The calyptra is either simply mitre-shaped, or mítriform (Pl. III. fig. 24), or it is half-cleft, or dimid´iate (figs. 14, 36). When the capsule is ripe, the upper part usually separates at a circular horizontal line (fig. 8) as a kind of lid, which is called the oper´culum (operculum, a lid), and thus the spores are enabled to escape. The rim of the capsule, from which the operculum has separated, forms its mouth, and this often exhibits a fringe of teeth (figs. 15, 18, 31), arranged in one or more rows; sometimes the teeth are replaced by a membrane, or, again, both teeth and a membrane may be present. This mouth-fringe is the per´istome (περἰ, around, στὁμα, mouth). In many mosses, an elastic row or ring of cells is situated between the mouth of the capsule and its operculum, called the annulus (figs. 18 & 39); this, when the capsule is ripe, aids in throwing off the operculum.

It is important to become acquainted with the structure and arrangement of these parts, as they form characters by which the families and genera of mosses are distinguished.

The capsules of the mosses form very beautiful microscopic objects, especially those furnished with a toothed peristome.

Most of the mosses produce their fructification in the winter and spring.

The class of mosses is divided into two Orders, according to whether the fruit-stalk is terminal, i. e. arises from the end of the stem or its branches, or whether it is lateral, arising from the side of the stem. Those with the fruit-stalk terminal, or the end fruited (Pl. III. fig. 22), form the Ac´rocarpi (ἄκρα, summit, καρπὀς, fruit); while those with the fruit-stalks lateral, or the side-fruited mosses (fig. 43), constitute the Pleu´rocarpi (πλευρἀ, side). The new shoots or young branches of the stems of mosses are termed innovations.

We will now examine a few common mosses more in detail, beginning with the Acrocarpi.

Sphag´num acutifólium (Pl. III. fig. 4) is found in pools or bogs, growing at the margins so as to be partially immersed. In this moss, the upper branches are grouped into a head. The leaves are crowded, and overlapping or im´bricate (imbrex, a tile) on the elongate stems; they are egg-shaped (ovate) on the main stems (fig. 1), and narrower or ovate-lanceolate on the branches; they are nerveless, and finely toothed at the apex. The capsule (fig. 5) is roundish-ovate, without a peristome, and the operculum is flattened. The grouped arrangement of the upper branches renders the species of Sphagnum easily recognized. The structure of the leaves is also very peculiar and characteristic (fig. 2). The cells of which they consist are of two kinds, one (fig. 2 a) being colourless, elongate, pointed, and containing a spiral fibre; the other consisting of shorter and narrower obtuse cells, containing chlorophyll, and situated between the former. In many of the former kind of cells, little round apertures exist on the under surface, and minute animals may sometimes be found imprisoned in them.

Another species of Sphagnum, S. obtusifolium, is common, and greatly resembles the above, but has shorter and thicker stems, and rounded-ovate, very concave, and obtuse leaves.

Gymnos´tomum truncat´ulum (Pl. III. fig. 6) is a common little moss, found on banks and in fields and gardens.

In this there is no peristome, although, in the young condition, a membrane extends more or less over the interior of the mouth of the capsule. The stem is slender, rigid, and simple, or but little branched. The calyptra is dimidiate; the operculum is present (fig. 8), and terminates above in an oblique beak, or it is obliquely rostrate (ros´trum, a beak) as it is called. The leaves are obovate (fig. 7) or ovate with the broader part remote from the stem, and narrowed at the apex, where the nerve protrudes or is ex´current (excurro, to run out). The spores (fig. 9) are reddish brown and smooth.

Dicránum heteromal´lum (Pl. III. figs. 10 & 11) is probably the first moss the reader will meet with on banks and heaths in the early spring; and it will be sure to be noticed on account of the bright green colour of the patches and the beautiful orange-brown capsules.

In this moss the capsule is nodding (cer´nuous) (Pl. III. fig. 15), and has a single peristome, consisting of sixteen equidistant teeth, each being deeply cut or cleft longitudinally (fig. 15 a), so that there are thirty-two teeth altogether; and these are marked with internal cross-bars, or transverse ridges. The calyptra is dimidiate (fig. 14); and the lid is furnished with a long oblique beak (fig. 15 b). The leaves are crowded, strongly nerved (fig. 12), lanceolate at the base, and very narrow towards the apex, which is toothed; they are, moreover, curved, and bent towards one side, or sécund.

