The leaf-scars are semicircular, small and swollen.
The bud-rings are plain. The twigs make a very small growth in a season, so that the leaf-scars and rings make them exceedingly rough.
The flower-cluster scars are small circles, with a dot in the centre, in the leaf-axils. The flowers come before the leaves.
The leaf-arrangement is alternate on the 2/5 plan. The pupils may compare the branching with that of their other specimens.
RED MAPLE (Acer rubrum).
This is a good specimen for the study of accessory buds. There is usually a bud in the axil of each lower scale of the axillary buds, making three side by side. We have already noticed this as occurring sometimes in Lilac. It is habitually the case with the Red Maple. The middle bud, which is smaller and develops later, is a leaf-bud. The others are flower-buds.
The leaf-scars are small, with three dots on each scar. The rings are very plain. The flower-cluster leaves a round scar in the leaf-axil, as in Cherry.
The leaves are opposite and the tree branches freely. The twigs seem to be found just below the bud-rings, as the upper leaf-buds usually develop best and the lower buds are single, containing flowers only.
NORWAY SPRUCE (Picea excelsa).
The buds are terminal, and axillary, from the axils of the leaves of the preceding year, usually from those at the ends of the branchlets. They are covered with brown scales and contain many leaves.
The leaves are needle-shaped and short. They are arranged densely on the branches, alternately on the 8/21 plan (see section on phyllotaxy). When they drop off they leave a hard, blunt projection which makes the stem very rough. As the terminal bud always develops unless injured, the tree is excurrent, forming a straight trunk, throwing out branches on every side. The axillary buds develop near the ends of the branchlets, forming apparent whorls of branches around the trunk. In the smaller branches, as the tree grows older, the tendency is for only two buds to develop nearly opposite each other, forming a symmetrical branch.
[Footnote 1: The pupils should observe how much more crowded the leaves are than in the other trees they have studied. The leaves being smaller, it is necessary to have more of them. Large-leaved trees have longer internodes than those with small leaves.]
The bud-scales are persistent on the branches and the growth from year to year can be traced a long way back.
The cones hang on the ends of the upper branches. They are much larger than in our native species of Black and White Spruce.
The Evergreens are a very interesting study and an excellent exercise in morphology for the older scholars.
2. Vernation. This term signifies the disposition of leaves in the bud, either in respect to the way in which each leaf is folded, or to the manner in which the leaves are arranged with reference to each other. The pupils have described the folding of the leaves in some of their specimens.
In the Beech, the leaf is plicate, or plaited on the veins. In the Elm, Magnolia, and Tulip-tree, it is conduplicate, that is, folded on the midrib with the inner face within. In the Tulip-tree, it is also inflexed, the blade bent forwards on the petiole. In the Balm of Gilead, the leaf is involute, rolled towards the midrib on the upper face.
Other kinds of vernation are revolute, the opposite of involute, where the leaf is rolled backwards towards the midrib; circinate, rolled from the apex downwards, as we see in ferns; and corrugate, when the leaf is crumpled in the bud.
In all the trees we have studied, the leaves simply succeed each other, each leaf, or pair of leaves, overlapping the next in order. The names of the overlapping of the leaves among themselves, imbricated, convolute, etc., will not be treated here, as they are not needed. They will come under aestivation, the term used to describe the overlapping of the modified leaves, which make up the flower.
[Footnote 1: Reader in Botany. VIII. Young and Old Leaves.]
3. Phyllotaxy. The subject of leaf-arrangement is an extremely difficult one, and it is best, even with the older pupils, to touch it lightly. The point to be especially brought out is the disposition of the leaves so that each can get the benefit of the light. This can be seen in any plant and there are many ways in which the desired result is brought about. The chief way is the distribution of the leaves about the stem, and this is well studied from the leaf-scars.
The scholars should keep the branches they have studied. It is well to have them marked with the respective names, that the teacher may examine and return them without fear of mistakes.
