Structure of Phanerogamiæ or Flowering Plants.
The two great divisions of the vegetable kingdom are known as Cryptogamia and Phanerogamia. It does not follow, however, that there is any abrupt break between the two, as will appear from the context. Although it is customary to speak of the flowering plants as a higher grade of life, yet there is an intermediary class of Phanerogamiæ in which the conspicuous parts of the generative system partake of a condition closely resembling those of the higher Cryptogamiæ, observed in Gymnosperms, Coniferæ, and Cycadæ. So it may be said the distinctive character of the former is that of reproduction by seeds rather than flowers. The progress of botanical science during the latter half of the Victorian reign has been quite as remarkable as that of histology; while the comparative physiology and morphology of plants have perhaps advanced even more rapidly because the ground was newer. The consequence is that the specialisation of botanical science has been brought about con-currently with a more comprehensive nomenclature. The chief cause in this instance of modern specialisation is utility. “If we look at the great groups of plants from a broad point of view, it will be seen that the fungi and the phanerogams occupy public attention on other grounds than do the algæ, mosses and ferns. Algæ are especially a physiologist’s group, employed in questions on nutrition, reproduction, and cell division and growth. The Bryophyta and Pteridophyta, are, on the other hand, the domain of the morphologist concerned with such questions as the alternations of generations and the evolution of the higher plants.
“Fungi and phanerogams, while equally or even more employed by specialists in morphology and physiology, appeal widely to general interest, and evidently so on the ground of utility. Without saying that this enhances the importance of either group, it certainly attracts scientific attention to them. However, the histology of the minute cell, in addition to its importance from an academical point of view, has a special interest for the microscopist.”
It would be impossible to find anything more remarkable in histology than the detailed agreement in the structure and behaviour of the nucleus in the higher plants and the higher animals, an agreement which is conspicuously manifest in those special divisions which take place during the maturation of the sexual cells.
So with regard to the question of “alternation of generations.” We have known since the important discoveries of Hofmeister that the development of a large part of the vegetable kingdom involves a regular alternation of two distinct generations, the one which is sexual being constantly succeeded, so far as the normal cycle is concerned, by the other which is asexual. This alternation is most marked in the mosses and ferns. In the Bryophyta the ordinary moss or liverwort plant is the sexual generation of the ovum, which, when fertilised, gives rise to the moss-fruit, and represents the asexual stage. The latter is once more seen to form spores from which the sexual plant is again developed.
In the Pteridophyta the alternation is equally regular, but the relative development of the two generations is totally different, the sexual form being the insignificant prothallus, while the whole fern-plant, as we ordinarily know it, is the asexual generation.
The thallus of some of the lower Bryophyta is quite comparable with the prothallus of a fern, so as regards the sexual generation there is no difficulty in seeing the relation of the two classes; but when we come to the asexual generation or sporophyte the case is totally different. There is no appreciable resemblance between the fruit of any of the Bryophyta and the plant of any vascular Cryptogam.
“It is now known that in the higher plants a remarkable numerical change takes place in the constituents of the nucleus of the cell shortly before fertilisation. In angiospermous plants a reduction of the chromosomes occurs shortly before differentiation of the sexual cells. Thus, in the case of the lily, fertilisation is not the simple fusion of nuclear bodies. These spheres are seen to fuse in pairs, and then by position to determine the plane of first cleavage of the ovum; agreeing, in fact, closely with what is observed to take place in the animal kingdom.”
In the higher grades of plants it will be evident that the several tissues that compose their bodies are not found in the root, stem, and leaf without definite order and purpose, but that they are grouped into systems for the performance of different kinds of work. In all flowering plants at least three different systems may be clearly distinguished. These are the epidermal or boundary tissue system, the fundamental or ground tissue system, and the fibro-vascular or conducting system. All three systems of tissue originate from meristem cells, located at the growing point of the stem and root.
Although these systems characterise the higher types of plants, the elementary tissues (represented in [Plate XIII]. and in other figures) enter alike into the several component parts of nearly all plants. The stem, the branch, and the root, are alike constituted of an outer coating which affords a mechanical support, and once formed takes no further share in the economy of the plant, excepting that of assisting to convey fluid from the roots to the branches and leaves, an action more of a capillary nature than vital. The nourishment of the plant is brought about by other material structures, as the pith, the cortex, the cambium, and so forth, all of which greatly assist in the formative process. The woody portion of the plants is especially concerned in furnishing support to the softer pulpy textures, while the tissues of leaves and flowers are chiefly composed of cells compactly held together by protoplasmic or albuminoid matter. Water, of course, enters largely into the constituents of all plants. Beneath the epidermis is another layer of importance, the parenchymatous, which becomes more or less solid with the growth of the pith and cellular wall. In the pulpy substance of some leaves the epidermis presents a thin lamina of palisade-tissue, the bulk of the mesophyll consisting of spongy parenchyma or sclerenchymatous fibres (seen in [Fig. 310]), which also serve to show the disposition of the several layers about to be brought under notice.
Development of the Tissue Systems.—In the growing plant the embryonic cells soon become differentiated into three primary meristem layers, known as dermatogen, periblem, and phloem, from which are developed respectively the primary cortex, epidermis, and the stele or vascular cylinder. The dermatogen forms the outermost layer of cells at the growing point, and when present always develops into true epidermal tissue. In stems the dermatogen is always single-layered, while in roots it consists of several layers and develops a many-layered epidermis.