Tor´tula murális (Pl. III. fig. 16) may be found on the top of almost every wall and on waste ground.

In this moss the peristome is single (fig. 18), consisting of thirty-two spirally twisted teeth, arranged in pairs. They are narrow and slender, and each is composed of two longitudinal portions (fig. 18 a), one of which is pale yellow, the other reddish brown, like the capsule, and both are fringed and covered with very minute papillæ. The capsule (fig. 18) is oblong, the ring or annulus remaining for some time. The lid is conical (fig. 19), with a longish somewhat oblique beak, and the calyptra is dimidiate (fig. 20). The stems are very short; the leaves (fig. 17) are oblong, obtuse; the nerve strong, and projecting as a colourless spirally striated bristle. The bristles often give the patches of the moss a hoary appearance on wall-tops. The margins of the leaves are folded back or recurved, giving them a peculiar thickened appearance.

The largest of our mosses are contained in the next genus, viz. Polyt´richum, some of them having the stems from 2 to 4 inches, or even more, in height; they are common on heaths and in woods.

Polyt´richum pilif´erum (Pl. III. fig. 22) is very common on open dry heaths. This moss has simple stems, with the leaves crowded on the lower part of those which are fertile or fruit-bearing. The fruit-stalk is terminal (acrocarpous); the capsule ovate, 4-sided or quadrangular, with a knob or struma (strúma, a swelling) at the base, the lid having a short beak. The calyptra (fig. 24) is half-cleft (dimidiate) and very hairy. The peristome is single, and consists of sixty-four teeth. The leaves (fig. 23) are lanceolate, nearly upright, the margins folded inwards or inflexed; and they end abruptly in a saw-edged or serrated hair-like point.

Poly´trichum commúne, which is also very common, is larger than the last species, and may easily be distinguished by the curved and serrate leaves, which have no bristle-point.

In the early spring, patches of both these mosses may be found, in which the stems are terminated by little rosettes (figs. 25 & 26); these will be referred to presently.

Keeping still to the end-fruited or Acrocarpous mosses, we have next to mention Funa´ria hygromet´rica (Pl. III. figs. 28, 29), which is readily distinguished from most other mosses by the pale apple-green colour which it possesses before the capsule ripens. It is extremely common on walls and waste ground.

The capsule of this moss (fig. 31) differs from those of the preceding mosses in the peristome being double (fig. 42), or composed of an outer and an inner row of teeth. The outer row consists of sixteen oblique reddish teeth, which are marked with transverse bars or trabe´culæ (trabecula, a little beam), and their points are connected by a net-like thin plate. The inner row contains also sixteen teeth, arising from the division of the membrane lining the capsule; these are yellowish, thin, and placed opposite the outer teeth. The capsule itself is pear-shaped or pyriform, orange-red when ripe, curved, and with the mouth oblique. The calyptra (fig. 36) is half-cleft, and expanded as if blown out below. The lid (fig. 31 a) is convex and obtuse; and the annulus (fig. 39) is large and easily separable. The fruit-stalks are curved near the top. The leaves (fig. 30) are ovate, concave, entire, with a nerve reaching the apex, which is acute and prolonged into a little point, or apic´ulate. The spores (fig. 38) are small and reddish brown. The specific name (hygromet´rica) of this moss expresses its hygrometric property; for if either the recent and moist moss be dried or the dry moss wetted, the fruit-stalk gradually twists in opposite directions in the two cases.

The last of the Acrocarpous mosses which we shall notice, Bry´um capil´lare (Pl. III. fig. 49), is tolerably common on trunks of trees, on the ground, and sometimes on walls.

The capsule of this moss has a double peristome or mouth-fringe; the outer consisting of sixteen reddish-brown, equidistant, transversely striped teeth; the inner composed of sixteen thin keeled teeth, more or less split down the middle, and with two or three intermediate cilia. The capsule is nodding, smooth, oblong, pear-shaped, slightly narrowed below the mouth; the lid being somewhat convex, and furnished with a short slender beak. The calyptra is dimidiate. The leaves (fig. 50) are obovate, the nerve extending beyond the point, rendering them bristle-pointed. The seeds (fig. 49 a) are small and green.

