Examine a fresh potato.' Notice the marks or eyes on its surface. Find a little projection within each eye. This is a bud. Immediately under it you will find a tiny scale which represents a leaf. Later we shall see that a bud on a stem always has the same relation to a leaf as does this bud to the tiny scale. In other words, the position is the same in each case, and the structure may be said to be homologous to that of the bud and leaf of an ordinary stem.

Try to find out the arrangement of the leaves on the potato; they may be either opposite or alternately placed on the stem. See, also, if you can find the point at which the stem was attached to the parent plant.

If a potato is cut in cross section it will be possible to find all the parts of a stem. The pith occupies the central

Out

portion; around this is the wood, which here looks like a dark band. side the wood we find the cortex, the potato being protected by the rather delicate epidermis.

BULBS.

Cut a potato and a sweet potato in cross section; place each in red ink overnight. It will then be easy to compare the course taken by fluids in passing up a root and a stem.

Cut out several eyes from healthy potatoes. From one or two remove all of the flesh of the potato, and in the other specimens leave food supply intact. Does the stored food help the young plant in its growth? Test for food stuffs in the potato. What foods are present?

REDUCED STEMS. In some plants the stem is so reduced as to be almost lost. This may be of a distinct advantage to the plant in enabling it to escape destruction from enemies. Such a plant is the common dandelion, which, because of its short stem, escapes grazing animals and the knives of lawn mowers. Many other low-lying weeds are partly immune from dangers which beset taller plants.

In bulbs the stem is covered with thickened leaves, the whole making a compact and reduced plant which, because of its stored food, enables the plant to make an early start in the spring.

Cut an onion in longitudinal sections; draw, showing the scalelike, thickened leaves, the greatly reduced stem, and the roots. Test for food stuffs. What foods do you find present in the onion?

1 See Hunter and Valentine, Manual, page 29.

CLIMBING STEMS. FIELD WORK.

A field excursion may be made for the purpose of obtaining as many kinds of climbing stems as possible. Place them in one of the following classes:

(1) Stems which twist or twine.

(2) Stems having roots as holdfasts.

(3) Stems having parts of leaves or branches as holdfasts.

the morning glory.

Stems may twist around an object in order to climb. Such a plant is Here the stimulus which draws the plant upward is evidently the sun. In stems which make use of this method of climbing, it is noticed that each stem twines around the support in a given direction, some revolving with the course of the sun, others in the opposite direction. When such a stem touches an object during its first growth, it is immediately stimulated to turn toward the object and coil around it.

Catbrier; the tendrils (T) are modified stipules (parts of leaves); Th, thorn.

LEAVES AND STEMS MODIFIED AS HOLDFASTS. In the common nasturtium (Tropaeolum) the leaves revolve in much the same manner as do the stems mentioned above. This movement results in some of the leafstalks fastening around supports, thus drawing the stem up.

TENDRILS. - In some plants definite climbing organs, known as tendrils, are developed. A tendril, which has the appearance of a much-twisted stem, may be modified from part of a leaf, as an entire leaf or as part of a branch. Tendrils have the habit of at first stretching out as far from the main stem as possible, then slowly revolving. After a support is touched they immediately coil around it and then begin to curl up somewhat after the manner of a watch spring. This draws up the stem of which they are a part.

Examine the tendrils of wild grape, pea, Virginia creeper, catbrier, white bryony, or any others you may find. Notice the position of the tendril on the main stem and try to decide of what part of the plant it is a modification. Notice the suckers or disks at the ends of the tendrils of the Virginia creeper. Can you discover their use?

STEMS MODIFIED AS THORNS. Leaves and parts of leaves may be changed into thorns for the protection of the plant. In some instances the stem becomes a spine or thorn. Such is the case in the honey locust.

Compare it with the black locust, in which a part of the leaf, the stipule, becomes the thorn. All such modifications seem to result in the better protection of tender parts which might otherwise suffer from the attack of animals.

LEAFLIKE STEMS.-An examination of the hothouse smilax (Myrsiphyllum) shows us that the structures which at first sight appear to be leaves are really green, leaflike branches. This we know, because immediately below each cladophyll, as the leaflike branch is called, we find a tiny scale, evidently the leaf. The cladophyll occupies the same position with relation to the leaf that a bud which develops into a lateral branch would occupy. Thus we see it is homologous to a branch.

ROOTS AND STEMS AS FOOD. Underground stems and roots form some of the most important sources of man's food supply. Our commonest foods, as the potato, sweet potato, carrot, parsnip, turnip, and beet, are well-known examples. Onions contain considerable proteid material. The sago palm is the chief support of many of the natives of Africa. Each adult tree will furnish 700 pounds of sago meal, 2 pounds being enough to support a man one day. The cassava root, from which tapioca is made, is one of the main supports of African natives. Sugar, obtained from the stem

of the sugar cane and from the beet root, is a world-known commodity. The following table shows the proportion of foods in some of the commoner roots and stems:

REFERENCE BOOKS

FOR THE PUPIL

Andrews, Botany, All the Year Round, Chaps. VI, VII. American Book Company.
Dana, Plants and Their Children, pages 99–129. American Book Company.
Goff and Mayne, First Principles of Agriculture. American Book Company.
Atkinson, First Studies of Plant Life, Chaps. IV, V, VI, VIII, XXI.

Company.

Ginn and

Hunter and Valentine, Laboratory Manual of Biology. Henry Holt and Company. MacDougal, The Nature and Work of Plants. The Macmillan Company.

FOR THE TEACHER

Apgar, Trees of the United States, Chaps. II, V, VI. American Book Company
Goodale, Physiological Botany. American Book Company.

Gray, Structural Botany, Chap. V. American Book Company.
Leavitt, Outlines of Botany. American Book Company.
Goebel, Organography of Plants, Part V. Clarendon Press.
Ganong, The Teaching Botanist. The Macmillan Company.
Lubbock, Buds and Stipules. D. Appleton and Company.

Pinchot, A Primer of Forestry.

of Agriculture.

Bul. No. 24, Division of Forestry, U.S. Department

Strasburger, Noll, Schenck, and Schimper, A Text-book of Botany. The Macmillan

Company.

Ward, The Oak. D. Appleton and Company.

Year Book, U.S. Department of Agriculture, 1894, 1895, 1898.

IX. LEAVES AND THEIR FUNCTIONS

IN the horse-chestnut bud previously studied the brown scales which cover the green scales of the bud can be shown to be like in structure to a leaf (homologous to a leaf). This is seen better in a near relative of the horse-chestnut, the buckeye, in which a series of changes in form from brown scales to green leaves may be followed.

Stipules. The bud of the tulip tree (Liriodendron tulipifera) is an admirable source of information as to the structure of a leaf. In the buds of a tulip tree, however, the scales seem to act as wrappers around the little leaves and not to correspond to the blade of the leaf, but to an outgrowth of the leafstalk or petiole below it. The outgrowths at the base of a leaf are known as stipules. The scales in this case are stipules which have come to protect the bud at a time when the delicate parts need protection most. These stipules are present as scales in many other buds. Frequently, when the leaf becomes able to care for itself, the stipules fall off and disappear. Such a bud is the elm, as can be seen by a careful dissection. Stipules usually are paired. Notice the notched appearance of the scales as you go inward. Most leaves do not show the stipules well, although a complete leaf is supposed to be supplied with them. They are well developed in the rose leaf.

Palmately-veined leaf of the maple.

1 See Hunter and Valentine, Manual, page 37.