In the various branches that the pupils have studied, they have seen that the arrangement of the leaves differs greatly. The arrangement of leaves is usually classed under three modes: the alternate, the opposite, and the whorled; but the opposite is the simplest form of the whorled arrangement, the leaves being in circles of two. In this arrangement, the leaves of each whorl stand over the spaces of the whorl just below. The pupils have observed and noted this in Horsechestnut and Lilac. In these there are four vertical rows or ranks of leaves. In whorls of three leaves there would be six ranks, in whorls of four, eight, and so on.
When the leaves are alternate, or single at each node of the stem, they are arranged in many different ways. Ask the pupils to look at all the branches with alternate leaves that they have studied, and determine in each case what leaves stand directly over each other. That is, beginning with any leaf, count the number of leaves passed on the stem, till one is reached that stands directly over the first. In the Beech and the Elm the leaves are on opposite sides of the stem, so that the third stands directly over the first. This makes two vertical ranks, or rows, of leaves, dividing the circle into halves. It is, therefore, called the 1/2 arrangement. Another way of expressing it is to say that the angular divergence between the leaves is 180 deg., or one-half the circumference.
[Footnote 1: The pupils must be careful not to pass the bud-rings when they are counting the leaves.]
The 1/3 arrangement, with the leaves in three vertical ranks, is not very common. It may be seen in Sedges, in the Orange-tree, and in Black Alder (Ilex verticillata). In this arrangement, there are three ranks of leaves, and each leaf diverges from the next at an angle of 120 deg., or one-third of the circumference.
By far the commonest arrangement is with the leaves in five vertical ranks. The Cherry, the Poplar, the Larch, the Oak, and many other trees exhibit this. In this arrangement there are five leaves necessary to complete the circle. We might expect, then, that each leaf would occupy one-fifth of the circle. This would be the case were it not for the fact that we have to pass twice around the stem in counting them, so that each leaf has twice as much room, or two-fifths of the circle, to itself. This is, therefore, the 2/5 arrangement. This can be shown by winding a thread around the stem, passing it over each leaf-scar. In the Beech we make one turn of the stem before reaching the third leaf which stands over the first. In the Apple the thread will wind twice about the stem, before coming to the sixth leaf, which is over the first.
Another arrangement, not very common, is found in the Magnolia, the Holly, and the radical leaves of the common Plantain and Tobacco. The thread makes three turns of the stem before reaching the eighth leaf which stands over the first. This is the 3/8 arrangement. It is well seen in the Marguerite, a greenhouse plant which is very easily grown in the house.
Look now at these fractions, 1/2, 1/3, 2/5, and 3/8. The numerator of the third is the sum of the numerators of the first and second, its denominator, the sum of the two denominators. The same is true of the fourth fraction and the two immediately preceding it. Continuing the series, we get the fractions 5/13, 8/21, 13/34. These arrangements can be found in nature in cones, the scales of which are modified leaves and follow the laws of leaf-arrangement.
[Footnote 1: See the uses and origin of the arrangement of leaves in plants. By Chauncey Wright. Memoirs Amer. Acad., IX, p. 389. This essay is an abstruse mathematical treatise on the theory of phyllotaxy. The fractions are treated as successive approximations to a theoretical angle, which represents the best possible exposure to air and light.
Modern authors, however, do not generally accept this mathematical view of leaf-arrangement.]
"It is to be noted that the distichous or 1/2 variety gives the maximum divergence, namely 180 deg., and that the tristichous, or 1/3, gives the least, or 120 deg.; that the pentastichous, or 2/5, is nearly the mean between the first two; that of the 3/8, nearly the mean between the two preceding, etc. The disadvantage of the two-ranked arrangement is that the leaves are soon superposed and so overshadow each other. This is commonly obviated by the length of the internodes, which is apt to be much greater in this than in the more complex arrangements, therefore placing them vertically further apart; or else, as in Elms, Beeches, and the like, the branchlets take a horizontal position and the petioles a quarter twist, which gives full exposure of the upper face of all the leaves to the light. The 1/3 and 2/5, with diminished divergence, increase the number of ranks; the 3/8 and all beyond, with mean divergence of successive leaves, effect a more thorough distribution, but with less and less angular distance between the vertical ranks."
