The Oak (Ward)/Chapter IX
THE TREE (continued). INFLORESCENCE AND FLOWERS—FRUIT AND SEED.
The oak flowers in May in this country, the young inflorescences developing as the leaves unfold. The flowers are unisexual, both male and female appearing on the same branches—i.e., the tree is monœcious—and even on the same twigs of the current year. The rule is that the apical bud of a last year's twig produces a few male inflorescences from between the axils of the upper scales, and then grows out into a green twig bearing about six to ten normal leaves, the female inflorescences arising from the axils of two or three of the upper leaves (Figs. 31 and 32). Lateral buds below the terminal bud of the last year's twig usually produce male inflorescences only—a phenomenon in accordance with their feeble development generally. Thus the male inflorescences are produced first—a common occurrence in forest trees.
Since the inflorescences arise from the axils of leaves, their arrangement accords with the phyllotaxis of the tree—i.e., ⅖—so far as it goes. It should be borne in mind that the bud-scales are stipules.
The male inflorescences hang down from between the bud-scales as simple catkin-like spikes, each bearing about a dozen flowers. Each male flower springs
Fig. 31.—A sprig of oak in May, with the pendent male catkin below, and the minute spikes of female flowers just showing above. (Th. Hartig.)
from the axil of a tiny scale-like bract, and consists of a shallow perianth, unequally divided into about five to seven small linear-lanceolate lobes, inclosing about five to twelve stamens; there is no trace of an ovary. The number of lobes of the perianth varies, as also does the number of stamens; the former are covered with short hairs.
Each of the stamens consists of a slender thread (filament) bearing on its top a four-chambered swollen anther. This contains a yellow dust, the pollen, composed of round grains (pollen grains), each with three thinner spots in its otherwise thick wall. Each of these pollen grains consists of a membrane inclosing nucleated protoplasm and food materials. When ripe the wind blows the pollen as it scatters from the dangling stamens, and some of the grains reach the stigmas of the female flowers; here they germinate, each pollen grain sending a delicate pollen-tube down the style into the ovary of the flower. This process of application of the pollen grains to the stigma is termed pollination, and depends on the wind.
The female inflorescences are also spikes (Fig. 32, A), but they bear only one to five flowers, and stand off from the axils of the foliage leaves. In the commonest English variety (Q. pedunculata) the spikes are rather long, obliquely erect, and the flowers are scattered on the upper end of the rachis of the spike; in other varieties the flowers are more clustered in the axils of the leaves. Here, as in one or two other details, minute differences are apparent in different individuals; similar trifling differences are met with in the structure of the male flowers.
Each female flower springs (like the male) from the axil of a small bract: in other respects it is very unlike the male flower. In the first place, the ovary is inferior, being sunk in and fused into a six-partite perigone, the teeth of which project some distance up and surround a trifid stigma (Figs. 33 and 34, c). One of the lobes of the perigone will be found opposite to the bract; the three lobes of the stigma are superposed on three alternate (outer) lobes of the perigone.
Fig. 32.—A, Flowering twig and inflorescences, male (♂), and female (♀), semi-diagrammatic. B, Diagram of plan of a similar but lateral twig. F. Leaf from axil of which the twig arises: x, parent stem; α and β, bracts. The numbers 1-11 denote pairs of stipules acting as budscales, some with male inflorescences (♂) springing from between them; the continued numbers 12-21 also denote pairs of stipules, but these have their accompanying leaves, with or without female inflorescences (♀) in the axils. (Eichler.)
There is yet a further covering to the female flower. The somewhat irregular margins of a minute cup-like investment are to be seen arising from beneath and around the perigone: this is the scaly cupula, the future "cup" in which the "acorn" is inserted (Fig. 34, m).
If the young female flower is carefully bisected longitudinally this cupule will be seen to consist of a ring of tissue, arising from beneath the ovary, and with its margin notched into scales. As the ovule enlarges the minute scales become more numerous, new ones arising at the inner margin of the up-growing cupule.
Fig. 33.—A group of female flowers
(slightly magnified). Each has a
spreading stigma above and the
commencing cupule below, and arises
from the axil of a pointed bract.
A transverse section across the female flower at a slightly later period shows that the inferior ovary is divided into three chambers (loculi), each corresponding to one of the lobes of the stigma, and each containing two ovules (Fig. 34). These ovules are inserted at the upper part of the inner angle of the chamber, and thus hang down in pairs. A curious point arises here. It seems that at the period when the female flower has just opened, but has not yet received any pollen on its stigma, neither the ovules nor the chambers are as yet formed, and the segments of the perigone spring from the lower portion of the flower, and this condition is not altered until pollination occurs; then the tissue below the stigma becomes the three-chambered ovary sunk in the perigone.
