The Oak (Ward)/Chapter VIII
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The Tree―Its Shoot System
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the tree—its shoot-system.
When we cut into an old branch or stem of the oak (Fig. 26) it is at once obvious that considerable changes have been produced since it was a twig or young shoot-axis, such as exists in the young plant. Of these changes the two following are the most conspicuous. The pith, instead of being surrounded by a cylinder of small vascular cords, the diameter of which hardly exceeds its own, as was the case in the one-year-old shoot-axis (Fig. 9), is now a mere speck in the middle of a huge mass of wood many hundreds of times as broad as itself, and the cambium cylinder which was developed, as we saw, in the primary vascular bundles, is now a large (though still thin) layer encircling this huge wood mass. Again, in place of a delicate epidermis surrounding a soft, green, cellular cortex, as we had in the young stem, there is here a hard, brown, rugged bark, splitting off in thick ridges on the outside.
The two chief series of change may be inferred from comparing the two conditions, and taking into consideration all we have learned so far. The pith is the same pith as before, and it is the cambium cylinder which has moved outwards, as it were, putting in all that solid-looking timber as it did so. The epidermis and
Fig. 26.—Photograph of the transverse section of a log of oak. about one sixth natural size. The cortex and bark are removed, and the outline is bounded by the cambium. The pith appears as a mere dot in the center; the medullary rays radiate from this, and the annual rings (about forty in number) are arranged concentrically around it. A large crack has formed along the plane of a medullary ray as the section dried. (Müller.)
the cortex of our young stem have disappeared, however, their place being taken by cork and bark. Closer inspection will show that a series of layers of phloëm has also been formed between these outer protective layers and the cambium.
We have now to obtain some ideas as to these curious processes of increase in thickness of the stems and branches.
The first thing to insure this is to understand the constitution and behavior of the cambium cylinder, for it is principally this tissue which brings about the changes we have to study.
We saw in Chapter IV that the xylem of each primary vascular bundle is separated from the phloem of the same bundle by a thin strand of cambium (Figs. 9 and 12); we also saw that the bundles are arranged in a closed ring round the pith, and are in their turn surrounded by the primary cortex, each being separated laterally from its neighbors by a primary medullary ray. The next point to bear in mind is that these medullary rays (like the pith and cortex) are merely parts of the general cell-tissue, or fundamental tissue, through which the vascular bundles run upwards and downwards with a tangentially sinuous course from the leaves. The primary medullary rays, therefore, are merely spokes, as it were, joining the pith and cortex; and if we could remove, the whole of the vascular bundles and epidermis from the young stem we should have left a solid cylinder of cell (pith) in the center, a hollow cylinder (cortex) concentric to this, and a space between the two bridged over at numerous places by cellular spokes (medullary rays) radiating from the pith to the cortex. Each spoke is very thin from side to side, and therefore stands out like a knife, with an upper and a lower edge (Fig. 27).
Now imagine the primary vascular bundles replaced.
Fig. 27.—Tangential longitudinal section of oak wood, magnified fifty diameters, and showing the transverse sections of the medullary rays, cut as they project towards the observer. (Müller.)
The first change is that the cambium in the vascular bundles becomes continuous across through the medullary rays, and so forms a complete thin cylinder, concentric to the pith—from which it is separated by the breadth of the xylem—and the cortex, from which it is separated by the breadth of the phloem.
The cells of this cambium cylinder go on dividing continuously during the whole summer, until the cylinder is, say, ten times as thick as it was before. Now suppose it to rest during the winter and go on again next season, and so on during each successive period of growth. Obviously this would realize one fact in the process we are considering—namely, that the stem would grow in thickness year by year, its diameter being increased by twice the thickness of the added cylinder.
But to make the above supposition accord with the facts, we must further picture to ourselves that when the thickening cylinder has attained a certain thickness, a large proportion of those of its cells which lie on the inside—i. e., nearest the pith, and therefore abutting on, lose their cambial nature and the xylem—become converted into elements of the wood; while a smaller proportion of those on the outer side (beneath the phloëm) become new phloem elements. In this way it will be seen that the thin cylinder of active cambium cell travels outwards; ever receding radially farther from the pith, and leaving xylem between itself and the primary Vascular bundles next the pith, and ever driving outwards the primary phloem and cortex, adding new phloëm elements (but in far less proportion) to the inside of the phloëm. Each winter it pauses in this process, and each spring it renews its activity. Further peculiarities will be noticed as we proceed.
Now let us see what the cambium cells are, and how they change into new elements of the xylem and phloëm, etc., respectively.