This moss serves to illustrate a great difficulty, which will often occur to the student, in determining whether a moss is end-fruited or side-fruited. For in this, as in many other end-fruited mosses, a little side-shoot or young branch (innovation) grows from the main stem immediately below the leaves surrounding the base of the fruit-stalk, so that the fruit-stalk appears to arise from the side of the stem. The only method of overcoming the difficulty is to examine carefully the comparative size and thickness of the stem and the shoot, and to determine which is the weaker and so the newer. The leaves surrounding the base of the fruit-stalk, which are mostly somewhat different in structure from the stem-leaves, are called the perichæ´tial (περἰ, around, χαἱτη, bristle) leaves.

From among the side-fruited or Pleurocarpous mosses we shall select one only, Hyp´num rutab´ulum (Pl. III. fig. 43), which is common on the trunks of trees and on banks.

In this moss, the nodding unequal curved capsule (fig. 45) has a double peristome, resembling that of Bryum (fig. 47). The calyptra is half-cleft, and the lid conical and shortly beaked. The stem is reclining or procumbent, and the pale green imbricated leaves (fig. 41) are ovate and pointed, faintly saw-edged, the nerve becoming indistinct at about the middle. It will be noticed that the cells of the leaf (fig. 48) have the prosenchymatous form, or are elongate with pointed ends; and that the fruit-stalk (fig. 45) is rough with little grains.

Fructification.—The fruit-producing organs of the mosses are of two kinds, comparable to those of the flowering plants, but with their names changed, as in the case of the ferns; the representatives of the anther being called antheridia, and those of the pistil archegonia. The antheridia may be best examined in Polytrichum piliferum or commune, the patches of stems with red rosette-like heads (figs. 25, 26) being readily found in the spring on open heaths. The coloured leaves forming these heads differ in form from those of the stem, being broader and very sharp-pointed, and have received the distinctive name of perigónial (περἰ, around, γνος, offspring) leaves. In the centre of these leaves, which must be separated with mounted needles in a drop of water, the antheridia (fig. 27), forming oblong cellular green sacs, will be seen; and intermingled with them will be found some slender pale filaments, composed of mostly two rows of cells, which are the paraph´yses (παρἁφυσις, a side growth). If the antheridia are quite ripe, they swell somewhat in the water, and from the free or unattached end a very delicate, colourless, cellular mass gradually escapes. If the antheridia are not quite ripe, the mass must be liberated by dissection.

On carefully examining this mass under a high power, it will be seen to consist of very delicate rounded cells (fig. 3 a), each containing a coiled filament, revolving more or less rapidly. After a time, these filaments (fig. 3 b) escape, so that they may be examined more minutely. They are excessively delicate, and are best seen when dried on the slide. Each consists of a very slender curved filament, with a still finer filament, or cilium, arising from it on each side. These are the spermatozóa or spermatozóids (στἑρμα, seed, ξῶον, animal, εἶδος, resemblance).

In Funária the antheridia (fig. 37) may also be found, by careful examination, in the little green heads terminating some of the stems (fig. 41, of the natural size). In this moss, the paraphyses are inflated at the summit into little knobs, or they are capitate (fig. 37). The pistil-like organs of mosses, or the archegónia, from which the capsule is formed, must be looked for in the winter or early spring. They occur in the parts of the stems from which the fruit-stalk subsequently arises, and are surrounded by perichætial leaves, so as to resemble in general aspect the antheridial heads. They are readily found in Tortula and Funaria, which are always at hand.

The archegonia (Pl. III. fig. 21) differ in form from the antheridia, being flask-shaped, with a neck and a dilated base. The neck contains a slender canal, and within the base is a special embryonal cell, from which the capsule is subsequently formed. The spermatozoa of the antheridia pass down the canals of the archegonia, and fertilize the embryonal cells; but one archegonium only comes to maturity in each head, the others ceasing to grow, and withering, in which condition they are found at the base of the fruit-stalk when the capsule is fully formed. The embryonal cell grows by subdivision, so as to form a stalk-like body, which as it rises extends the archegonium upwards until it splits across near the base. Thus the archegonium becomes split horizontally into two parts, the upper and longer of which forms the calyptra, whilst the lower remains as a very short tube or sheath (vagi´nula) surrounding the base of the fruit-stalk. The cellular stalk-like body then swells at the summit, the swollen portion gradually becoming developed into the capsule, by resolving itself into an outer wall lined inside with a coat forming the outer row of teeth at the top, and within this a thinner membrane or spore-sac, the cleft upper margin of which forms the inner teeth; and within this are contained the spores. The mass of cells within the spore-sac remains, forming a central column, called the columella.

