Outlines of Lessons in Botany Part 10

[Footnote 1: "Plants growing in pots were protected wholly from the light, or had light admitted from above or on one side as the case might require, and were covered above by a large horizontal sheet of gla.s.s, and with another vertical sheet on one side. A gla.s.s filament, not thicker than a horsehair, and from a quarter to three-quarters of an inch in length, was affixed to the part to be observed by means of sh.e.l.lac dissolved in alcohol. The solution was allowed to evaporate until it became so thick that it set hard in two or three seconds, and it never injured the tissues, even the tips of tender radicles, to which it was applied. To the end of the gla.s.s filament an excessively minute bead of black sealing-wax was cemented, below or behind which a bit of card with a black dot was fixed to a stick driven into the ground.... The bead and the dot on the card were viewed through the horizontal or vertical gla.s.s-plate (according to the position of the object) and when one exactly covered the other, a dot was made on the gla.s.s plate with a sharply pointed stick dipped in thick India ink. Other dots were made at short intervals of time and these were afterwards joined by straight lines. The figures thus traced were therefore angular, but if dots had been made every one or two minutes, the lines would have been more curvilinear."--The Power of Movement in Plants, p. 6.]

The use of the gla.s.s filament is simply to increase the size of the circle described, and thus make visible the movements of the stem. All young parts of stems are continually moving in circles or ellipses. "To learn how the sweeps are made, one has only to mark a line of dots along the upper side of the outstretched revolving end of such a stem, and to note that when it has moved round a quarter of a circle, these dots will be on one side; when half round, the dots occupy the lower side; and when the revolution is completed, they are again on the upper side. That is, the stem revolves by bowing itself over to one side,--is either pulled over or pushed over, or both, by some internal force, which acts in turn all round the stem in the direction in which it sweeps; and so the stem makes its circuits without twisting."[1]

[Footnote 1: How Plants Behave. By Asa Gray. Ivison, Blakeman, Taylor & Co., New York, 1872. Page 13.]

The nature of the movement is thus a successive nodding to all the points of the compa.s.s, whence it is called by Darwin _circ.u.mnutation_. The movement belongs to all young growing parts of plants. The great sweeps of a twining stem, like that of the Morning-Glory, are only an increase in the size of the circle or ellipse described.[1]

[Footnote 1: "In the course of the present volume it will be shown that apparently every growing part of every plant is continually circ.u.mnutating, though often on a small scale. Even the stems of seedlings before they have broken through the ground, as well as their buried radicles, circ.u.mnutate, as far as the pressure of the surrounding earth permits. In this universally present movement we have the basis or groundwork for the acquirement, according to the requirements of the plant, of the most diversified movements. Thus the great sweeps made by the stems of the twining plants, and by the tendrils of other climbers, result from a mere increase in the amplitude of the ordinary movement of circ.u.mnutation."--The Power of Movement in Plants, p. 3.]

When a young stem of a Morning-Glory, thus revolving, comes in contact with a support, it will twist around it, unless the surface is too smooth to present any resistance to the movement of the plant. Try to make it twine up a gla.s.s rod. It will slip up the rod and fall off. The Morning-Glory and most twiners move around from left to right like the hands of a clock, but a few turn from right to left.

While this subject is under consideration, the tendrils of the Pea and Bean and the twining petioles of the Nasturtium will be interesting for comparison. The movements can be made visible by the same method as was used for the stem of the Morning-Glory. Tendrils and leaf petioles are often sensitive to the touch. If a young leaf stalk of Clematis be rubbed for a few moments, especially on the under side, it will be found in a day or two to be turned inward, and the tendrils of the Cuc.u.mber vine will coil in a few minutes after being thus irritated.[1] The movements of tendrils are charmingly described in the chapter ent.i.tled "How Plants Climb," in the little treatise by Dr. Gray, already mentioned.

[Footnote 1: Reader in Botany. X. Climbing Plants.]

The so-called "sleep of plants" is another similar movement. The Oxalis is a good example. The leaves droop and close together at night, protecting them from being chilled by too great radiation.