Each cell of the cambium is a thin-walled prism, many times longer than broad or thick, and with its ends brought to an edge like that of a thick chisel, and so arranged that these edges run radially and fit in between those of cambium cells at higher and lower levels. As we have seen, the prism is oblong in transverse section. Each of these cells contains protoplasm and a nucleus, surrounding a sap-cavity, and they are nourished like other cells by the substances brought down from the leaves and up from the roots, taking what they need from the sap.
When a given cambium cell has taken into its protoplasm sufficient food materials, and has accomplished other life-processes under the action of oxygen, which it absorbs dissolved in the water of the sap, it grows larger, especially in the radial direction, and then it divides into two cells; then each of these may repeat these processes, and so on. At last the older ones can no longer grow and divide, but become changed into elements of the xylem or phloëm, according to their position. All the xylem thus produced by the cambium is called secondary xylem, and the phloem secondary phloëm, and so on, to distinguish them from the primary structures found in the early stage. I now proceed to some further details, which could only be rendered intelligible in the light of the preceding preliminary remarks.
After the cambium ring is once formed the daughter-cells cut off on the inside of the cambium always become transformed into one or more of the following elements:(1) Some cambium cells which lie on the radial continuation of a medullary ray undergo a few horizontal divisions across the long axis, and then simply pass over as constituents of a medullary ray; as the cambium ring moves outward, in consequence of the repeated formation of thickening rings, the periphery of the cylinder of course increases, and this allows of more space tangentially. One consequence of this is the occasional and gradual widening of the medullary ray in process of lengthening; this takes place to a small extent only. Another consequence of the increased space is the occasional interpolation of new medullary rays. Radial rows of cambial cells at points which lie between the planes of two gradually diverging medullary rays suddenly commence to form new medullary rays. Hence, as the wood mass increases in radial thickness, more and more of these interpolated medullary rays appear, cutting up the wood proper into partial sections. In succeeding years the cambium keeps adding to the length of these rays, as it does to that of the older rays, and again forms new ones between as space increases. In the same ring about
Fig. 28.—The various chief elements of the wood of the oak, isolated by maceration, and highly magnified: f, a fiber, distinguished by its thick walls, simple slit-like pits, and no contents; w.p, part of a row of wood-parenchyma cells, with simple pits, and containing starch in winter; tr, a tracheid, distinguished from the fiber especially by its bordered pits; p.v, part of a rather large pitted vessel, made up of communicating segments, each of which corresponds to a tracheid, and has bordered pits on its walls; sp, part of a spiral vessel.
teen rays to the millimetre may be counted on the transverse section of the wood.
(2) The cambium cells situated between the rays—except when they suddenly commence to form a new ray, as just described—pass over into one or more of the following elements of the wood proper—viz., wood-parenchyma, libriform fibers, tracheids, segments of the vessels (see Fig. 28).
When a cambium cell passes over into wood-parenchyma it first undergoes a few horizontal divisions transverse to its long axis, and then we have a vertical row of five or six parenchymatous cells, the walls of which do not thicken much, but obtain small simple pits, and retain part of their living contents—protoplasm, nucleus, starch-forming corpuscles, etc.—and indeed present much resemblance to the cells of the medullary rays themselves.
When the cambial cell becomes transformed into a libriform fiber it does this simply by thickening its walls at the expense of the living contents, etc., which soon disappear. The cell undergoes no horizontal divisions, and probably elongates very slightly. The thickened walls become pitted with minute simple pits, and are stratified and eventually lignified.
In the case of the transformation of a cambial cell into a tracheid everything is essentially as described in the last paragraph, except that the diameter increases and the thickening walls become marked with bordered pits, quite similar to those of the pine, except that they are more numerous, are not confined to the radial walls, and they are not quite circular, but have an oval shape with a slit-like aperture to the border, the long axis of the slit being nearly transverse to the long axis of the tracheid.
In the conversion of cambium cells into vessels the chief point to note is that the vessel is essentially a vertical row of superposed tracheids—each of which has been developed from a cambium cell as just described—the oblique separating walls of which become almost entirely obliterated. The markings, thickening, and want of contents are as in the case of tracheids, the chief difference being the more pronounced growth in diameter of the vessel segments, especially those formed in the spring wood.
It will readily be understood that the growth in diameter of these vessel elements exerts a disturbing effect on the radial arrangement of the other elements of the wood, and the displacements and compression of the latter are considerable and various, so that, at length, very little trace of the original order is observable. It not unfrequently happens, however, that many successive rows of the fibers or tracheids are formed in the outer parts of the annual ring, and in such cases the original radial series can be detected.