These stages of growth may be readily traced in Funaria. In Plate III., fig. 40 represents two fertilized archegonia of the natural size, surrounded by the perichætial leaves; fig. 33 is a still more advanced archegone. In fig. 34 the calyptra has separated from the vaginule, and contains the stalk-like body, which is represented alone in fig. 32, the dark summit indicating the commencing formation of the capsule. Fig. 35 represents the young capsule, in which all the parts are more advanced in growth.

When the seeds of mosses germinate, they produce at first a green Conferva-like filament, which branches at one end, the cells containing green endochrome, while brownish little roots are given off from the other end. The young leafy buds or young stems arise from these confervoid filaments.

Examination.—In the examination of the mosses, the capsules should be viewed as opake objects while fixed in the forceps; and to discover the minute structure of the teeth of the peristome, a capsule should be wetted with spirit, then immersed in water, slit up with fine scissors, and spread out with the mounted needles, so as to form a transparent object. In this way, the curious structure of the teeth becomes very distinct.

It must be noticed that, in the mosses, the antheridia and the archegonia usually occur in separate flower-like heads; or the mosses are either monœcious or diœcious (p. 38).

Preservation.—The mosses may be dried under pressure, and preserved entire in the same manner as the ferns or the flowering plants. If simply dried without pressure, their structure can be readily made out at any future time, by immersing them in water, or by keeping them for a few hours in a moist atmosphere. The minute structures of mosses may be mounted in solution of chloride of calcium, or in glycerine; they keep extremely well without closing the cells.

CHAPTER VII.

ALGÆ (ALGA, SEA-WEED).

Fucoid´eæ, Fucoid Algæ, or Melanospor´eæ (μἑλας, black or dark). The plants composing this order form our largest sea-weeds, and are found everywhere in the sea and on the sea-shore. They are of an olive-green or olive-brown colour, and usually become darker on drying.

Fúcus vesiculósus, with its parallel-sided or linear olive-brown fronds, is known to every one as the seaweed which is hung up to act as a weather-glass. The fronds have a central stout vein, or midrib, and scattered air-bladders, mostly in pairs.

The fructification consists of yellowish oval enlargements of the ends of the fronds, called the receptacles (fig. 16); but these are somewhat variable in form, being often angular or truncate. On holding one of the receptacles to the light, it will appear to contain a number of little grains imbedded in its substance, slightly projecting above the surface, and in the centre of each is a minute dot or pore. These grains are the capsules, or conceptacles, and contain the spores. The substance of the receptacles is composed of a beautiful network of colourless, jointed, cellular fibres (figs. 17 a and 18 a), the meshes of which are filled with a transparent gelatinous substance; but immediately around the conceptacles the cells are shorter and more closely packed. The spores (fig. 19) are arranged in the conceptacles in a radiate manner; they are brown, and surrounded by a colourless sac, called the perispore (περἰ, around, σπορἀ, seed); and between them are numerous slender, colourless, jointed filaments, the paraph´yses. The spores are not, however, truly single spores, for they ultimately divide into eight segments or sporules, each of which is capable of producing a new plant.

In the conceptacles of some fronds of Fucus no spores will be found, the conceptacles (fig. 18) being filled with elegantly branched colourless filaments (fig. 20), the ends of many of them being distended into little yellowish sacs; these are the antherid´ia. The antheridia contain large numbers of exceedingly minute spermatozoa, furnished with two cilia, and very similar to those existing in the antheridia of the mosses; these, escaping through the pore of the conceptacle, fertilize the spores.

The figure (20) in the plate was drawn from a conceptacle of Fucus serrátus, another common species, differing from F. vesiculosus in having the margins of the frond serrate; the antheridia of the two species do not, however, differ in any important respect. To examine the conceptacles of Fucus and their contents, the receptacles should be soaked in water, if not fresh, and thin sections made with a sharp knife. They form very beautiful objects, and may be preserved in chloride of calcium or glycerine.

Florid´eæ, or Rhodosper´meæ (ῥὁδον, rose, σπἑρμα, seed).—The second Order of Algæ, forming the Florideæ (flos, a flower), comprises the red sea-weeds; a few of them are purple, or greenish-red; so that by the colour alone they may be readily distinguished from the Fucoids, and from nearly all those of the next Order, the Confer´voids. A few of them are leaflike, or possess flat fronds; but most of them consist of finely divided or feathery fronds. They are often found upon the sea-shore of a dirty white colour, the colouring matter having been decomposed or washed out by rain.