[Footnote 1: Gray's Structural Botany, Chap, iv, p. 126.]
For directions for finding the arrangement of cones, see Gray's Structural Botany, Chap. IV, Sect. 1.
The subject appears easy when stated in a text-book, but, practically, it is often exceedingly difficult to determine the arrangement. Stems often twist so as to alter entirely the apparent disposition of the leaves. The general principle, however, that the leaves are disposed so as to get the best exposure to air and light is clear. This cannot be shown by the study of the naked branches merely, because these do not show the beautiful result of the distribution. Many house plants can be found, which will afford excellent illustrations (Fig. 21). The Marguerite and Tobacco, both easily grown in the house, are on the 3/8 plan. The latter shows the eight ranks most plainly in the rosette of its lower leaves. The distribution is often brought about by differences in the lengths of the petioles, as in a Horsechestnut branch (Fig. 22) where the lower, larger leaves stand out further from the branch than the upper ones; or by a twist in the petioles, so that the upper faces of the leaves are turned up to the light, as in Beech (Fig. 23). If it is springtime when the lessons are given, endless adaptations can be found.
[Footnote 1: Reader in Botany. IX. Leaf-Arrangement.]
Gray's First Lessons. Sect. IV. VII, sec. 4. How Plants Grow. Chap. I, 51-62; I, 153.
The stem, as the scholars have already learned, is the axis of the plant. The leaves are produced at certain definite points called nodes, and the portions of stem between these points are internodes. The internode, node, and leaf make a single plant-part, and the plant is made up of a succession of such parts.
The stem, as well as the root and leaves, may bear plant-hairs. The accepted theory of plant structure assumes that these four parts, root, stem, leaves, and plant-hairs, are the only members of a flowering plant, and that all other forms, as flowers, tendrils, etc., are modified from these. While this idea is at the foundation of all our teaching, causing us to lead the pupil to recognize as modified leaves the cotyledons of a seedling and the scales of a bud, it is difficult to state it directly so as to be understood, except by mature minds. I have been frequently surprised at the failure of even bright and advanced pupils to grasp this idea, and believe it is better to let them first imbibe it unconsciously in their study. Whenever their minds are ready for it, it will be readily understood. The chief difficulty is that they imagine that there is a direct metamorphosis of a leaf to a petal or a stamen.
Briefly, the theory is this: the beginnings of leaf, petal, tendril, etc., are the same. At an early stage of their growth it is impossible to tell what they are to become. They develop into the organ needed for the particular work required of them to do. The organ, that under other circumstances might develop into a leaf, is capable of developing into a petal, a stamen, or a pistil, according to the requirements of the plant, but no actual metamorphosis takes place. Sometimes, instead of developing into the form we should normally find, the organ develops into another form, as when a petal stands in the place of a stamen, or the pistil reverts to a leafy branch. This will be more fully treated under flowers. The study of the different forms in which an organ may appear is the study of morphology.
