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.

III. 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 alternate arrangement. At the base of each leaf? Com- leaf stalk is a pair of small stipules. (Bailey.) pare this

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?

H

0000000

FIG. 20. Forms of leaves.

(Courtesy of Furman and Miller, Botanical Aid

Western Publishing Co., Chicago, Ill.)

E

2. Look carefully at the scales of the terminal bud to see if they have any definite arrangement. State whether or not this arrangement corresponds to that of the leaf scars.

3. (Demonstration.) Examine a terminal bud from which one or two scales have been removed. Bud-scales are modified leaves. How do these scales differ from ordinary leaves? What is the use of the scales to the bud? How are they adapted for this use?

C. Bud-scale scars. (These are also called annual scars because they are formed at the beginning of the growing season of

each year when the terminal bud opens and its scales fall off. To prove this, remove one or two outside scales from a terminal bud, and note the scar thus formed.)

1. How many groups of bud-scale scars or annual scars do you find on the twig you are studying?

2. Since one set is formed each spring, how many years of growth are shown on the twig?

D. Breathing pores or lenticels. Look for small elevations on the bark. These locate the lenticels. Describe the lenticels. E. Make a careful outline drawing of the twig, showing its form, the position and shape of the leaf scars with their woody bundles, the terminal and lateral buds, bud-scales, budscale scars, and lenticels. Label each of the structures shown in your drawing.

67. The structure of leaves. Laboratory Study No. 35. A. Parts of a leaf.

1. Examine a simple leaf, e.g. maple, geranium, or lilac, and note that it is made up of the following parts: a leaf-stalk, which attaches the main part of the leaf to the stem of the plant, and the blade, the flat, expanded portion.

a. How does the blade differ in form from the leafstalk?

b. Hold the leaf to the light. How many main veins. do you find? Where are they smallest? By what are the larger veins connected?

c. Make a drawing, natural size, by tracing the outline of the leaf-stalk and blade. Draw carefully

the principal veins and a few of their branches, being careful to show their relative size and their connections. Label leaf-stalk, blade, main veins, network of veins.

2. (Optional.) Examine a compound leaf, e.g. rose, clover,
locust, pea, horse-chestnut. Notice that the blade is
divided into three or more parts known as leaflets,
which are attached either to the end of the leaf-
stalk or on either side of the mid-vein of the com-
pound leaf.

a. In what respect, therefore, does the blade of a com-
pound leaf differ from the blade of a simple leaf?
b. Compare the arrangement of the leaflets in a leaf like
the rose, locust, or pea with that in the Virginia
creeper or horse-chestnut. Which leaves have the
leaflets arranged like the bones in the palm of the
hand (palmately compound), and which have the
leaflets arranged along the side of the mid-vein as in
a feather (pinnately compound, from Latin, pinna
feather)?
c. At the base of the leaf-stalk of the rose, clover, or pea leaf,
notice two leaf-like objects (small in the case of the
rose). These are known as stipules. Stipules are also
found as a part of many simple leaves. How do
stipules differ from the other parts of leaves? (See
Fig. 19.)

=

B. Gross structure of leaves. Secure thick leaves such as sedum, tulip, hyacinth, or onion.

1. Peel off from the upper and lower surface a thin membrane known as the epidermis. Hold the epidermis between yourself and the light. Tell what you have done and state two characteristics of epidermis.