We shall consider a few of the genera and species under the heads of the families to which they belong.

Corallina´ceæ, the Corallines, or calcareous Algæ.—In this family we have the beautiful Corallína officinális (Pl. IV. fig. 28), the common Coralline, which is very abundant on the sea-shore, attached to larger sea-weeds, shells, and rocks. It is hard and chalky, from the presence of a large proportion of carbonate of lime in its minute cells. The fronds are composed of jointed and branched filaments. The fructification (figs. 29 and 30) consists of ovate cellular capsules, or ceramid´ia (κερἁμιον, earthen vessel), placed mostly at the ends of pinnate stalks, and containing a tuft of somewhat club-shaped jointed spores, springing from the base of the capsules (fig. 30). When ripe, the spores escape from a pore or hole in the end of the capsules. The spores are 4-jointed, and hence are called tet´raspores (τἑτρα, four).

To observe these spores, the capsules must be soaked in strong vinegar for some hours, and then washed with water, to dissolve the calcareous matter.

Jánia rúbens (Pl. IV. figs. 7 and 8) is another common and very elegant little coralline, and is of a pale red colour. It differs from the last in the branches being dichot´omous (δἱχα, in two, τομὀς, cutting) or forked, instead of pinnate. The capsules, or ceramidia, have also two short horn-like branchlets, placed one on each side, near the end.

The genus Melobésia has the frond crustaceous, i. e. forming a hard crust or layer. M. polymor´pha (Pl. IV. figs. 5 and 6) is common on shells, stones, &c. The capsules (ceramidia) here form little blunt cones, scattered over the crusts, and containing the tufted tetraspores, as in Corallina.

Lithocys´tis Allman´ni (fig. 9) is very minute, and not uncommon upon sea-weeds, stones, &c. It consists of a single fan-shaped crustaceous layer of cells, closely investing the body to which it is attached; its fructification is unknown.

Leaving the family of crustaceous Florideæ, we shall now pass to those of softer consistence, although all the marine Algæ contain a considerable quantity of calcareous matter.

Rhodomela´ceæ.—In this family we have the large genus Polysiphónia, in which the frond (Pl. IV. figs. 25 and 26) is filamentous, the filaments being apparently jointed and longitudinally striated. The filaments are composed of rings of cells (fig. 27), arranged end to end, and containing dark endochrome. The ends of the colourless cell-walls separating the endochromes of the cells of adjacent rings produce the jointed appearance; while the striated appearance is caused by the dark cells being elongate and the cell-walls thick, so as to form white interspaces.

The fructification consists of capsules (ceramidia), attached to the sides of the branches, containing pear-shaped spores, with tetraspores imbedded in swollen branches of separate plants.

Polysiphónia fastigiáta (fig. 25, a small piece) is common, attached to the fronds of Fucus. Its filaments are rigid, bristle-like, of the same breadth throughout, forked, and forming globular brown or yellowish tufts, from 2 to 4 inches long. The joints are broader than long, each with 16-18 of the dark cells. In the centre of the branches of this sea-weed is a row of curious objects (fig. 26 a), consisting of a dark-coloured body surrounded with irregular spiny marginal processes, and with a colourless short process above and below. These require further investigation.

P. nigres´cens is also common among masses of seaweeds. Its filaments are brown, pinnate, the branches awl-shaped, and the joints about as long as broad.

Dásya coccin´ea (Pl. IV. figs. 1 and 2, representing small portions of a filament) is a very common filamentous red sea-weed of the same family. The filaments are 6-8 inches long, and bipinnate,—the larger ones somewhat resembling those of Polysiphonia, in being composed of parallel longitudinal cells, arranged round the centre, but containing also smaller intermediate cells; while the smallest branches (fig. 2), which arise in tufts, consist of a single row of cells, little longer than broad. The fruit consists of ovate capsules (ceramidia), placed at the base of the branches, and containing a round mass of spores. There is also another kind of fructification, occurring on distinct plants; this is formed of one or two rows of tetraspores, immersed in pod-like capsules, called stichid´ia (στἱχος, row).