1. Forms of Stems.—Stems may grow in many ways. Let the pupils compare the habits of growth of the seedlings they have studied. The Sunflower and Corn are erect. This is the most usual habit, as with our common trees. The Morning Glory is twining, the stem itself twists about a support. The Bean, Pea and Nasturtium are climbing. The stems are weak, and are held up, in the first two by tendrils, in the last by the twining leaf-stalks. The English Ivy, as we have seen, is also climbing, by means of its aerial roots. The Red Clover is ascending, the branches rising obliquely from the base. Some kinds of Clover, as the White Clover, are creeping, that is, with prostrate branches rooting at the nodes and forming new plants. Such rooting branches are called stolons, or when the stem runs underground, suckers. The gardener imitates them in the process called layering, that is, bending down an erect branch and covering it with soil, causing it to strike root. When the connecting stem is cut, a new plant is formed. Long and leafless stolons, like those of the Strawberry are called runners. Stems creep below the ground as well as above. Probably the pupil will think of some examples. The pretty little Gold Thread is so named from the yellow running stems, which grow beneath the ground and send up shoots, or suckers, which make new plants. Many grasses propagate themselves in this way. Such stems are called rootstocks. "That these are really stems, and not roots, is evident from the way in which they grow; from their consisting of a succession of joints; and from the leaves which they bear on each node, in the form of small scales, just like the lowest ones on the upright stem next the ground. They also produce buds in the axils of these scales, showing the scales to be leaves; whereas real roots bear neither leaves nor axillary buds." Rootstocks are often stored with nourishment. We have already taken up this subject in the potato, but it is well to repeat the distinction between stems and roots. A thick, short rootstock provided with buds, like the potato, is called a tuber. Compare again the corm of Crocus and the bulb of Onion to find the stem in each. In the former, it makes the bulk of the whole; in the latter, it is a mere plate holding the fleshy bases of the leaves.
[Footnote 1: Gray's First Lessons, revised edition, 1887, page 42.]
2. Movements of Stems.—Let a glass thread, no larger than a coarse hair, be affixed by means of some quickly drying varnish to the tip of the laterally inclined stem of one of the young Morning-Glory plants in the schoolroom. Stand a piece of cardboard beside the pot, at right angles to the stem, so that the end of the glass will be near the surface of the card. Make a dot upon the card opposite the tip of the filament, taking care not to disturb the position of either. In a few minutes observe that the filament is no longer opposite the dot. Mark its position anew, and continue thus until a circle is completed on the cardboard. This is a rough way of conducting the experiment. Darwin's method will be found in the footnote.
[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 glass, and with another vertical sheet on one side. A glass 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 shellac 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 glass 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 glass-plate (according to the position of the object) and when one exactly covered the other, a dot was made on the glass 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 glass 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."
[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 compass, whence it is called by Darwin circumnutation. 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.
[Footnote 1: "In the course of the present volume it will be shown that apparently every growing part of every plant is continually circumnutating, though often on a small scale. Even the stems of seedlings before they have broken through the ground, as well as their buried radicles, circumnutate, 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 circumnutation."—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 glass 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 Cucumber vine will coil in a few minutes after being thus irritated. The movements of tendrils are charmingly described in the chapter entitled "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. 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." 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 illustration 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 glass 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 class, 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 unassisted 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, constitute cellular tissue (parenchyma). This was well seen in the stem of the cutting of Bean in which the roots had begun to form. The strengthening fabric in almost all flowering plants is made up of woody bundles, or woody tissue. 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 associated 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.
[Footnote 1: See page 46.]
[Footnote 2: If elements of the same kind are untied, they constitute 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. Scraping the bark of an old tree, we find the bark homogeneous. 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. 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 class.
He also issues a series of books on woods illustrated 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. 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, sec. 1, 401-13. sec. 3. sec. 6, 465-74.
How Plants Grow. Chap. 1, 82, 90-118.
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, illustrating 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 constitutes a single leaf? 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.
[Footnote 1: See page 31.]
[Footnote 2: With one class 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 illustrations 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.
Let the pupils compare the distribution of the veins in their specimens. They have already distinguished parallel-veined from netted-veined leaves, and learned that this difference is a secondary distinction between monocotyledons and dicotyledons. The veins in netted-veined leaves are arranged in two ways. The veins start from either side of a single midrib (feather-veined or pinnately-veined), or they branch from a number of ribs which all start from the top of the petiole, like the fingers from the palm of the hand (palmately-veined). The compound leaves correspond to these modes of venation; they are either pinnately or palmately compound.
