Chapter I (5)

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.

_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.

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.

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.

_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.”

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.

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.

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.

_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 1/2-inch and then a 1/6-inch objective.

_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.

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.

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.

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.

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.]

_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]