Fig. 310.—Section of Leaf of Piper.
c. Cortex; ep. Epidermis; pal. Palisade-tissue; scl f. Sclerenchymatous fibres of pericycle; o. Oil gland.
The periblem occurs immediately beneath the dermatogen, forming a hollow cylinder of tissue, which surrounds the phloem. From the periblem is developed the fundamental tissue of the primary cortex. When no dermatogen is present in the growing-point (stems of vascular cryptogams) the external layer of the periblem develops cells which perform epidermal functions. The phloem occupies the centre of the growing-point, and consists of a solid mass of somewhat elongated cells. From the phloem are developed the fibro-vascular and fundamental tissues of the vascular-cylinder or stele.
PLATE XIII.
ELEMENTARY PLANT TISSUES.
Epidermal or Boundary Tissue System.—This system constitutes the external covering of the plant, and is commonly called the epidermis. It includes, besides the ordinary epidermal cells, the guard-cells of the stomata and water pores, the plant hairs or trichomes, and the epidermal or external glands. The epidermal tissues are chiefly protective in function, serving to prevent excessive evaporation from the interior tissues of the plant.
Fig. 311.
a. Epidermis, reticulated ducts, and conjunctive palisade cells; b. Vertical section of alder root, woody layer, and boundary ducts.
In stems the external layer of cells, whatever its origin, is known as the epidermis, while in roots it is called the epiblema. The epidermis usually consists of a single layer of cells, but in some cases it is two or three-layered, as in the leaves of figs and begonias.
In land plants the epidermis is usually strongly cutinised, while in submerged plants it is never cutinised. The epidermis of land plants is also often waxy, the wax occurring on the surface as minute grains, rods or flakes, constituting the so-called bloom of leaves and fruits, and giving to them their glaucous appearance. Chlorophyll bodies are usually absent from the ordinary epidermal cells of land plants, while they commonly occur in the epidermal cells of aquatic plants.
Ordinary epidermal cells are usually thin-walled and transparent, and contain a nucleus and colourless watery protoplasm, but are destitute of both chlorophyll-bodies and starch-grains.
The external layers of the outer walls constitute the cuticle of the plant, while the internal layers and the radial and inner walls are composed of cellulose. The cells of the epidermis are always very compactly arranged, having their walls so closely adherent that the intercellular spaces are entirely obliterated except at the stomata and water-pores.
Fig. 312.
1. Vertical section of leaf of Iris germanica; a, a. Elongated cells of the epiderm; b. Stomata cut through longitudinally; c, c. Green cells of the parenchyma; d, d. Colourless tissue of the interior of the leaf. 2. Portion of leaf torn from its surface; a. Elongated cells of the cuticle; b. Cells of the stomata; c. Cells of the parenchyma; d. Limiting wall of the epidermic cell; e. Lacunæ or openings in the parenchyma corresponding to the stomata.
There are exceptions to this rule, as, for example, in Cinchona calisaya, which shows no trace of epidermis, this being replaced by a corky layer of tubular cells. Where this occurs in a plant to any extent, the whole of the outer tissues are displaced, and the bark consists exclusively of phloem tissues. This, although of constant occurrence in C. calisaya, is not so common in other species, as C. succirubia, the middle structure of which consists of parenchyma in which appear more or less numerous isolated store-cells, and when these are absent there is a formation of rhytidoma and displacement of the tissues containing the store-cells and ducts. The chlorophyll of C. succirubia is very marked, and its spectrum presents seven distinct absorption bands.
The epidermal system of plants in general includes other tissues than those already named, as the guard-cells of the stomata, the water-pores, plant-hairs or trichomes, and the external or epidermal glands, all of which are but modifications of ordinary epidermal tissue.
The Stomata or Breathing Pores are apertures in the epidermal which lie over large intercellular spaces (Fig 312, 2, b). These are usually bordered by two modified epidermal cells, called guard-cells. Stomata are formed in the following manner: A young epidermal cell divides into two equal portions by the formation of a septum across its middle, each half developing into a guard-cell; the septum now splits lengthwise and separates the guard-cells, leaving an aperture or stoma between them.
In the higher plants the guard-cells of the stomata are crescent-shaped and occur in pairs, the concave sides of the cells facing each other with the aperture between, while in mosses the stomata possesses but a single annular guard-cell which surrounds the aperture. The guard-cells of stomata usually contain chlorophyll-bodies in addition to the ordinary protoplasm. They have the power of increasing or diminishing the size of the aperture under the influence of light and moisture, thus regulating the amount of evaporation from the internal tissues of the plant.
Water Pores or Water Stomata are apertures in the epidermis, similar in structure to ordinary stomata, but differ from them both in function and distribution. Water-pores excrete water in the form of drops, and have their guard-cells fixed and immovable. They always occur at the ends of vasal bundles, and are found on the margin and at the apex of leaves.
Plant Hairs or Trichomes are modified epidermal cells prolonged externally, and may be either unicellular or multicellular. Each hair consists of a basal portion, or foot, which is embedded among the ordinary epidermal cells, and an apical portion or body, which is prolonged externally. Ordinary epidermal hairs are usually thin-walled, the inner layers of the wall being composed of cellulose, while the outer layer is more or less strongly cutinised. The walls may become hardened by deposits of lime-salts or silica. Sometimes the cells become glandular and secrete oily, resinous, or irritating matters, as in stinging-nettle hairs ([Plate XIII]., No. 19), when they are known as glandular hairs. The development of resin-passages may be observed in transverse sections of the stem of the ivy (Hedera helix) cut from a young succulent stem, and mounted in glycerine. The resin is seen scattered through the cortex and pith, and in the soft bast which lies outside the cambium in various stages of development, starting from a group of four cells without intercellular spaces.