The cause of these movements is believed to lie in changes of tension preceding growth in the tissues of the stem.[1] Every stem is in a state of constant tension. Naudin has thus expressed it, "the interior of every stem is too large for its Jacket."[2] If a leaf-stalk of Nasturtium be slit vertically for an inch or two, the two halves will spring back abruptly. This is because the outer tissues of the stem are stretched, and spring back like india-rubber when released. If two stalks twining in opposite directions be slit as above described, the side of the stem towards which each stalk is bent will spring back more than the other, showing the tension to be greater on that side. A familiar ill.u.s.tration of this tension will be found in the Dandelion curls of our childhood.

[Footnote 1: See Physiological Botany. By Geo. L. Goodale. Ivison & Co., New York, 1885. Page 406.]

[Footnote 2: The following experiment exhibits the phenomenon of tension very strikingly. "From a long and thrifty young internode of grapevine cut a piece that shall measure exactly one hundred units, for instance, millimeters. From this section, which measures exactly one hundred millimeters, carefully separate the epidermal structures in strips, and place the strips at once under an inverted gla.s.s to prevent drying; next, separate the pith in a single unbroken piece wholly freed from the ligneous tissue. Finally, remeasure the isolated portions, and compare with the original measure of the internode. There will be found an appreciable shortening of the epidermal tissues and a marked increase in length of the pith."--Physiological Botany, p. 391.]

The movements of the Sensitive Plant are always very interesting to pupils, and it is said not to be difficult to raise the plants in the schoolroom. The whole subject, indeed, is one of the most fascinating that can be found, and its literature is available, both for students and teachers. Darwin's essay on "Climbing Plants," and his later work on the "Power of Movement in Plants," Dr. Gray's "How Plants Behave," and the chapter on "Movements" in the "Physiological Botany," will offer a wide field for study and experiment.

3. _Structure of Stems_.--Let the pupils collect a series of branches of some common tree or shrub, from the youngest twig up to as large a branch as they can cut, and describe them. Poplar, Elm, Oak, Lilac, etc., will be found excellent for the purpose.

While discussing these descriptions, a brief explanation of plant-structure may be given. In treating this subject, the teacher must govern himself by the needs of his cla.s.s, and the means at his command.

Explanations requiring the use of a compound microscope do not enter necessarily into these lessons. The object aimed at is to teach the pupils about the things which they can see and handle for themselves. Looking at sections that others have prepared is like looking at pictures; and, while useful in opening their eyes and minds to the wonders hidden from our una.s.sisted sight, fails to give the real benefit of scientific training.

Plants are built up of cells. The delicate-walled spherical, or polygonal, cells which make up the bulk of an herbaceous stem, const.i.tute cellular tissue (_parenchyma_). This was well seen in the stem of the cutting of Bean in which the roots had begun to form.[1] The strengthening fabric in almost all flowering plants is made up of woody bundles, or woody tissue.[2] The wood-cells are cells which are elongated and with thickened walls. There are many kinds of them. Those where the walls are very thick and the cavity within extremely small are _fibres_. A kind of cell, not strictly woody, is where many cells form long vessels by the breaking away of the connecting walls. These are _ducts_. These two kinds of cells are generally a.s.sociated together in woody bundles, called therefore fibro-vascular bundles. We have already spoken of them as making the dots on the leaf-scars, and forming the strengthening fabric of the leaves.[3]

[Footnote 1: See page 46.]

[Footnote 2: If elements of the same kind are untied, they const.i.tute a tissue to which is given the name of those elements; thus parenchyma cells form parenchyma tissue or simply parenchyma; cork-cells form cork, etc. A tissue can therefore be defined as a fabric of united cells which have had a common origin and obeyed a common law of growth.--Physiological Botany.

p. 102.]

[Footnote 3: See page 58.]

We will now examine our series of branches. The youngest twigs, in spring or early summer, are covered with a delicate, nearly colorless skin.

