In [Figure 75], B, is shown such a flower as it appears in the winter and early spring following. The leaves are thick and fleshy, closely pressed together, as is seen by dividing the flower lengthwise, and each leaf ends in a long point (D). The ovules are still very small. As the growth of the tree is resumed in the spring, the flower (cone) increases rapidly in size and becomes decidedly green in color, the ovules increasing also very much in size. If a scale from such a cone is examined about the first of June, the ovules will probably be nearly full-grown, oval, whitish bodies two to three millimetres in length. A careful longitudinal section of the scale through the ovule will show the general structure. Such a section is shown in [Figure 77], G. Comparing this with the sporangia of the pteridophytes, the first difference that strikes us is the presence of an outer coat or integument (in.), which is absent in the latter. The single macrospore (sp.) is very large and does not lie free in the cavity of the sporangium, but is in close contact with its wall. It is filled with a colorless tissue, the prothallium, and if mature, with care it is possible to see, even with a hand lens, two or more denser oval bodies (ar.), the egg cells of the archegonia, which here are very large. The integument is not entirely closed at the top, but leaves a little opening through which the pollen spores entered when the flower was first formed.
After the archegonia are fertilized the outer parts of the ovule become hard and brown, and serve to protect the embryo plant, which reaches a considerable size before the sporangium falls off. As the walls of the ovule harden, the carpel or leaf bearing it undergoes a similar change, becoming extremely hard and woody, and as each one ends in a sharp spine, and they are tightly packed together, it is almost impossible to separate them. The ripe cone ([Fig. 75], A) remains closed during the winter, but in the spring, about the time the flowers are mature, the scales open spontaneously and discharge the ripened ovules, now called seeds. Each seed (E, s) is surrounded by a membranous envelope derived from the scale to which it is attached, which becomes easily separated from the seed. The opening of the cones is caused by drying, and if a number of ripe cones are gathered in the winter or early spring, and allowed to dry in an ordinary room, they will in a day or two open, often with a sharp, crackling sound, and scatter the ripe seeds.
A section of a ripe seed (F) shows the embryo (em.) surrounded by a dense, white, starch-bearing tissue derived from the prothallium cells, and called the “endosperm.” This fills up the whole seed which is surrounded by the hardened shell derived from the integument and wall of the ovule. The embryo is elongated with a circle of small leaves at the end away from the opening of the ovule toward which is directed the root of the embryo.
The seed may remain unchanged for months, or even years, without losing its vitality, but if the proper conditions are provided, the embryo will develop into a new plant. To follow the further growth of the embryo, the ripe seeds should be planted in good soil and kept moderately warm and moist. At the end of a week or two some of the seeds will probably have sprouted. The seed absorbs water, and the protoplasm of the embryo renews its activity, beginning to feed upon the nourishing substances in the cells of the endosperm. The embryo rapidly increases in length, and the root pushes out of the seed growing rapidly downward and fastening itself in the soil (G, r). Cutting the seed lengthwise we find that the leaves have increased much in length and become green (one of the few cases where chlorophyll is formed in the absence of light). As these leaves (called “cotyledons” or seed leaves) increase in length, they gradually withdraw from the seed whose contents they have exhausted, and the young plant enters upon an independent existence.
The young plant has a circle of leaves, about six in number, surrounding a bud which is the growing point of the stem, and in many conifers persists as long as the stem grows ([Fig. 75], K, b). A cross-section of the young stem shows about six separate fibro-vascular bundles arranged in a circle (S, fb.). The root shows a central fibro-vascular cylinder surrounded by a dark-colored ground tissue. Growing from its surface are numerous root hairs ([Fig. 75], M).
For examining the microscopic structure of the pine, fresh material is for most purposes to be preferred, but alcoholic material will answer, and as the alcohol hardens the resin, it is for that reason preferable.