[Footnote 1: See page 34.]
These ribs and veins are the woody framework of the leaf, supporting the soft green pulp. The woody bundles are continuous with those of the stem, and carry the crude sap, brought from the roots, into the cells of every part of the leaf, where it is brought into contact with the external air, and the process of making food (Assimilation 4) is carried on. "Physiologically, leaves are green expansions borne by the stern, outspread in the air and light, in which assimilation and the processes connected with it are carried on."
[Footnote 1: Gray's Structural Botany, p. 85.]
The whole leaf is covered with a delicate skin, or epidermis, continuous with that of the stem.
[Footnote 1: Reader in Botany. XI. Protection of Leaves from the Attacks of Animals.]
2. Descriptions.—As yet the pupils have had no practice in writing technical descriptions. This sort of work may be begun when they come to the study of leaves. In winter a collection of pressed specimens will be useful. Do not attach importance to the memorizing of terms. Let them be looked up as they are needed, and they will become fixed by practice. The pupils may fill out such schedules as the following with any leaves that are at hand.
SCHEDULE FOR LEAVES.
Simple or compound. Simple (arr. and no. of leaflets) Venation Netted and feather-veined Shape Oval 1. BLADE < Apex Acute Base Oblique Margin Slightly wavy Surface Smooth
2. PETIOLE Short; hairy
3. STIPULES Deciduous
Remarks. Veins prominent and very straight.
[Footnote 1: The specimen described is a leaf of Copper Beech.]
In describing shapes, etc., the pupils can find the terms in the book as they need them. It is desirable at first to give leaves that are easily matched with the terms, keeping those which need compound words, such as lance-ovate, etc., to come later. The pupils are more interested if they are allowed to press and keep the specimens they have described. It is not well to put the pressed leaves in their note books, as it is difficult to write in the books without spoiling the specimens. It is better to mount the specimens on white paper, keeping these sheets in brown paper covers. The pupils can make illustrations for themselves by sorting leaves according to the shapes, outlines, etc., and mounting them.
3. Transpiration.—This term is used to denote the evaporation of water from a plant. The evaporation takes place principally through breathing pores, which are scattered all over the surface of leaves and young stems. The breathing pores, or stomata, of the leaves, are small openings in the epidermis through which the air can pass into the interior of the plant. Each of these openings is called a stoma. "They are formed by a transformation of some of the cells of the epidermis; and consist usually of a pair of cells (called guardian cells), with an opening between them, which communicates with an air-chamber within, and thence with the irregular intercellular spaces which permeate the interior of the leaf. Through the stomata, when open, free interchange may take place between the external air and that within the leaf, and thus transpiration be much facilitated. When closed, this interchange will be interrupted or impeded."
[Footnote 1: Gray's Structural Botany, page 89. For a description of the mechanism of the stomata, see Physiological Botany, p. 269.]
In these lessons, however, it is not desirable to enter upon subjects involving the use of the compound microscope. Dr. Goodale says: "Whether it is best to try to explain to the pupils the structure of these valves, or stomata, must be left to each teacher. It would seem advisable to pass by the subject untouched, unless the teacher has become reasonably familiar with it by practical microscopical study of leaves. For a teacher to endeavor to explain the complex structure of the leaf, without having seen it for himself, is open to the same objection which could be urged against the attempted explanation of complicated machinery by one who has never seen it, but has heard about it. What is here said with regard to stomata applies to all the more recondite matters connected with plant structure."
[Footnote 1: Concerning a few Common Plants, p. 29.]
There are many simple experiments which can be used to illustrate the subject.
(1) Pass the stem of a cutting through a cork, fitting tightly into the neck of a bottle of water. Make the cork perfectly air-tight by coating it with beeswax or paraffine. The level of the liquid in the bottle will be lowered by the escape of water through the stem and leaves of the cutting into the atmosphere.
