sawdust received their light by an indirect course.
The young seedling in this case responded to the influence of the stimulus of light so as to grow out finally through the hole in the box into the open air. Make this experiment, and see if you get the same result. Vary the apparatus in any manner you can, and plant some other seeds or grains.
This experiment shows that another factor besides gravity influences the stem to grow upward. This growth of the stem to the
light is of very great importance to a growing plant, because, as we shall see later, food-making depends largely on the amount of sunlight the leaves receive.
Structure of a Bud. If we cut a head of cabbage so that the knife blade cuts through the long axis of the stem, we find that the stem is much shortened or dwarfed, and that the leaves are so placed as to cover it entirely. The cabbage is a big bud. If we carry out our definition of a bud, starting with what we have seen in the cabbage, we might say that a bud is a very much shortened branch, or in reality "the promise of a branch."
Cabbage head cut lengthwise to show stem and leaves. Photographed by Overton.
Factors which influence
the Opening of a Bud. - A bud responds to the same stimuli that we have seen call a young plant into active life from the seed. If a branch containing unopened buds (such as horsechestnut or willow) is placed in water in a moderately warm room, it will respond to the factors without it and begin to open. The tips of branches, still attached to the tree outdoors, may be introduced into a warm room through a hole bored in the window sash. They will open to bear flowers and leaves during the
coldest months of the year. The factors which influence the germination of seeds also act on the bud.
Position of the Bud on the Stem. - The growth of the stem from the bud can best be observed in a very young seedling. If, for example, we examine a pea seedling, it will be seen that the epicotyl is, in truth, the first bud of the plant. It produces the first stem and leaves. The position of the most active buds determines the form of the future tree. Buds come out at the ends of branches (terminal) and at the sides (lateral).
Deliquescent Tree. If you examine a winter branch of the apple, elm, or oak tree, you will find that the lateral buds have
Excurrent trees (at right) and deliquescent tree (at left). In background a row of cedars which were planted by birds roosting on a fence along a roadside.
developed more strongly and more rapidly than the terminal bud. Thus the tree has come to assume during its growth a rounded shape. Such a tree, having a rather stout, short trunk, with many low, spreading, lateral branches, is said to be deliquescent.
Excurrent Tree. If, on the other hand, the terminal buds of the tree get a better supply of light, food, or if other factors aid its growth, the tree will be tall and have but one main trunk, such as the Lombardy poplar, and pines and cedars. Such a tree is named excurrent. The picture shows trees of these two shapes. Structure and Functions of the Parts of a Winter Twig. The best way for us to understand the growth of a bud into a branch