Root hairs spring from the epiblema and are never cutinised, but are frequently more or less mucilaginous. The root-hairs are the principal absorbing organs of the plant, and are confined to the younger roots, occurring just above their tips. Root-hairs are never present in aquatic plants, and are absent from the roots of certain of the Coniferæ. It is a curious fact with regard to bell-heather growing in higher latitudes, that the plants possess a peculiar root structure as a protection against droughts. In most of them the sustentation of life depends upon the formation of a number of long thin filaments on their roots resembling root-hairs, which penetrate the root, forming nodular masses within it. These filaments belong to a fungus entirely parasitic to the root, and yet different from a common parasite, inasmuch as the plant in this way obtains so much of its nourishment, and when the fungus is not present, or is removed, the plant can no longer live on a peaty soil. The leaf-blade of the coarse moorland grass Nardus is likewise endowed with a singular property—that of rolling up cylindrically and spreading out again to adapt itself to the dry and wet weather of the moorlands of Scotland.
Fig. 313.
a. Section of the testa of Gourd Seed, showing communicating cells filled with colouring matter; b. Section of stem of Clematis, three pores separated and more highly magnified; c. Transverse section of same, showing medullary rays.
Fundamental or Ground Tissue System.—This system constitutes the groundwork of plants, and is the system through which the vasal bundles are distributed. The fundamental tissues are composed largely, though not wholly, of parenchyma, and are chiefly concerned in the metabolic work of plant life.
Ground tissue includes, besides ordinary parenchyma, collenchyma, selerenchymatous parenchyma, fibrous tissue, cork, laticiferous and glandular tissues. To the fundamental system also belongs the chlorophyll cells of leaves, the thin-walled cells of the pith and medullary rays, the cells of the cortex of stems and roots, and most of the soft cellular tissues in all plants.
The lower plants consist almost entirely of fundamental tissue. In the herbaceous forms of the higher plants the ground tissues largely predominate, while in woody plants they are present in much smaller proportion, the vascular tissues being the most abundant. In aquatic plants generally, the fundamental tissues constitute the principal system.
The hypoderma occurs immediately beneath the epidermis, and consists of several layers of cells. A collenchymatous hypoderma is found in the stems and petioles of most herbaceous dicotyls, and frequently occurs next the mid-rib of leaves, where it forms a strengthening tissue. A sclerenchymatous hypoderma occurs either as a continuous layer beneath the epidermis, as in the stems of some ferns, Pteris aquilina, and in leaves of the pine; or it may form numerous isolated strands beneath the epidermis, as in the stems of horsetails and in certain Umbelliferæ.
Fig. 314.
a. Tangential section of Taxus baccata (Yew), showing the woody fibre; b. Vertical section of same, spiral fibres, and ducts; c. Vertical section of Elm, showing ducts and dotted cells.
The endodermis is the innermost layer of the extra-stelar fundamental tissues, and always abuts on the stele or steles. In monocotyls it marks the boundary between the cortex and the central cylinder, and it is sometimes spoken of as the nucleus sheath.
In stems the endodermal cells are usually thin-walled and unlignified, having a suberous thickening band extending round the upper, lower and lateral surfaces, which in cross-section appears as a black dot on the radial wall ([Fig. 314], c.)
According to its position in the stele, the conjunctive tissue is divided into three principal portions, viz., that portion which invests the vasal bundles, the pericycle; that portion which lies between the bundles of the stele, the interfascicular conjunctive tissue; and that which occupies the centre of the stele, the medullary conjunctive tissue. The pericycle, formerly called the pericambium, is the outermost layer of the conjunctive tissue of the stele.
The bundle-sheath of the young stem is more easily recognised than in the older stem. It is, in fact, a continuous layer of cells, whose radial walls have a characteristic dark spot on each radial wall. The bundle-sheath lies immediately outside the vascular bundles, curving slightly towards the centre of the stem in the spaces between the bundles. It is more prominent in the stem when very young, as the cells are then filled with starch granules. This layer of cells will be readily seen in sections treated with iodine.
In dicotyls and gymnosperms the medullary rays consist essentially of interfascicular ground tissue. The medullary conjunctive tissue occupies the centre of the stele, constituting the so-called pith, and usually consists of parenchymatous cells, but may contain, in addition, either stone cells, sclerenchyma fibres, laticiferous or glandular tissues.
The Fibro-vascular or Conducting Tissue System.—This system constitutes the fibrous framework of the plant, and is the system by means of which fluids are conducted from one part of the plant to another. Its function is partly to give strength and support, but principally to conduct the crude and elaborated juices to and from the leaves. It is found only in the higher plants, constituting the tough and stringy tissues in stems and roots, and the system of veins in leaves. The fibro-vascular system consists essentially of vascular tissue (ducts, tracheids, and sieve-tubes), and forms long strands—the fibro-vascular bundles—which extend vertically through the fundamental tissues of the plant. The term “fibro-vascular,” as applied to the conducting system, is not strictly correct, since fibres do not always accompany the vascular elements, hence this system is often spoken of as the vascular system, and the bundles as vascular, or more briefly as vasal bundles.