There are several other points also to be noted in the development of secondary wood. In the first place, the various elements do not maintain an exact vertical position, but may lean over both in the radial and in the tangential directions. These slight displacements from the vertical are chiefly due to the fact that the elements—fibers, tracheids, and vertical groups of wood-parenchyma—have not finished their growth in length when they pass over from the cambial condition; consequently the pointed ends of the elongating fibers, etc., push themselves between the ends of others which lie above and below them, and a slight tilting from the vertical results. This may be sufficient to produce a twisting of the stems and branches which is visible even to the unaided eye.
Another important point is that the length of the elements, as well as their diameters, vary at different periods in the life of the tree.
First as to the diameter. The fibers and tracheids developed in the autumn have a relatively smaller radial diameter than those formed earlier, and this, combined with the fact that those elements which develop in the spring have the relatively largest diameters, alone would suffice to mark the boundary between any two annual rings. But the same holds good for the vessels; those formed in the spring wood are very large compared with those formed later—the latter are also more sparely developed—whence the contrast at the boundary between the annual rings is intensified. With the diminution in relative diameter of the tracheids and fibers a corresponding increase in the thickness of their walls is connected—a phenomenon which again intensifies the contrast between adjacent annual rings.
But, in addition to these differences in diameter within one and the same annual ring, a gradual increment in the average size of certain of the elements (both in length and diameter) occurs as the tree becomes older—in other words, the average width and length of the elements increases year by year up to a certain age; after reaching a definite size they enlarge no more. These changes differ according to the part of the tree concerned. In the stem of the oak the chief changes in this connection are:
The fibers increase in length as follows, according to Sanio's measurements: While they average 0.42 mm. in length in the first annual ring, they increase to 0.60 mm. in the second, 0.74 mm. in the fourth, and go up to 1.22 mm. after a great age (one hundred and thirty years?) The tracheids in the same annual rings were found to average 0.39, 0.43, 0.53, and 0.72 mm. respectively; and the individual members or segments of the argar vessels averaged 0.25 mm. in the second annual ring, 0.26 mm. in the fourth, and 0.36 mm. in the three outer rings. The mean radial diameter of these vessels also increased: in the third year it was 0.08 mm., and it rose year by year until in the sixth year the definitive width of 0.31 to 0.33 mm. was attained. After this the width of these vessels is practically constant. These increments in size appear to take place after the element has passed out of the strictly cambial condition.
The passage of the older wood in the center of the stem into the condition known as "heart-wood" ( duramen) as opposed to "sap-wood" (alburnum) is not attended with any profound anatomical changes; the chief alterations are of the nature of infiltration by foreign chemical substances, and alteration in the physical properties of the cell-walls and in the contents. These changes are somewhat sudden, and the fact that starch ceases to be deposited in this altered wood helps to indicate that the change is one of degradation—the cells of the softer tissues have ceased to be "alive," and the "heart" commences to undergo degradation. At the same time, although we must regard the "heart" as dead, it is very resistant, perhaps owing to the preservative action of infiltrated bodies.
A remarkable phenomenon which may be noticed here is the filling up of the older large vessels with tyloses. These are thin-walled, bladder-like vesicles projecting into the cavity of the vessel from the bordered pits, and are, in fact, due to the protrusion into the cavity of the thin-walled parenchyma cells, which drive the pit membrane in and then swell up. At the planes of contact between various tyloses from opposite points on the wall of the vessel the tyloses are flattened, and the appearance is very like that of a parenchymatous tissue (Fig. 29, d). When young the tyloses are found to contain a nucleus, protoplasm, and cell-sap, and they are known to form division membranes and divide like cells of the pith or cortex; later on they lose their contents and form a sort of packing in the by this time functionless vessel.
During the whole time of the activity of the cambium ring and the formation of wood on its interior, it must not be forgotten that the outer rows of cambial cells are passing over into the tissue known as bast or secondary phloëm (also called secondary cortex); the chief differences in the process being (1) that much
Fig. 29.—A small piece of one annual ring of old oak wood (magnified twenty diameters): a, boundary of the autumn wood of the preceding (older) ring; b, that between the zone shown and the next youngest ring. In the annual ring shown the spring wood begins with large vessels, c and d, some with tyloses, d, in them, and passes gradually into autumn wood, with smaller vessels, e, e, and more tracheids and fibers, g. Only small medullary rays, i, are shown. (Hartig.)
less phloëm than xylem is formed; (2) that the elements do not become lignified; and (3) that the disturbances in the arrangement of the elements are more profound from the continued pressure exerted upon them between the resistant wood and the elastic periderm and bark, on the one hand, and the increased extension tangentially which it undergoes as the thickening mass of wood drives it outwards, on the other. The other differences chiefly concern tlie individual elements now to be described.