(2) Cut two shoots of any plant, leave one on the table and place the other in a glass of water. The first will soon wilt, while the other will remain fresh. If the latter shoot be a cutting from some plant that will root in water, such as Ivy, it will not fade at all. Also, leave one of the plants in the schoolroom unwatered for a day or two, till it begins to wilt. If the plant be now thoroughly watered, it will recover and the leaves will resume their normal appearance.
[Footnote 1: Lessons in Elementary Botany, by Daniel Oliver, London. Macmillan & Co., 1864, pp. 14-15.]
Evaporation is thus constantly taking place from the leaves, and if there is no moisture to supply the place of what is lost, the cells collapse and the leaf, as we say, wilts. When water is again supplied the cells swell and the leaf becomes fresh.
(3) Place two seedlings in water, one with its top, the other with its roots in the jar. The latter will remain fresh while the first wilts and dies.
Absorption takes place through the roots. The water absorbed is drawn up through the woody tissues of the stem (4), and the veins of the leaves (5), whence it escapes into the air (6).
(4) Plunge a cut branch immediately into a colored solution, such as aniline red, and after a time make sections in the stem above the liquid to see what tissues have been stained.
[Footnote 1: The Essentials of Botany, by Charles E. Bessey. New York, Henry Holt & Co., 1884. Page 74. See also Physiological Botany, pp. 259-260.]
(5) "That water finds its way by preference through the fibro-vascular bundles even in the more delicate parts, is shown by placing the cut peduncle of a white tulip, or other large white flower, in a harmless dye, and then again cutting off its end in order to bring a fresh surface in contact with the solution, when after a short time the dye will mount through the flower-stalk and tinge the parts of the perianth according to the course of the bundles."
[Footnote 1: If the stems of flowers are cut under water they will last a wonderfully long time. "One of the most interesting characteristics of the woody tissues in relation to the transfer of water is the immediate change which the cut surface of a stem undergoes upon exposure to the air, unfitting it for its full conductive work. De Vries has shown that when a shoot of a vigorous plant, for instance a Helianthus, is bent down under water, care being taken not to break it even in the slightest degree, a clean, sharp cut will give a surface which will retain the power of absorbing water for a long time; while a similar shoot cut in the open air, even if the end is instantly plunged under water, will wither much sooner than the first."—Physiological Botany, p. 263.]
[Footnote 2: Physiological Botany, p. 260.]
(6) Let the leaves of a growing plant rest against the window-pane. Moisture will be condensed on the cold surface of the glass, wherever the leaf is in contact with it. This is especially well seen in Nasturtium (Tropaeolum) leaves, which grow directly against a window, and leave the marks even of their veining on the glass, because the moisture is only given out from the green tissue, and where the ribs are pressed against the glass it is left dry.
Sometimes the water is drawn up into the cells of the leaves faster than it can escape into the atmosphere. This is prettily shown if we place some of our Nasturtium seedlings under a ward-case. The air in the case is saturated with moisture, so that evaporation cannot take place, but the water is, nevertheless, drawn up from the roots and through the branches, and appears as little drops on the margins of the leaves. That this is owing to the absorbing power of the roots, may be shown by breaking off the seedling, and putting the slip in water. No drops now appear on the leaves, but as soon as the cutting has formed new roots, the drops again appear.
[Footnote 1: See Lectures on the Physiology of Plants. By Sidney Howard Vines, Cambridge, England. University Press, 1886. Page 92.]
This constant escape of water from the leaves causes a current to flow from the roots through the stem into the cells of the leaves. The dilute mineral solutions absorbed by the roots are thus brought where they are in contact with the external air, concentrated by the evaporation of water, and converted in these cells into food materials, such as starch. The presence of certain mineral matters, as potassium, iron, etc., are necessary to this assimilating process, but the reason of their necessity is imperfectly understood, as they do not enter in the products formed.