That the arrangement, and course of the vascular bundles in dicotyledous stems are connected with those of the leaves is an obvious fact. It may be seen in sections of Helianthus, but is more markedly shown in plants with regularly decussate leaves, as Cerastium, Clematis, &c. Still, the arrangement of the bundles may differ radically from that of the leaves, and is, to a certain extent, independent of them. This will be noticed in sections of Iberis amara, where the bundles do not run longitudinally, but in tangential spirals. These, as Nägeli pointed out, have no direct relation with the leaves; and he recommends a series of types for investigation, in which it will be seen how closely the arrangement of the bundles is connected with the arrangement of the leaves, and the number of bundles entering the stem from each leaf: Iberis amara, leaves alternate, leaf-trace with one bundle; Lupinus, leaves alternate, leaf-trace with three bundles; Cerastium, leaves opposite, leaf-trace with one bundle; Clematis, leaves opposite, leaf-trace with three bundles; Stachys, leaves opposite, leaf-trace with two bundles.
Fig. 315.
1. Transverse section of the stem of Cedar, showing xylem or wood; 2. Section of stem of Conifer, the phloem and zones of annual growth; 3. Section of an Ivory Nut, cells, and radiating pores; 4. Section of the outer or ligneous portion of same, with radiating cells.
The connection of the leaf and stem will be best seen by cutting longitudinal sections through a young node of Helianthus, so as to include the median plane of the leaf, or of both leaves if opposite to each other, as they often are; steep them in dilute potash for twenty-four hours and mount in glycerine. A medium power will serve for their examination. The course of the vascular bundles will appear dark through the more transparent parenchyma. The continuity of the tissues of the stem and petiole if followed will be seen to have no definite boundary between the two parts; the bundles from the petiole pass into the stem, and no bundle of the upper internode lies in the same vertical plane as that which enters from the petiole between two successive bundles of the vascular ring.
Every complete vasal bundle consists of xylem or wood and phloem or bast.
The former consists essentially of trachery tissue (ducts and tracheids), and may contain in addition both wood fibres and wood parenchyma. The phloem or bast consists essentially of sieve tissue, and usually contains some ordinary parenchyma. In angiosperms companion-cells always accompany the sieve-tubes in the phloem, while in gymnosperms they are absent.
According to the relative positions of the xylem and phloem elements, there are two principal kinds of conjoint bundles—the collateral and the concentric. Of these again there are three varieties, but the experiments with leaves bring out parallel facts; that in ordinary stems the staining of the wood by an ascending coloured liquid is due, not to the passage of the coloured liquid up the substance of the wood, but to the permeability of its ducts and such of its pitted cells as are united into regular canals; and the facts showing this at the same time indicate with tolerable clearness the process by which wood is formed, for what in these cases is seen to take place with dye may be fairly presumed to take place with sap.
Taking it, then, as a fact that the vessels and ducts are the channels through which the sap is distributed, the varying permeability of their walls, and consequent formation of wood, is due to the exposure of the plant to intermittent mechanical strains, actual or potential, or both, in this way. If a trunk, a bough, shoot, or a petiole is bent by a gust of wind, the substance of its convex side is subject to longitudinal tension, the substance of its concave side being at the same time compressed. This is the primary mechanical effect. The secondary is when the tissues of the convex side are stretched, and also produce lateral compression. In short, the formation of wood is dependent upon transverse strains, such as are produced in the aerial parts of upright plants by the action of the wind.
Fig. 316.—Termination of Vascular System.
1.—Absorbent organ from the leaf of Euphorbia neriifolia. The cluster of fibrous cells forming one of the terminations of the vascular system is here embedded in a solid parenchyma.
2.—A structure of analogous kind from the leaf of Ficus elastica. Here the expanded terminations of the vessels are embedded in the network parenchyma, the cells of which unite to form envelopes for them.
3.—End view of an absorbent organ from the root of a turnip. It is taken from the outermost layer of vessels. Its funnel-shaped interior is drawn as it presents itself when looked at from the outside of this layer, its narrow end being directed towards the centre of the turnip.
4.—Shows on a larger scale one of these absorbents from the leaf of Panax Lessonii. In this figure is clearly seen the way in which the cells of the network parenchyma unite into a closely-fitting case for the spiral cells.
5.—A less-developed absorbent, showing its approximate connection with a duct. In their simplest forms these structures consist of only two fenestrated cells, with their ends bent round so as to meet. Such types occur in the central mass of the turnip, where the vascular system is relatively imperfect. Besides the comparatively regular forms of these absorbents, there are forms composed of amorphous masses of fenestrated cells. It should be added that both the regular and irregular kinds are very variable in their numbers: in some turnips they are abundant, and in others scarcely to be found. Possibly their presence depends on the age of the turnip.
6.—Represents a much more massive absorbent from the same leaf, the surrounding tissues being omitted.
7.—Similarly represents, without its sheath, an absorbent from the leaf of Clusia flava.
8.—A longitudinal section through the axis of another such organ, showing its annuli of reticulated cells when cut through. The cellular tissue which fills the interior is supposed to be removed.
In concentric bundles one of the elements, either the xylem or the phloem, occupies the centre, and is more or less surrounded by the other, as seen in [Fig. 310]. Meristem tissue is never present, hence concentric bundles are always closed. They, however, occur in the stems of most ferns, and are always surrounded by a pericycle and endodermis, and should be regarded as steles. Concentric bundles with a central phloem occur in the rhizomes of some monocotyles, as Calamus, Iris, Convallaria, &c.