All that was said of the medullary rays in the wood applies also to those in the bast; the cambium in keeping open or originating new medullary rays does so on both sides, and therefore the medullary rays are to be traced radially through the cambium from wood to cortex. The rays in the bast are termed “bast rays”; the broader ones contain isolated groups of sclerotic cells and cells containing crystals.
The changes which the radial rows of cells on the exterior of the cambium zone undergo to form the elements of the secondary phloëm are as follows:
(1) Bast parenchyma (Fig. 17, b p) is developed, like the wood parenchyma, from cambium cells which undergo a few transverse divisions and then pass over as longitudinal groups of cells, which retain their living contents, etc. From these longitudinal groups, accompanying the sieve-tubes as parallel series, they are called companion cells (cambiform cells).
(2) Sieve-tubes (Fig. 18, b p), which may be regarded as homologous with the vessels of the wood, and, like those, are constituted of series of segments. Each segment corresponds to a cambium cell, and is obliquely tapering at the end where it fits on to another segment. These dividing septa are not completely broken through, as in the case of the wood-vessels, however, but are pierced by a grating-like series of holes (the sieve) through which the protoplasmic and other contents of the continuous segments pass uninterruptedly. Similar sieve-plates occur on the lateral walls of the segments also. The walls are not thickened and not lignified, and thus the morphological similarities between the sieve-tubes of the bast and the vessels of the wood (which only contain air and water, have their septa absorbed, and their walls lignified and covered with bordered and simple pits) depend almost entirely on the similar development. The sieve-pores are very fine, and easily overlooked.
(3) The bast fibers (Figs. 17 and 18 b), which are homologous with the libriform fibers of the wood, and are developed in the same way from single cells of the cambium. They are short, blunt, very thick-walled fibers, grouped in strands which appear on the transverse section of the bast as tangential bands 2-4 deep, alternating (in the radial direction) with broader bands of sieve-tubes and parenchyma. These bands of fibers (hard bast) are accompanied at their outer and inner boundaries by parenchyma-like cells arranged in vertical rows, each of which contains a large simple crystal of calcium oxalate imbedded in yellowish substance, and the walls of which are slightly sclerotic. Similar vertical series of cells are found in the soft bast, but they contain compound (clustered) crystals of the same salt (Figs. 17 and 18, e).
The soft bast also contains scattered roundish groups of short sclerenchyma cells, the thickened walls of which are traversed by very numerous pit-canals; cells containing crystals also accompany these groups.
In consequence of the above arrangements the secondary cortex presents a more or less stratified appearance on the transverse section, the strata consisting chiefly of alternate tangential layers of hard bast and soft bast (Fig. 17); the elements of the latter also showing a decided tendency to be arranged in layers.
After the first year the young stem or branches covered with thin periderm are seen to be dotted with lenticels or cortical pores. Structures similar in every respect and subserving the same function—viz., the exchange of gases with the environment—are formed on the roots as soon as the periderm is developed.
The lenticel is a local interruption of the periderm, where the cells are loosened so as to allow air to pass between the loosened cells into the intercellular spaces between the cortical cells. Each lenticel may be described as a biconvex projecting swelling of the periderm, the swelling being caused by the increased radial diameter of the loosened cells. This is the condition during the spring and summer, but in the winter the cork-cambium is continuous across beneath the lenticel, and forms periderm in an uninterrupted sheet, to be ruptured again at the lenticel during the formation and swelling of the looser cells (complementary or packing cells) in the following spring. These loose packing-cells are at first quite similar to young cork-cells, and are developed as such, but they loosen and round off, and their cell-walls do not become completely suberized for a long time, but are capable of swelling; in fact, the rounding off depends on the absorption of water by the cellulose walls and contents. The outer parts of the older lenticel openings are thrown off with the bark-scales, but the inner parts remain, and can be found between the scales in older branches, in the fissures.
The first points of origin of lenticels are usually beneath the stomata, and the lenticels may be regarded as devices for prolonging the passages of the stomata through the thickening periderm year by year. The cortical cells beneath the stoma become meristematic—in effect they continue the phellogen below the stoma, only they divide less regularly and in all directions. The daughter-cells thrown off externally swell up and protrude, driving the stomatic cells outwards and apart, and emerging between the ruptured guard-cells as the first packing-tissue. The phellogen or cambium of the lenticel forms phelloderm on its interior in continuation of that formed by the rest of the cork-cambium. The protruding packing-cells dry up eventually, and form the powdery substance seen between the gaping lips of older lenticels. In the autumn the cells formed by the meristem below the packing-cells do not separate, but are suberized and closely and radially arranged like the rest of the cork: in fact, they continue the cork layer as a closing layer beneath the lenticel, thus protecting the tissues beneath through the winter. In the following spring new layers of loose, swelling packing-cells are developed again, and these absorb water and bulge, bursting the closing layer and reopening the lenticel for the season. As the branch ages and its surface increases new lenticels are developed between the earlier ones, and, of course, with no reference to stomata.