[Footnote 1: See page 48.]
The amount of water exhaled is often very great. Certain plants are used for this reason for the drainage of wet and marshy places. The most important of these is the Eucalyptus tree.
[Footnote 1: Reader in Botany. XII. Transpiration.]
"The amount of water taken from the soil by the trees of a forest and passed into the air by transpiration is not so large as that accumulated in the soil by the diminished evaporation under the branches. Hence, there is an accumulation of water in the shade of forests which is released slowly by drainage. But if the trees are so scattered as not materially to reduce evaporation from the ground, the effect of transpiration in diminishing the moisture of the soil is readily shown. It is noted, especially in case of large plants having a great extent of exhaling surface, such, for instance, as the common sunflower. Among the plants which have been successfully employed in the drainage of marshy soil by transpiration probably the species of Eucalyptus (notably E. globulus) are most efficient."
[Footnote 1: Reader in Botany. XIII. Uses of the Forests.]
[Footnote 2: Physiological Botany, page 283.]
4. Assimilation.—It is not easy to find practical experiments on assimilation. Those which follow are taken from "Physiological Botany" (p. 305).
Fill a five-inch test tube, provided with a foot, with fresh drinking water. In this place a sprig of one of the following water plants,—Elodea Canadensis, Myriophyllum spicatum, M. verticillatum, or any leafy Myriophyllum (in fact, any small- leaved water plant with rather crowded foliage). This sprig should be prepared as follows: Cut the stem squarely off, four inches or so from the tip, dry the cut surface quickly with blotting paper, then cover the end of the stein with a quickly drying varnish, for instance, asphalt-varnish, and let it dry perfectly, keeping the rest of the stem, if possible, moist by means of a wet cloth. When the varnish is dry, puncture it with a needle, and immerse the stem in the water in the test tube, keeping the varnished larger end uppermost. If the submerged plant be now exposed to the strong rays of the sun, bubbles of oxygen gas will begin to pass off at a rapid and even rate, but not too fast to be easily counted. If the simple apparatus has begun to give off a regular succession of small bubbles, the following experiments can be at once conducted:
(1) Substitute for the fresh water some which has been boiled a few minutes before, and then allowed to completely cool: by the boiling, all the carbonic acid has been expelled. If the plant is immersed in this water and exposed to the sun's rays, no bubbles will be evolved; there is no carbonic acid within reach of the plant for the assimilative process. But,
(2) If breath from the lungs be passed by means of a slender glass tube through the water, a part of the carbonic acid exhaled from the lungs will be dissolved in it, and with this supply of the gas the plant begins the work of assimilation immediately.
(3) If the light be shut off, the evolution of bubbles will presently cease, being resumed soon after light again has access to the plant.
(5) Place round the base of the test tube a few fragments of ice, in order to appreciably lower the temperature of the water. At a certain point it will be observed that no bubbles are given off, and their evolution does not begin again until the water becomes warm.
The evolution of bubbles shows that the process of making food is going on. The materials for this process are carbonic acid gas and water. The carbonic acid dissolved in the surrounding water is absorbed, the carbon unites with the elements of water in the cells of the leaves, forming starch, etc., and most of the oxygen is set free, making the stream of bubbles. When the water is boiled, the dissolved gas is driven off and assimilation cannot go on; but as soon as more carbonic acid gas is supplied, the process again begins. We have seen by these experiments that sunlight and sufficient heat are necessary to assimilation, and that carbonic acid gas and water must be present. The presence of the green coloring matter of the leaves (chlorophyll) is also essential, and some salts, such as potassium, iron, etc., are needful, though they may not enter into the compounds formed.
The food products are stored in various parts of the plant for future use, or are expended immediately in the growth and movements of the plant. In order that they shall be used for growth, free oxygen is required, and this is supplied by the respiration of the plant.