The pollination takes place in May-June, and fertilization soon afterwards; in July the young acorns can be made out peeping from the cupules in which they had hitherto been inclosed. The acorn reaches its full size towards the end of September, and ripens and falls in
Fig. 34.—Female flower in section. To the left three transverse sections through the young ovary; the lower one showing the three placentas, each with two ovules. To the right, three longitudinal median sections through the whole flower at successive periods: a, stigma; b, carpal; c, perianth; d, cavity of ovary with ovules; m, the cupule. (Th. Hartig.)
October. When ripe the acorn is, as we have seen, an ovoid, smooth, olive-brown nut, with the broad end inserted into the cupule, and the narrower, somewhat tapering end projecting free.
It will be interesting, in the light of the foregoing remarks, to examine one of the stronger lateral buds of the oak towards the end of April, before it unfolds. A transverse section of such a bud shows the following structures: In the center is the axis of the young shoot, represented by the small central dot in the diagram (Fig. 32, B). Surrounding this are about eight to ten green leaves in section, and folded on their midribs in such a way that the two halves of the upper surface are face to face and somewhat crumpled; some of these are turned so that their edges are directed one way, others with them directed the other.
Each of these leaves has a pair of small stipules, also cut across, and rather difficult to identify (Fig 32, 12-20). Some of the foliage leaves bear female inflorescences in their axils, as indicated by the sign ♀ in the figure. Following on these stipulate leaves are a number of pairs of larger stipules, devoid of foliage leaves and constituting the bud-scales (Fig. 32, 1-11). Some of these bear male inflorescences (♂) between them—i.e., in the position corresponding to the axil of the leaf.
It will be understood that in this diagram the parts are all represented on a ground-plan, but that as the bud opens the inner leaves and stipules are on higher levels than the outer scales. In fact, proceeding in the order of the numerals, we pass in an ascending spiral from the outermost lower pair (1) of scales (stipules) to the innermost upper pair (21) with their leaf.
If we suppose the female inflorescences removed, the above diagram will serve to represent the lateral buds which develop male inflorescences only, or if we suppose the three bracts F, a, and β away, it would serve for a terminal bud.
Each single female flower stands in the axil of a minute scale on the floral axis, as said, and its general structure has been described. When the pollen grains have been dusted on to the trifid stigma, about the end of May or beginning of June, each grain germinates and sends a minute tube down the style, and this pollen-tube soon reaches the cavity of the ovary, and its end becomes applied to one of the ovules. While the pollen-tube is descending the style, the ovules have arisen as minute cellular outgrowths from the angles of the three chambers of the ovary (Fig. 34, d). There are two in each chamber. Each ovule is at first a mere solid lump of cells (nucellus), which curves and becomes enveloped in two thin investing layers, called integuments, as shown in the figures A-D (in Fig. 35). Inside the solid nucellus, n, of the ovule there soon arises a small cavity filled with nucleated protoplasm, and termed the embryo-sac, e, because the embryo is to be developed in it.
This embryo-sac contains, among other structures, a minute, nucleated, naked mass of protoplasm, called the oösphore, or egg-cell. The pollen-tube has carried down in its apex also a nucleated mass of protoplasm, and it passes this over into the egg-cell in the embryo-sac; the union of the nucleus from the pollen-tube with the nucleus of the egg-cell constitutes the act of fertilization, and the fertilized egg-cell is now termed the oöspore, and at once begins to grow into the embryo.
Fig. 35.—Various stages in the development of the ovule: n, nucellus; i, i' integuments; p, point of attachment to placenta; e, embryo-sac; r, vascular cord supplying ovule; m, micropyle; x, young embryo. (Partly after Th. Hartig.)
It would be very interesting to describe at length all the remarkable details of these processes, and their morphological meaning in the light of modern biology, but the limits and purpose of this little book will not admit of that, and I must content myself with this brief résumé.
During this process of fertilization the cupule has grown up like a scaly wall round the ovary (Fig. 34), and the tip of the latter is seen peeping out from its orifice.
We are now in a position to understand generally the changes that convert the female flower into the cupped acorn. The fertilized oöspore becomes the embryo (Fig. 35, x); it grows at the expense of the contents of the embryo-sac, and develops a radicle, a plumule, and two relatively large cotyledons, which soon become so big that they occupy the whole space in the sac (Fig. 36). Moreover, the embryo-sac increases to make more room for this growing embryo. And now comes in a curious point. We saw that the ovary consisted of three chambers, each containing two ovules; each of these six ovules also had its embryo-sac, containing an egg-cell, etc., and each of the total of six egg-cells may be fertilized by the contents of so many pollen-tubes coming from pollen grains on the stigmas. But the rule is that five of the ovules with their contents perish at an early period, because one strong one takes the lead in development, and starves the rest by taking all the available nourishment to itself. Consequently the advancing ovary is soon filled by one ovule—the other five and two of the chambers being pressed to one side by it.