Beneath this is a layer of bark, usually green, which gives the color to the stem, an inner layer of bark, the wood and the pith. The pith is soft, spongy and somewhat sappy. There is also sap between the bark and the wood. An older twig has changed its color. There is a layer of brown bark, which has replaced the colorless skin. In a twig a year old the wood is thicker and the pith is dryer. Comparing sections of older branches with these twigs, we find that the pith has shrunk and become quite dry, and that the wood is in rings. It is not practicable for the pupils to compare the number of these rings with the bud-rings, and so find out for themselves that the age of the branch can be determined from the wood, for in young stems the successive layers are not generally distinct. But, in all the specimens, the sap is found just between the wood and the bark, and here, where the supply of food is, is where the growth is taking place. Each year new wood and new bark are formed in this _cambium-layer_, as it is called, new wood on its inner, new bark on its outer face. Trees which thus form a new ring of wood every year are called _exogenous_, or outside-growing.

Ask the pupils to separate the bark into its three layers and to try the strength of each. The two outer will easily break, but the inner is generally tough and flexible. It is this inner bark, which makes the Poplar and Willow branches so hard to break. These strong, woody fibres of the inner bark give us many of our textile fabrics. Flax and Hemp come from the inner bark of their respective plants (_Linum usitatissimum_ and _Cannabis sativa_), and Russia matting is made from the bark of the Linden (_Tilia Americana_).

We have found, in comparing the bark of specimens of branches of various ages, that, in the youngest stems, the whole is covered with a skin, or _epidermis_, which is soon replaced by a brown outer layer of bark, called the _corky layer_; the latter gives the distinctive color to the tree.

While this grows, it increases by a living layer of cork-cambium on its inner face, but it usually dies after a few years. In some trees it goes on growing for many years. It forms the layers of bark in the Paper Birch and the cork of commerce is taken from the Cork Oak of Spain. The green bark is of cellular tissue, with some green coloring matter like that of the leaves; it is at first the outer layer, but soon becomes covered with cork. It does not usually grow after the first year. Sc.r.a.ping the bark of an old tree, we find the bark h.o.m.ogeneous. The outer layers have perished and been cast off. As the tree grows from within, the bark is stretched and, if not replaced, cracks and falls away piecemeal. So, in most old trees, the bark consists of successive layers of the inner woody bark.

Stems can be well studied from pieces of wood from the woodpile. The ends of the log will show the concentric rings. These can be traced as long, wavy lines in vertical sections of the log, especially if the surface is smooth. If the pupils can whittle off different planes for themselves, they will form a good idea of the formation of the wood. In many of the specimens there will be knots, and the nature of these will be an interesting subject for questions. If the knot is near the centre of the log, lead back their thoughts to the time when the tree was as small as the annular ring on which the centre of the knot lies. Draw a line on this ring to represent the tree at this period of its growth. What could the knot have been? It has concentric circles like the tree itself. It was a branch which decayed, or was cut off. Year after year, new rings of wood formed themselves round this broken branch, till it was covered from sight, and every year left it more deeply buried in the trunk.

Extremely interesting material for the study of wood will be found in thin sections prepared for veneers. Packages of such sections will be of great use to the teacher.[1] They show well the reason of the formation of a dividing line between the wood of successive seasons. In a cross section of Oak or Chestnut the wood is first very open and porous and then close.

This is owing to the presence of ducts in the wood formed in the spring.

In other woods there are no ducts, or they are evenly distributed, but the transition from the close autumn wood, consisting of smaller and more closely packed cells, to the wood of looser texture, formed in the following spring, makes a line that marks the season's growth.

[Footnote 1: Mr. Romeyn B. Hough, of Lowville, N.Y., will supply a package of such sections for one dollar. The package will consist of several different woods, in both cross and vertical section and will contain enough duplicates for an ordinary cla.s.s.

He also issues a series of books on woods ill.u.s.trated by actual and neatly mounted specimens, showing in each case three distinct views of the grain.

The work is issued in parts, each representing twenty-five species, and selling with text at $5, expressage prepaid; the mounted specimens alone at 25 cts. per species or twenty-five in neat box for $4. He has also a line of specimens prepared for the stereopticon and another for the microscope. They are very useful and sell at 50 cts. per species or twenty-five for $10.]

Let each of the scholars take one of the sections of Oak and write a description of its markings. The age is easily determined; the pith rays, or _medullary rays_, are also plain. These form what is called the silver grain of the wood. The ducts, also, are clear in the Oak and Chestnut.

