Imágenes de páginas
PDF
EPUB

also be supplied with water and other materials from the soil. Our experiment with red ink (see 61) showed that the soil-water is carried upward through the woody portions of stems. A microscopical examination of thin sections of a stem (see Fig. 15) shows the presence of tubular cells

[ocr errors]

B

known as ducts, similar to those found in the central cylinder of roots with which they are connected. These are the parts of the wood through which the soil-water passes most readily up to the leaves.

After the raw materials have been changed into the plant foods by green leaves, these plant foods, by the process of digestion, are changed into such a form that they can pass from the leaves into the fibrous bark in which are tubular cells known as sieve-tubes. (See Figs. 15 and 16.) Through these the liquid food FIG. 16.- Sieve tube, passes down the stem to be stored away or used in the growth of root or stem. In young stems the pith rays or medullary rays (59, B, 3), the fine lines extending from the bark toward the center of the stem, are supposed to serve as chan

[graphic]

that conveys sap downward through the leaf, stem, and root. A, longituing edge view of sieve plate (in the middle); B, surface view of sieve plate.

dinal section show

nels for the passage of food across the stem and also for the storage of food.

In the type of stem represented in the corn, lilies, and palm trees, the woody material through which sap passes is not arranged in the form of annual rings, but the woody bundles are scattered through the pith. Each bundle consists of ducts that carry the soil-water up through the stem out into the leaves, of sieve-tubes that convey downward from the leaves the manufac

tured food substances, and of wood cells that help to strengthen the bundle.

[ocr errors]

64. Changes in stems during their growth. In our discussion thus far, we have considered the adaptations of stems for exposing leaves to the light and for transmitting food materials to and from the leaves. But the stem has other important functions which we are now to consider. In a young twig, before the brown bark thickens and shuts out the light, the green bark, on account of the presence of chlorophyll, is enabled to carry on the manufacture of carbohydrates. In a very young stem the surface is covered by thin epidermis which helps to prevent

the undue escape of moisture. In this layer are tiny openings that allow the inward and outward passage of gases that occur in breathing and food manufacture. Later this epidermis is replaced by the outer or brown bark, which serves as a means of protection against unfavorable weather conditions and insects. In this brown bark the tiny openings referred to above are developed into large openings known as lenticels which carry on the same functions. In an old tree the outer bark becomes very thick and corky and the green layer disappears entirely.

[graphic]

FIG. 17. Cross section of a tree trunk showing bark, wood (with its annual rings and medullary rays), and pith at center. - (Courtesy of New York Botanical Garden.)

The growth of the tree in thickness, as already stated, is due to the activity of a layer of cells between the wood and the fibrous bark. This is the cambium layer (Fig. 15). In early spring the cambium cells by rapid growth and division form on their innermost surface a new layer of wood (which appears

FIG. 18. Cross section of young bamboo, showing hard outer rind, woody bundles, scattered through the pith. The center of the stem is hollow. (Courtesy of New York Botanical Garden.)

III.

as a ring in cross section), and on their outer surface more fibrous bark. As the season advances, the activity of these cells becomes less and less, and finally growth ceases during the winter.1

Stems of plants like the corn, bamboo, and palm have no true cambium layer, and therefore even in the case of plants of this type that live on from year to year no annual rings are formed. In the growth of these stems, new bundles develop in the pith between those already formed.

[graphic]

THE STRUCTURE AND ADAPTATIONS OF LEAVES

65. Leaf arrangement.2-Along the sides of twigs leaves are arranged in such a way as to secure as much light as possible without being shaded by the leaves above them. Thus in plants like the horse-chestnut, maple, and lilac, the leaves are arranged so that at a given level on the twig two leaves are opposite each other, while the next pair are at right angles to the first pair. This is known as an opposite arrangement. The beech, elm, and rose, on the other hand, have an alternate arrangement, only one leaf being found at a given level on the twig.

66. External structure of a horse-chestnut twig. - Laboratory Study No. 34. (Optional.) (Maple, beech, or other woody twig may be used with slight verbal changes.)

1 Sometimes trees form more than one ring during a season. 2 Before assigning this section for study, the teacher should demonstrate from leafy twigs (e.g. maple, horse-chestnut, lilac, elm, apple) the characteristic differences between the opposite and. alternate arrangement of leaves.

A. Leaf scars.

(The horseshoe-shaped scars with the raised dots like horseshoe nails indicate the places where the stalks of the leaves were attached.)

1. Do the leaf scars occur in pairs, or is there only one scar at a given level? How, therefore, were the leaves arranged on the

stem?

2. Count the num

ber of dots on
several differ-

ent leaf scars;
these dots are

the ends of
the wood

bundles that

carried sap to
the various
leaflets. Look
at the picture
of horse-chest-
nut leaves.
(See Fig. 20,

K.) How
many main

veins do you

find in one

compound FIG. 19.-Spray of young apple tree, showing

leaf? Com

pare this

alternate arrangement. At the base of each leaf stalk is a pair of small stipules. — (Bailey.)

number with the number of dots on the leaf scars; what do you conclude?

B. Buds. (At the end of most twigs is a single terminal bud;

the buds along the side of the twig are lateral buds. Each bud is covered with bud-scales.)

1. State the position of each kind of bud on the twig. Where are the lateral buds found with reference to the leaf scars?

[blocks in formation]

FIG. 20. Forms of leaves.

(Courtesy of Furman and Miller, Botanical Aid

Western Publishing Co., Chicago, Ill.)

[blocks in formation]
« AnteriorContinuar »