In a few weeks the ovary and its cupule have increased considerably in size, and the one successful ovule, with the rapidly developing embryo in the embryo-sac in its interior, occupies nearly the whole of its cavity; the remains of the two aborted chambers and
Fig. 36.—Sections of acorns in three planes at right angles to one another. A, transverse; B, longitudinal in the plane of the cotyledons (l); C, longitudinal across the plane of the cotyledons; c, cotyledons; t, testa; p, pericarp; s, scar, and r, radicle; pl, plumule. The radicle, plumule, and cotyledons together constitute the embryo. The embryonic tissue is at r and pl. The dots in A, and the delicate veins in B and C, are the vascular bundles.
the five unsuccessful ovules being traceable as tiny, shriveled remnants in one corner. The walls of the ovary then gradually change into the polished brown walls (pericarp) of the fruit; the walls of the ovule become the coat (testa) of the seed; and the embryo developed from the fertilized egg-cell fills up the interior of the latter, as described in Chapter II.
The ripe fruit is the acorn, and we may regard it apart from the cupule; it contains the seed.
The acorn is an egg-shaped, nut-like fruit (glans), about 18 mm. long and 8–10 mm. broad (Fig. 36); the apex is somewhat pointed with a hard remnant of the stigma, the base is broader, and marked with the circular scar which denotes where it was inserted in the cupule. The trifid character of the stigma can often be observed even on the ripe fruit, which is smooth (or with fine longitudinal striæ), and olive-brown in color when ripe. The ripe acorn may thus be regarded as consisting of the pericarp (to which the calyx or perianth is fused) and the seed.
The pericarp (Fig. 36, p) is a thin, hard shell, comprised of four layers: (1) An epidermis of small, cuboidal cells with their external walls much thickened (Fig. 37, e). (2) Four or five series of very thick-walled and pitted sclerenchyma cells (Fig. 37, 1). (3) Then follow numerous rows of thin-walled parenchyma cells, comprising the chief thickness of the pericarp (Fig. 37). It is in this tissue that the small vascular bundles supplying the pericarp run, and here and there nests of sclerenchyma cells are scattered. The parenchyma cells may contain minute starch grains, in addition to the remains of chlorophyll corpuscles, even when ripe; they also contain tannin, and, here and there, crystals of calcium oxalate. (4) The internal epidermis consists of elongated cells in one layer.The seed proper fills up the entire cavity inclosed by the fruit-wall above described. It consists of a relatively very thin testa, or seed-coat, closely enveloping the large,
Fig. 37.—Transverse sections of the pericarp (III) and seed (VI) of the oak. E, epidermis; i, thick layer of sclerenchyma; under this come the parenchyma cells, with a few sclerenchyma cells here and there. T, testa of seed; G, vascular bundles; e, the outer layer or epidermis of the cotyledon; Co, thin- walled cells of cotyledons (cf. Figs. 35 and 36) filled with starch, etc. (Harz.)
straight embryo (Fig. 36, t). At the broad end the funicle can be observed attaching the seed to the base of the acorn; it is inserted laterally, and traces of the aborted ovules may sometimes be found at the point of insertion. The vessels from the funiculus branca at the chalaza and ramify in the testa.
The testa is a shining, pale-brown or yellowish skin, consisting only of a few rows of cuboidal, thin-walled parenchyma cells, the outer rows of which may be the integuments, and the innermost possibly belong to the remains of the nucellus; or the latter may be represented by the outer portion of the thin membrane which includes all that remains of the embryo-sac. A few feeble vascular bundles run through the testa (Fig. 37, g).
The testa is closely applied to the surface of the two stout cotyledons. These fill up by far the greater part of the space inclosed by the thin testa and pericarp, and their shape is almost described in saying that. Each is a colorless, hard, plano-convex body, face to face with the other by the flat surface (Fig. 36); a transverse section of the acorn shows each cotyledon occupying half the circle. At the more pointed end of the acorn these two cotyledons will be found to be joined to the very small embryo (plumule and radicle) by what will on germination lengthen into very short stalks (petioles), but which are at present mere bridges of tissue, across which minute vascular bundles run from the embryo into the cotyledons. If the shell-like investments described above are removed from the embryo, it is then possible to gently separate the cotyledons and see the minute plumule and radicle to which they are joined (Fig. 36); on removing one cotyledon the plumule will be seen imbedded in a slight depression at the base. At this point there is a little room to spare, not quite filled up by the radicle and plumule; a minute remnant of endosperm may occasionally be found here, not having been entirely absorbed by the developing embryo.
The cotyledons and embryo are composed of a delicate epidermis inclosing the whole (Fig. 37, e), and very thin-walled cells forming the main mass of tissue in which the vascular bundles run. These bundles are scattered in the thickness of the cotyledons, ready to convey fluids to and fro on germination, and already contain lignified vessels in the xylem and sieve-tubes in the phloëm.
The iso-diametric, closely-packed cells of the cotyledons are filled with reserve materials, consisting of large quantities of starch grains imbedded in proteids and tannin. Here and there are scattered cells filled with brown pigments and containing tannin; some cells also contain oil-drops. Traces of sugar (quercite), certain bitter principles, acids, and mineral substances also occur in the tissues.