Some plants steal their food ready-made. Such a one is the Dodder, which sends its roots directly into the plant on which it feeds. This is a parasite. It has no need of leaves to carry on the process of making food. Some parasites with green leaves, like the mistletoe, take the crude sap from the host-plant and assimilate it in their own green leaves. Plants that are nourished by decaying matter in the soil are called saprophytes. Indian Pipe and Beech-Drops are examples of this. They need no green leaves as do plants that are obliged to support themselves.
[Footnote 1: Reader in Botany. XIV. Parasitic Plants.]
Some plants are so made that they can use animal matter for food. This subject of insectivorous plants is always of great interest to pupils. If some Sundew (Drosera) can be obtained and kept in the schoolroom, it will supply material for many interesting experiments. That plants should possess the power of catching insects by specialized movements and afterwards should digest them by means of a gastric juice like that of animals, is one of the most interesting of the discoveries that have been worked out during the last thirty years.
[Footnote 1: See Insectivorous Plants, by Charles Darwin. New York: D. Appleton and Co., 1875.
How Plants Behave, Chap. III.
A bibliography of the most important works on the subject will be found in Physiological Botany, page 351, note.]
[Footnote 2: Reader in Botany. XV. Insectivorous Plants.]
5. Respiration.—Try the following experiment in germination.
Place some seeds on a sponge under an air-tight glass. Will they grow? What causes them to mould?
Seeds will not germinate without free access of air. They must have free oxygen to breathe, as must every living thing. We know that an animal breathes in oxygen, that the oxygen unites with particles of carbon within the body and that the resulting carbonic acid gas is exhaled. The same process goes on in plants, but it was until recently entirely unknown, because it was completely masked during the daytime by the process of assimilation, which causes carbonic acid to be inhaled and decomposed, and oxygen to be exhaled. In the night time the plants are not assimilating and the process of breathing is not covered up. It has, therefore, long been known that carbonic acid gas is given off at night. The amount, however, is so small that it could not injure the air of the room, as is popularly supposed. Respiration takes place principally through the stomata of the leaves. We often see plants killed by the wayside dust, and we all know that on this account it is very difficult to make a hedge grow well by a dusty road. The dust chokes up the breathing pores of the leaves, interfering with the action of the plant. It is suffocated.
The oxygen absorbed decomposes starch, or some other food product of the plant, and carbonic acid gas and water are formed. It is a process of slow combustion. The energy set free is expended in growth, that is, in the formation of new cells, and the increase in size of the old ones, and in the various movements of the plant.
[Footnote 1: See page 13.]
[Footnote 2: This table illustrates the differences between the processes.
ASSIMILATION PROPER. RESPIRATION.
Takes place only in cells Takes place in all active cells. containing chlorophyll.
Requires light. Can proceed in darkness.
Carbonic acid absorbed, Oxygen absorbed, carbonic oxygen set free. acid set free.
Carbohydrates formed. Carbohydrates consumed.
Energy of motion becomes Energy of position becomes energy of position. energy of motion.
The plant gains in dry The plant loses dry weight. weight.
Physiological Botany, page 356.]
[Transcriber's Note: Two footnote marks  and  above in original text, but no footnote text was found in the book]
This process of growth can take place only when living protoplasm is present in the cells of the plant. The substance we call protoplasm is an albuminoid, like the white of an egg, and it forms the flesh of both plants and animals. A living plant can assimilate its own protoplasm, an animal must take it ready-made from plants. But a plant can assimilate its food and grow only under the mysterious influence we call life. Life alone brings forth life, and we are as far as ever from understanding its nature. Around our little island of knowledge, built up through the centuries by the labor of countless workers, stretches the infinite ocean of the unknown.
Gray's First Lessons. Sect. VII, XVI, sec. 2, sec. 4, sec. 5, sec. 6, 476-480.
How Plants Grow. Chap. I, 119-153, Chap. III, 261-280.