Fig. 317.—Vertical section of Sugar-cane Stem showing parachyma and crystalline cells, × 200 diameters.
The Stele, or Vascular Cylinder, is developed from the phloem of the growing plant, and consists of one or more vasal bundles imbedded in fundamental tissue, the whole being enclosed by a pericycle and an endoderm. The typical stele includes all the tissues evolved by the endodermis, which, however, forms no part of the vascular cylinder itself, but merely surrounds it. The pericycle is always the outermost layer of the tissues of the stele, while the endodermis is the innermost layer of the extra-stelar tissues.
The arboreus type of stem can be best followed by making sections of a twig of the elm (Ulmus campestris), which will be found to be cylindrical hirsute, green or brown according to age, the latter colour being due to the formation of cork. Small brown excrescences are scattered over its surface; these are termed lenticels. The cork will be seen to lie immediately below the epidermis, and to consist of cubical cells, with little or no cell contents; they are arranged in radial rows, without intercellular spaces. The walls of these cork cells will stain yellowish-brown with Schultze’s solution. Treat a thin section with sulphuric acid and the walls will swell out and gradually lose their sharpness of outline, with the exception of the cuticularised outer wall of the epidermis and the cork. This material is occasionally found developed in the twigs of the elm, so that it can be separated as thick radial plates of tissue.
“By comparing sections of twigs cut of various ages, the following information may be gleaned: That cork cambium, or phellogen, appears as a layer of cortical cells below the epidermis, and that these divide parallel to the surface of the stem. The result of successive divisions in this direction is the formation of secondary tissue, which develops externally as cork, internally as phelloderm. The true cork cambium consists of only a single cell in each radial row, from which, by successive division, all these secondary tissues are derived—i.e., cambium of vascular bundles. As stems grow older, layers of cork appear successively further and further from the external surface; not only the cortex, but also the outer portions of the phloem are thus cut off from physiological connection with the inner tissue. The term bark is applied to tissues thus cut off, together with the cork which forms the physiological boundary. The stem of Vitis affords a good example of such successive layers of cork.”
Fig. 318.
1. Laticiferous Tissue; 2. Vertical section of a Leaf of the India-rubber Tree, with a central gland; 3. Vertical cast of spiral tubes of Opuntia.
For the study of sieve-tubes take the vegetable marrow, in which they are of extraordinary size. Cut transverse sections of the stem and stain with eosin, and mount them in glycerine. The general arrangement will be seen to differ from that of most other herbaceous plants. Below the epidermis a thick walled band of sclerenchyma with lignified walls will be seen distinct from the vascular bundles, which readily take a stain. The vascular bundles are separate and distinct, and the structure of the bundle is abnormal, there being in each a separate central mass of xylem, with the phloem masses lying, the one central, the other in the peripheral side. Between the xylem and the phloem masses is the cambium layer. The structure being the same in both will serve for the study of the punctate sieve-plates; these are readily stained with eosin, as shown in Sach’s text-book.
Laticiferous Tissues ([Fig. 318]).—In cutting sections of latex care must be taken to at once transfer them to alcohol so as to prevent the flow of the latex from the cells, otherwise the laticiferous vessels will be much less easily traced. The better method is to plunge the root of the dandelion (Leontodon taraxacum), after cleaning, into alcohol, and there let it remain until it has become hardened; then cut thin tangential sections from the phloem, and longitudinal sections through the cambium, and mount them in potash and glycerine. The laticiferous vessels appear circular in the transverse sections with brown contents; these are distributed in groups round the central xylem. Observe in such sections the presence of sphere crystals of inulin. These are formed quite irrespective of the cell-walls.
Laticiferous cells are readily seen in the cortex of Euphorbia splendens, cut just outside the vascular ring. Long tubes will be seen to run through the cortical parenchyma, with thick cellulose walls and granular contents. These are the laticiferous cells, the branching of which distinguishes them from the preceding structure. Included in the granular contents are starch grains of a peculiar dumb-bell form.
Leaf or Petiole.—The general morphology of leaf tissue is essentially the same as that of the stem from which it proceeds. In the typical monostotic stem of Phanerogamæ each leaf receives a portion of the stele or central cylinder of the stem. Such portion is termed a meristele, and may be either entire or split up into a number of schizosteles.
The microscopical structure of leaves should be studied in the whole organ, and by the aid of isolating elements. The whole or portion of a leaf should be soaked in chloral hydrate solution; this will render it transparent, whereby the internal structure can be studied as a whole. Sections should be prepared from fresh leaves, or dried ones softened by soaking in water. Cut them transversely, both in the direction of the mid-rib and at right angles to it. This is best effected by placing the material between two pieces of elder pith or fresh carrot. Sections of the whole are made and transferred to a dish of water. Leaf sections are easily made for examination by macerating the leaves in solution of caustic potash varying in strength from one to five per cent. The epidermis on both sides may be detached, and the elements of the mesophyll and vascular bundles isolated for separate examination.
Potassium permanganate proves to be a useful reagent. A weak solution causes the protoplasmic structures to swell up, thus assisting in the observation of the structure of the chromatophores. This solution may also be employed as a macerating fluid. Beautiful preparations are obtained in this way of the sieve-tubes of Vitis.