There is a difference in color between the outer and inner wood, the older wood becomes darker and is called the _heart-wood_, the outer is the _sap-wood_. In Birds-eye Maple, and some other woods, the abortive buds are seen. They are buried in the wood, and make the disturbance which produces the ornamental grain. In sections of Pine or Spruce, no ducts can be found. The wood consists entirely of elongated, thickened cells or fibres. In some of the trees the pith rays cannot be seen with the naked eye.

Let the pupils compare the branches which they have described, with a stalk of Asparagus, Rattan, or Lily. A cross section of one of these shows dots among the soft tissue. These are ends of the fibro-vascular bundles, which in these plants are scattered through the cellular tissue instead of being brought together in a cylinder outside of the pith. In a vertical section they appear as lines. There are no annular rings.

If possible, let the pupils compare the leaves belonging to these different types of stems. The parallel-veined leaves of monocotyledons have stems without distinction of wood, bark and pith; the netted-veined leaves of dicotyledons have exogenous stems.

Dicotyledons have bark, wood, and pith, and grow by producing a new ring of wood outside the old. They also increase by the growth of the woody bundles of the leaves, which mingle with those of the stem.[1] Twist off the leaf-stalk of any leaf, and trace the bundles into the stem.

[Footnote 1: See note, p. 127, Physiological Botany.]

Monocotyledons have no layer which has the power of producing new wood, and their growth takes place entirely from the intercalation of new bundles, which originate at the bases of the leaves. The lower part of a stem of a Palm, for instance, does not increase in size after it has lost its crown of leaves. This is carried up gradually. The upper part of the stem is a cone, having fronds, and below this cone the stem does not increase in diameter. The word _endogenous_, inside-growing, is not, therefore, a correct one to describe the growth of most monocotyledons, for the growth takes place where the leaves originate, near the exterior of the stem.

_Gray's First Lessons_. Sect. VI. Sect, XVI, --1, 401-13. --3. --6, 465-74.

_How Plants Grow_. Chap. 1, 82, 90-118.

VI.

LEAVES.

We have studied leaves as cotyledons, bud-scales, etc., but when we speak of _leaves_, we do not think of these adapted forms, but of the green foliage of the plant.

1. _Forms and Structure_.--Provide the pupils with a number of green leaves, ill.u.s.trating simple and compound, pinnate and palmate, sessile and petioled leaves. They must first decide the question, _What are the parts of a leaf_? All the specimens have a green _blade_ which, in ordinary speech, we call the leaf. Some have a stalk, or _petiole_, others are joined directly to the stem. In some of them, as a rose-leaf, for instance, there are two appendages at the base of the petiole, called _stipules_. These three parts are all that any leaf has, and a leaf that has them all is complete.

Let us examine the blade. Those leaves which have the blade in one piece are called _simple_; those with the blade in separate pieces are _compound_. We have already answered the question, _What const.i.tutes a single leaf_?[1] Let the pupils repeat the experiment of cutting off the top of a seedling Pea, if it is not already clear in their minds, and find buds in the leaf-axils of other plants.[2]

[Footnote 1: See page 31.]

[Footnote 2: With one cla.s.s of children, I had much difficulty in making them understand the difference between simple and compound leaves. I did not tell them that the way to tell a single leaf was to look for buds in the axils, but incautiously drew their attention to the stipules at the base of a rose leaf as a means of knowing that the whole was one. Soon after, they had a locust leaf to describe; and, immediately, with the acuteness that children are apt to develop so inconveniently to their teacher, they triumphantly refuted my statement that it was one leaf, by pointing to the stiples. There was no getting over the difficulty; and although I afterwards explained to them about the position of the buds, and showed them examples, they clung with true childlike tenacity to their first impression and always insisted that they could not see why each leaflet was not a separate leaf.]

An excellent way to show the nature of compound leaves is to mount a series showing every gradation of cutting, from a simple, serrate leaf to a compound one (Figs. 24 and 25). A teacher, who would prepare in summer such ill.u.s.trations as these, would find them of great use in his winter lessons. The actual objects make an impression that the cuts in the book cannot give.

[Ill.u.s.tration: FIG. 24.--Series of palmately-veined leaves.]

[Ill.u.s.tration: FIG. 25.--Series of pinnately-veined leaves.]

Let the pupils compare the distribution of the veins in their specimens.