The exterior of the very young stem or branch is smooth or slightly pubescent, the green color gradually passing into a silver-gray as the periderm develops, and in a few years (when the shoot is from five to twenty years old, or thereabout) the gradually thickening bark is shining and turning browner, flecked with lenticels and lichens. Later still the bark is rugged, brown, and fissured, and usually covered with small lichens and fungi. Bark begins to exfoliate at about the thirtieth year.
The epidermis cracks and peels off when the twigs are a year old, and shreds of the dead membrane may be detected on the outside of the young cork, which begins to form very early during the first year. It is, in fact, owing to the impervious nature of this cork that the epidermis dies, and to the stretching of the cortex as the stem grows in thickness that the dead membrane cracks and peels off (see Figs. 17 and 18).
The first indication of the development of the cork is the conversion of the sub-epidermal layer of cortex-cells into a meristem — i. e., the cells become capable of active growth and division.
Each cell of the layer referred to may be termed an initial cell of the cork-cambium (or phellogen), and the layer may be called the initial layer. This layer behaves essentially like the cambium of a fibro-vascular bundle, except that its daughter-cells become cork and phelloderm instead of phlöem and xylem.
The first event to notice is that each of the initial cells grows radially, and divides by a tangential wall into an inner cell nearest the axis of the branch and an outer cell nearer the epidermis; the outer cell becomes forthwith a cork-cell — i. e., its contents die and mostly disappear, and the cellulose cell-wall becomes suberized — the inner cell remains capable of repeating the process. But this is not the only case. After the division, as before, of the initial cell, it may happen that the inner cell becomes transformed into a collenchymatous cortical cell containing chlorophyll, and it is the outer of the daughter-cells which retains the meristem character and acts again as a phellogen cell, cutting off daughter-cells sometimes on one side and at others on the other. Thus, in the oak, the phellogen gives rise to permanent tissue on both side, of the initial layer: those cells which lie on the inside become phelloderm (cortical cells), those on the outside become transformed into phellem (cork). The three tissues, phelloderm, phellogen, and phellem, are called the periderm.
It is obvious that the cork-cambium, by thus adding to the cortical parenchyma, is gradually driven radially outwards from the center of the stem. This means that it obtains room to extend tangentially, and it does this by its cells occasionally dividing by walls perpendicular to the far more numerous tangential walls. It is also easy to see that the cork-cells must be arranged in radial rows, and this arrangement is very conspicuous (Fig. 18). The earlier cork-cells have very thin walls, later ones have the walls thicker.
After the development of the first layer of cork the stretched epidermis dies, and forms simply a dead membrane outside the thin cork. In succeeding years layers of phellogen are formed annually beneath the older ones, and thus the cork layers increase. Moreover, since the successive layers cut out thin, scale-like areas of cortex, trapping them, as it were, between the present and the preceding cork, the thickening corky covering is stratified — consists of successive and obliquely overlying thin sheets of dead cortex and cork proper (Fig. 30). Again, since the increase in thickness of the stem or branch is continually driving these corky and dead structures outwards, they at length crack, and form the fissured bark found on older parts. Bark is thus seen to be something more than cork, or even periderm, and it is defined to be all the dead tissues cut out by the phellogen.It is also to be noticed that the successive phellogen layers of different years are not concentric, but the new ones cut the old ones at acute angles (Fig. 30), thus cutting out scale-like areas of cortex; the consequence of this is the formation of the very irregular scales of bark thrown off from the older stems and branches of the oak. It follows from what has been said that in older
Fig. 30.—Transverse section of bark of oak. The successive cork-layers (Perid.) cut out masses of the cortex, and since everything which is thus separated from the underlying tissues dies, scales of bark, consisting of various kinds of tissues—sclerenchyma, d; parenchyma, c; bast-fibers, e; crystal cells, f; etc.—are cut off periodically. All that lies outside the innermost sheet of cork is comprised under the term bark. (Kny.)
trees the phellogen layers may be formed so far down in the cortex that they cut out tissues of the secondary cortex—i.e., phloëm and bast fibers. It is, of course, this gradual exfoliation of the cut-out areas of bark that explains the relative thinness of the bark in very old stems and branches; the whole of the primary cortex, and most of that formed from the cambium, have been thrown off as bark long before.