Special structural peculiarities are to be observed in the leaves of various plants in which the epidermis consists of more than a single layer of cells (e.g., the leaves of Ficus, Peperaceæ, Begoniaceæ, &c.), cystoleths in the cells of the epidermis of Urtica; glandular structure in Ruta, Psorales; the coriaceous leaves of the Cherry Laurel, and the cylindrical leaves of Stonecrop (Sedum acre).
Reproductive Organs.—The development of the rudiments of flowers is of an extremely interesting nature, and the complete flower should be carefully studied. Median sections are best suited for the purpose. In the large majority of plants the calyx is developed first, then the corolla, and next the stamens. Preparations should be made from materials hardened in alcohol, or first fixed with a strong solution of picric acid and then hardened in alcohol.
Pollen-grains.—Microspores are found lying free in sections made of the reproductive organs; these may be transferred to a glycerine fluid and examined under a high power. They are mostly spherical, with granular protoplasmic contents, in which with much difficulty two nuclei can be made out. Mount and examine, as types of the various forms of granules, the pollen of Helianthus, Althœa, Cucurbita, Ænothera, Orchis, Mimosa, Tulipa, &c. Mount any of these pollen-grains in a weak solution of cane-sugar (about five per cent.), examine with a high power, and note the configuration of their walls with a medium power under polarised light. If transverse sections be made from very young buds, the development of the anther and the pollen may be traced. The material should be preserved in strong alcohol, and the sections treated with equal parts of alcohol and glycerine, and exposed in a watch-glass that the alcohol may evaporate. By this method sections may be prepared for illustrating the formation of the tapetum, special mother-cells, and division of the nucleus.
Fig. 319.—Pollen Grains.
A. Pollen-grain of Clove-pink; B. Poppy; C. Passion-flower (Passiflora cœrulea); D. Cobœa scandens.
Starch Granules.—One of the most universally distributed materials found in plants is starch composed of two substances, granulose, which constitutes by far the largest part, and a skeleton of farinose. It is only the former of these that stains blue with iodine solutions; the latter partially assumes a brownish colour. The structure of starch granules is not of equal density throughout; the hilum or nuclear portion is most conspicuous, around which the rest of the material is deposited in layers, indicative of stratification. The several layers next to the hilum are less dense than those farthest from it. The position of the hilum determines the form of the grain, a few being rounded, others oval or elongated. The grain also contains different proportions of water; this conveys the appearance of concentric lines or curves about the nucleus. The latter is more conspicuous in the potato starches, as seen in [Plate XIII]., Nos. 6-15. Starch grains, in nearly all cases, are formed by the agency of proteid bodies, either chloroplasts or amyloplasts, and under the action of sunlight are gradually broken up and employed in the process of growth. There are some plants, however, notably the Compositeæ, in which another carbohydrate, inulin, takes the place of starch from the first, and is used as a reserve food material. For this reason we look in vain for starch in the cells of Inula, Taraxacum, &c. From the whole group of fungi starch is absent; this seems to explain the fact that chlorophyll, or colouring matter, is rarely met with in the fungi, hence their inability to utilize, like green plants, carbon-dioxide as food.
Fig. 320.—Swollen Potato Starch, after the application of potassium hydrate. (Magnified 210 diameters.)
The tissues which most commonly contain starch, or which contain it in largest quantity, are those of the parenchymatous series, though it sometimes occurs in the latex of laticiferous tissues, and even in ducts and tracheids. In the stems of Dicotyledons it occurs chiefly in the parenchyma of the middle and inner bark, in the medullary ray cells, and in the cells of the pith. In the roots of these plants it has a similar distribution, being for the most part confined to the middle or inner bark and the medullary rays, pith not being present in these organs. In succulent stems and roots, of course, it also commonly occurs in the xylem tissues of the fibro-vascular bundles.
A study of the various kinds of starches is important, since this material is very largely used as an adulterant. Other than microscopical means of detecting frauds are practically useless; assaying is tedious and expensive, while the microscope is always available and at hand. The limits of variation should be studied in starches from the same species of plants; the variations are not very wide, but in most cases characteristic, so that the discrimination is at all times an easy task. The reagents required are simply iodine and dilute potassium hydrate, aided by polarised light.
Fig. 321.
a a a. Granules and cells of cocoa; b b b. Arrowroot, Tous-les-mois; c c c. Tapioca starch. (Magnified 300 diameters.)
The starch grains of the potato are the best to study in the first instance on account of their large size ([Fig. 320]).
In arrowroot starch ([Fig. 321]) the stratification is almost as distinct as in that of the potato; the grains much resemble each other. Although somewhat smaller, the grains of arrowroot are more uniform in size. The starches are much used as an adulterant of drugs and various articles sold as cocoas.
Wheat-starch ([Fig. 322]) consists of circular flattened grains varying much in size, the central nucleus and stratification of which are very difficult to distinguish.
In the smaller starches the hilum becomes more indistinct, and without stratification, as in rice-starch, the latter being angular in shape. The hilum in other leguminous plants forms a longitudinal cleft; white rye-starch exhibits distinct cracks. Compound grains are occasionally met with, as in the oat. In [Plate XIII]. will be found small groups of starches taken under the same medium power for the sake of comparison. In the microscopical examination of starches first use a 2⁄3-inch or a ½-inch and then a 1⁄6-inch objective.