Cross-sections of the leaf, when sufficiently magnified, show that the outer colorless border of the section is composed of two parts: the epidermis of a single row of regular cells with very thick outer walls, and irregular groups of cells lying below them. These latter have thick walls appearing silvery and clearer than the epidermal cells. They vary a good deal, in some leaves being reduced to a single row, in others forming very conspicuous groups of some size. The green tissue of the leaf is much more compact than in the fern we examined, and the cells are more nearly round and the intercellular spaces smaller. The chloroplasts are numerous and nearly round in shape.
Scattered through the green tissue are several resin passages (r), each surrounded by a circle of colorless, thick-walled cells, like those under the epidermis. At intervals in the latter are openings—breathing pores—([Fig. 76], J), below each of which is an intercellular space (i). They are in structure like those of the ferns, but the walls of the guard cells are much thickened like the other epidermal cells.
Each leaf is traversed by two fibro-vascular bundles of entirely different structure from those of the ferns. Each is divided into two nearly equal parts, the wood (x) lying toward the inner, flat side of the leaf, the bast (T) toward the outer, convex side. This type of bundle, called “collateral,” is the common form found in the stems and leaves of seed plants. The cells of the wood or xylem are rather larger than those of the bast or phloem, and have thicker walls than any of the phloem cells, except the outermost ones which are thick-walled fibres like those under the epidermis. Lying between the bundles are comparatively large colorless cells, and surrounding the whole central area is a single line of cells that separates it sharply from the surrounding green tissue.
In longitudinal sections, the cells, except of the mesophyll (green tissue) are much elongated. The mesophyll cells, however, are short and the intercellular spaces much more evident than in the cross-section. The colorless cells have frequently rounded depressions or pits upon their walls, and in the fibro-vascular bundle the difference between the two portions becomes more obvious. The wood is distinguished by the presence of vessels with close, spiral or ring-shaped thickenings, while in the phloem are found sieve tubes, not unlike those in the ferns.
The fibro-vascular bundles of the stem of the seedling plant show a structure quite similar to that of the leaf, but very soon a difference is manifested. Between the two parts of the bundle the cells continue to divide and add constantly to the size of the bundle, and at the same time the bundles become connected by a line of similar growing cells, so that very early we find a ring of growing cells extending completely around the stem. As the cells in this ring increase in number, owing to their rapid division, those on the borders of the ring lose the power of dividing, and gradually assume the character of the cells on which they border ([Fig. 76], B, cam.). The growth on the inside of the ring is more rapid than on the outer border, and the ring continues comparatively near the surface of the stem ([Fig. 76], A, cam.). The spaces between the bundles do not increase materially in breadth, and as the bundles increase in size become in comparison very small, appearing in older stems as mere lines between the solid masses of wood that make up the inner portion of the bundles. These are the primary medullary rays, and connect the pith in the centre of the stem with the bark. Later, similar plates of cells are formed, often only a single cell thick, and appearing when seen in cross-section as a single row of elongated cells (C, m).
As the stem increases in diameter the bundles become broader and broader toward the outside, and taper to a point toward the centre, appearing wedge-shaped, the inner ends projecting into the pith. The outer limits of the bundles are not nearly so distinct, and it is not easy to tell when the phloem of the bundles ends and the ground tissue of the bark begins.
A careful examination of a cross-section of the bark shows first, if taken from a branch not more than two or three years old, the epidermis composed of cells not unlike those of the leaf, but whose walls are usually browner. Underneath are cells with brownish walls, and often more or less dry and dead. These cells give the brown color to the bark, and later both epidermis and outer ground tissue become entirely dead and disappear. The bulk of the ground tissue is made up of rather large, loose cells, the outer ones containing a good deal of chlorophyll. Here and there are large resin ducts ([Fig. 76], H), appearing in cross-section as oval openings surrounded by several concentric rows of cells, the innermost smaller and with denser contents. These secrete the resin that fills the duct and oozes out when the stem is cut. All of the cells of the bark contain more or less starch.