Fig. 322.
a. Husks of Wheat-starch, swollen by reagents and heat; b. A portion of cellulose; c. Rice-starch, magnified 420 diameters.
The bran of the husk of wheat when broken by grinding is seen to be composed of two coats of hexagonal cells, the outer of which is detached by the roasting process. The hexagonal cell layer is, however, so little altered as to be perfectly distinguishable under the microscope. Thus even a small admixture of roasted corn with coffee or chicory can be detected without much difficulty. As to whether starch granules should be regarded as crystalline or colloid bodies, a difference of opinion still prevails. There are, however, reasons for believing that the polarisation effects produced by starch grains are not due to crystalline structure but to stress or strain, of the same nature as the polarisation of glass when it is subject to strain. The polarising phenomena are precisely such as would be induced in any transparent solid composed of layers, the inner of which being kept in a state of stress by the compression exerted by the outer layers. Moreover, when by use of a swelling reagent, such as caustic potash solution, the outer wall of the starch is made to expand by the imbibition of water, the polarisation effects immediately disappear. Were the solid particles of crystal thus forced apart by water each particle would still exhibit polarisation phenomena.
Want of space will not permit me to further enlarge upon other micro-chemical substances that enter into the composition of plants; as, for example, the oil secreting glands. These when present take the place of starch. There is, however, one product among the cell contents of plants of some interest to the microscopist—those extremely fine crystals known as raphides, composed of calcium-phosphate and oxalate. Mr. Gulliver insisted upon the value of raphides as characteristic of several families of plants. Schleiden states that “needle-formed crystals, in bundles of from twenty to thirty in a cell, are present in almost all plants,” and that so really practical is the presence or absence of raphides, that by studying them he has been able to pick out pots of seedling Onagraceæ, which had been accidentally mixed with pots of other seedlings of the same age, and at that period of growth when no other botanical character would have been so readily sufficient.
If we examine a portion of the layers of an onion ([Plate XIV]., No. 3), or a thin section of the stem or root of the garden rhubarb (No. 4), we shall find many cells in which either bundles of needle-shaped crystals or masses of a stellate form occur, not strictly raphides.
Raphides were first noticed by Malpighi in Opuntia, and subsequently described by Jurine and Raspail. According to the latter observer, the needle-shaped or acicular are composed of phosphate, and the stellate of oxalate of lime. There are others having lime as a basis, in combination with tartaric, malic, and citric acids, all of which are destroyed by acetic acid; others are soluble in many of the fluids employed in mounting. These crystals vary in size from the 1⁄40th of an inch, while others are as small as the 1⁄1000th. They occur in all parts of the plant; in the stem, bark, leaf, petals, fruit, root, and even in the pollen, and occasionally in the interior of cells. In certain species of aloe, as Aloe verrucosa, we are able to discern small silky filaments; these are bundles of the acicular form of raphides, and probably, as in sponges, act as a skeleton support to the internal soft pulpy mass.
PLATE XIV.
STELLATE AND CRYSTALLINE TISSUE OF PLANTS.
In portions of the cuticle of the medicinal squill (Scilla maritima) large cells are found full of needle-shaped crystals. These cells, however, do not lie in the same plane as the smaller cells of the cuticle. In the cuticle of an onion every cell is occupied either by an octahedral or a prismatic crystal of calcium oxalate. In some specimens the octahedral form predominates; in others, even from the same plant, the crystals are prismatic and arranged in a stellate form, as in that of the grass (Pharus cristatus). ([Plate XIV]., No. 6.)
Raphides of peculiar figure are found in the bark of certain trees. In the hickory (Carya alba) may be observed masses of flattened prisms having both extremities pointed. In vertical sections from the stem of Elæagnus angustifolia, numerous raphides of large size are embedded in the pith, and also found in the bark of the apple-tree, and in elm seeds, every cell containing two or more minute crystals.
In the Graminaceæ, especially the canes; in the Equisetum hyemale, or Dutch rush; in the husk of rice, wheat, and other grains, silica in some form or other is abundant. Some have beautifully-arranged masses of silica with raphides. The leaves of Deutzia scabia, No. 7, are remarkable for their stellate hairs, developed from the cuticle of both their upper and under surfaces; forming most interesting and attractive objects examined under polarised light. ([Plate VIII]., No. 173.)
Silica is found in the structure of Rubiaceæ both in the stem and leaves, and, if present in sufficient thickness, depolarises light. This is especially the case in the glandular hairs on the margins of the leaves. One of the order Compositæ, a plant popularly known as the “sneezewort” (Archillæ ptarmica), has a large amount of silica in the hairs found about the serratures of its leaves.
All plants are provided with hairs; some few with hairs of a defensive character. Those in the Urtica dioica, commonly called the Stinging-nettle, are glandular hairs, developed from the cuticle, and contain an irritating fluid; in other hairs a circulation is visible: examined under a power of 100 diameters, they present the appearance seen at [Plate XIII]., No. 19.
Fig. 323.