The phloem, when strongly magnified, is seen to be made up of cells arranged in nearly regular radiating rows. Their walls are not very thick and the cells are usually somewhat flattened in a radial direction.
Some of the cells are larger than the others, and these are found to be, when examined in longitudinal section, sieve tubes ([Fig. 76], E) with numerous lateral sieve plates quite similar to those found in the stems of ferns.
Fig. 76.—Scotch pine. A, cross-section of a two-year-old branch, × 3. p, pith. c, bark. The radiating lines are medullary rays. r, resin ducts. B, part of the same, × 150. cam. cambium cells. x, tracheids. C, cross-section of a two-year-old branch at the point where the two growth rings join: I, the cells of the first year’s growth; II, those of the second year. m, a medullary ray, × 150. D, longitudinal section of a branch, showing the form of the tracheids and the bordered pits upon their walls. m, medullary ray, × 150. E, part of a sieve tube, × 300. F, cross-section of a tracheid passing through two of the pits in the wall (p), × 300. G, longitudinal section of a branch, at right angles to the medullary rays (m). At y, the section has passed through the wall of a tracheid, bearing a row of pits, × 150. H, cross-section of a resin duct, × 150. I, cross-section of a leaf, × 20. fb. fibro-vascular bundle. r, resin duct. J, section of a breathing pore, × 150. i, the air space below it.
The growing tissue (cambium), separating the phloem from the wood, is made up of cells quite like those of the phloem, into which they insensibly merge, except that their walls are much thinner, as is always the case with rapidly growing cells. These cells (B, cam.) are arranged in radial rows and divide, mainly by walls, at right angles to the radii of the stem. If we examine the inner side of the ring, the change the cells undergo is more marked. They become of nearly equal diameter in all directions, and the walls become woody, showing at the same time distinct stratification (B, x).
On examining the xylem, where two growth rings are in contact, the reason of the sharply marked line seen when the stem is examined with the naked eye is obvious. On the inner side of this line (I), the wood cells are comparatively small and much flattened, while the walls are quite as heavy as those of the much larger cells (II) lying on the outer side of the line. The small cells show the point where growth ceased at the end of the season, the cells becoming smaller as growth was feebler. The following year when growth commenced again, the first wood cells formed by the cambium were much larger, as growth is most vigorous at this time, and the wood formed of these larger cells is softer and lighter colored than that formed of the smaller cells of the autumn growth.
The wood is mainly composed of tracheids, there being no vessels formed except the first year. These tracheids are characterized by the presence of peculiar pits upon their walls, best seen when thin longitudinal sections are made in a radial direction. These pits ([Fig. 76], D, p) appear in this view as double circles, but if cut across, as often happens in a cross-section of the stem, or in a longitudinal section at right angles to the radius (tangential), they are seen to be in shape something like an inverted saucer with a hole through the bottom. They are formed in pairs, one on each side of the wall of adjacent tracheids, and are separated by a very delicate membrane (F, p, G, y). These “bordered” pits are very characteristic of the wood of all conifers.
The structure of the root is best studied in the seedling plant, or in a rootlet of an older one. The general plan of the root is much like that of the pteridophytes. The fibro-vascular bundle ([Fig. 75], M, fb.) is of the so-called radial type, there being three xylem masses (x) alternating with as many phloem masses (ph.) in the root of the seedling. This regularity becomes destroyed as the root grows older by the formation of a cambium ring, something like that in the stem.
The development of the sporangia is on the whole much like that of the club mosses, and will not be examined here in detail. The microspores (pollen spores) are formed in groups of four in precisely the same way as the spores of the bryophytes and pteridophytes, and by collecting the male flowers as they begin to appear in the spring, and crushing the sporangia in water, the process of division may be seen. For more careful examination they may be crushed in a mixture of water and acetic acid, to which is added a little gentian violet. This mixture fixes and stains the nuclei of the spores, and very instructive preparations may thus be made.[11]