A. Cotton; B. Fibres of Flax; C. Filaments of Silk; D. Wool of Sheep.
The fibrous tissue of plants is of great value in many manufactures. It supplies material for our linens, cordage, paper, and other industries. This tissue is remarkable for toughness of fibre, and exhibits an approach to indestructibility, in the use it is put to in connection with the electric light. It is of importance, then, to be able to distinguish it from other fibres with which it is often mixed in various manufactures. Here the use of the microscope is found of considerable importance. In flax and hemp, in which the fibres are of great length, there are traces of transverse markings at short intervals. In the rough condition in which flax is imported into this country, the fibres have been separated, to a certain extent, by a process termed hackling, and further subjected to hackling, maceration, and bleaching, before it can be reduced to the white silky condition required by the spinner and weaver, and finally assumes the appearance of structureless tubes, [Fig. 323] B. China-grass, New Zealand flax, and some other plants produce a similar material, but are not so strong, in consequence of the outer membrane containing more lignine. It is important to the manufacturer that he should be able to determine the true character of some of the textures employed in articles of clothing; this he may do by the aid of the microscope. In linen we find each component thread made up of the longitudinal, unmarked fibres of flax; but if cotton has been mixed, we recognise a flattened, more or less rounded band, as in [Fig. 323] A, having a very striking resemblance to hair, which, in reality, it is; since, in the condition of elongated cells, it lines the inner surface of the pod. These, again, should he contrasted with the filaments of silk, [Fig. 323] C, and also of wool, [Fig. 323] D. The latter may be at once recognised by the zigzag transverse markings on its fibres. The surface of wool is covered with furrowed and twisted fine cross lines, of which there are from 2,000 to 4,000 in an inch. On this structure depends its felting property, in judging of fleeces, attention should be paid to the fineness and elasticity of the fibre—the furrowed and scaly surface, as shown by the microscope, the quantity of fibre in a given surface, the purity of the fleece, upon which depend the success of the scouring and subsequent operations.
Fig. 324.
1. Woody Fibre from the root of the Elder, exhibiting small pores; 2. Woody fibre of fossil wood, showing large pores; 3. Woody fibre of fossil wood, bordered with pores and spiral fibres; 4. Fossil wood from coal.
In the mummy-cloths of the Egyptians flax only was used, whereas the Peruvians used cotton alone. By the many improvements introduced into manufacturing processes, flax has been reduced to the fineness and texture of silk, and even made to resemble other materials.
Fossil Plants.—It is well known that the primordial forests furnish a number of families of plants familiar to the modern algæologist. The cord-like plant, Chorda filium, known as “dead men’s ropes,” from its proving fatal at times to the too adventurous swimmer who gets entangled in its thick wreaths, had a Lower Silurian representative, known to palæontologists as Palæochorda, or ancient chorda, which existed, apparently, in two species,—a larger and a smaller. The still better known Chondrus crispus, the Irish moss, or Carrageen moss, has likewise its apparent, though more distant representative, in chondritis, a Lower Silurian algal, of which there seems to exist at least three species. The fucoids, or kelpweeds, appear to have also their representatives in such plants as Fucoides gracilis, of the Lower Silurians of the Malverns; in short, the Thallogens of the first ages of vegetable life seem to have resembled in the group, and in at least their more prominent features, the algæ of the existing time. And with the first indications of land we pass from the thallogens to the acrogens—from the seaweeds to the fern-allies. The Lycopodiaceæ, or club-mosses, bear in the axils of their leaves minute circular cases, which form the receptacles of their spore-like seeds. And when high in the Upper Silurian system, and just when preparing to quit it for the Lower Old Red Sandstone, we detect our earliest terrestrial organisms, we find that they are composed exclusively of those little spore-receptacles.
The existing plants whence we derive our analogies in dealing with the vegetation of this early period contribute but little, if at all, to the support of animal life. The ferns and their allies remain untouched by the grazing animals. Our native club-mosses, though once used in medicine, are positively deleterious; horsetails (Equisetaceæ), though harmless, so abound in silex, which wrap them round with a cuticle of stone, that they are rarely cropped by cattle; while the thickets of fern which cover our hill and dell, and seem so temptingly rich and green in their season, scarce support the existence of a single creature, and remain untouched, in stem and leaf, from their first appearance in spring until they droop and wither under the frosts of early winter.
The flora of the coal measures was the richest and most luxuriant, in at least individual productions, with which the fossil botanist has formed an acquaintance. Never before or since did our planet bear so rank a vegetation as that of which the numerous coal seams and inflammable shales of the carboniferous period form but a portion of the remains—the portion spared, in the first instance, by dissipation and decay, and in the second by denuding agencies. Nevertheless almost all our coal—the stored-up fuel of a world—is not, as it is often said to be, the product of destroyed forests of conifers and flora of the profuse vegetation of the earliest periods in the history of our globe. Later investigations show that our coal measures are the compressed accumulations of peat-bogs which, layer by layer, have sunken down under the superimposed weight of the next. The vertical stems of coniferous trees became imbedded by a natural process of decay, and were subsequently overwhelmed in the erect position in which they are found. The true grasses scarcely appear in the fossil state at all. For the first time, amid the remains of a flora that seems to have had but few flowers—the Oolitic ages—do we detect, in a few broken fragments of the wings of butterflies, decided traces of the flower-sucking insects. Not, however, until we enter into the great Tertiary division do these become numerous. The first bee makes its appearance in the amber of the Eocene, locked up hermetically in its gem-like tomb—an embalmed corpse in a crystal coffin—along with fragments of flower-bearing herbs and trees. Her tomb remains to testify to the gradual fitting up of our earth as a place of habitation for creatures destined to seek delight for the mind and eye, as certainly as for the proper senses, and in especial marks the introduction of the stately forest trees, and the arrival of the charmingly beautiful flowers that now deck the earth.[62]