Delesseria´ceæ.—In this family, the typical or most highly developed genus of which, Delesséria, has beautiful leaf-like rose-red fronds, we shall examine the common Plocámium coccin´eum (Pl. IV. figs. 23 and 22). This is of a fine red colour; the fronds are from 2 to 12 inches long, and consist of numerous branched and bushy filaments. These are compressed, with the branchlets arranged in alternate rows on the two margins of the stem. The end branchlets are acute and pectinate (pecten, a comb), or arranged like the teeth of a comb. The cells of which the filaments consist are small and angular, giving the surface the appearance of being elegantly netted under a high power. The fruit (fig. 22) consists of globular capsules, called coccid´ia (κὁκκος, a berry), placed in the axils or forks at which two branches separate, and containing a mass of angular spores. There are also tetraspore-pods (stichidia), as in Dasya; and tetraspores (fig. 24) in little leaf-like altered branches (fig. 21), called spor´ophylles (σπορἁ, seed, φὑλλον, leaf), and antheridia are present.

Rhodymenia´ceæ.—In this family we have Hyp´nea purpuras´cens (Pl. IV. fig. 35). The filamentous pale purple frond of this sea-weed is from 6 inches to a foot or more in length, the branches being alternate and spreading. The fructification consists of capsules or coccidia (fig. 32), immersed in the branches, and containing the spores (fig. 34). Tetraspores also occur in the cells of the surface of the filaments.

Ceramia´ceæ.—This is the last family to be noticed. Cerámium nodósum (Pl. IV. figs. 10 and 11), which belongs to it, is a most delicate and elegant filamentous sea-weed, commonly found attached to other sea-weeds. The filaments are hair-like or capillary, irregularly dichot´omous; they consist of colourless cells, 3 or 4 times as long as broad, and with thick walls. The junctions of the cells are swollen (fig. 11), and covered with very minute dark red cells, giving them a knotty and jointed appearance to the naked eye or under a low power. The globular capsules, or favel’læ (favus, a honeycomb), containing the numerous spores, are situated at the ends of the branchlets, and the tetraspores (fig. 11) in twos or threes on the outer margins of them.

In Cerámium rúbrum, which is also very common, being found attached to stones, rocks, and the larger Algæ, the filaments (Pl. IV. fig. 12) are stouter than in C. nodosum, branched so as to form tufts from 2 to 10 inches long, and their ends forked, with the tips hooked inwards (fig. 14). The central cells of the filaments are large and rounded, and their walls are entirely covered with a layer of very small angular red cells. The globular capsules (fig. 15), or favellæ, are situated on the suter surface of the branches, stalked, and supported by 3 or 4 short branchlets. The tetraspores (fig. 13) are imbedded in the branches, towards the ends. The capsules called favellæ differ from the coccidia in the walls being simply membranous, while the walls of the coccidia, like those of the ceramidia, are composed of cells.

The tetraspores are usually imbedded, among the cells of the superficial layer of the filaments, and are not very easily recognized by an unpractised eye; it will be observed in the figures that they are sometimes cleft horizontally, at others obliquely.

Confervoid´eæ.—This Order consists principally of the green freshwater Algæ, although some of them are yellowish brown, purple, or red, and some are marine. Their general structure may be best illustrated by selecting certain common examples from the families composing the order. The families are 13 in number. The species which are figured in the plates are found in fresh water, except when otherwise stated.

Conferva´ceæ.—On removing some of the soft green matter found adhering to the stems of water-plants in any pool or pond, one of the species of Conferva, C. flocculosa, is almost sure to be met with. On close inspection with the naked eye, the green filaments of which it consists are just visible, as extremely fine, soft, silky threads; and, under a high power of the microscope, the filaments are seen to be unbranched, and composed of a single row of cells (Pl. V. fig. 1), or joints, as they are called in technical works; these are 2 or 3 times as long as broad. In some specimens the joints are swollen, so as to present a rounded outline. In another common species, C. bombyc´ina, the filaments are somewhat more slender, and the joints are from 3 to 5 times as long as broad.

Cladoph´ora crispáta.—This Confervoid forms large, entangled, dull-green masses, composed of branched, tufted, somewhat rigid and coarse filaments. It is often a troublesome overrunner of the fresh-water vivarium. The filaments are composed of thick-walled cells (Pl. V. fig. 6), from 4 to 6 times as long as broad, and often containing minute starch-granules.

The Confervaceæ have two modes of reproduction. The first of these consists in the division of the endochrome of the joints into a number of distinct segments, each of which becomes furnished at one end with two very slender cilia (Pl. V. fig. 6 a). After a time, these ciliated bodies, which are called zo´ospores (ξῶον, animal, σπορἀ, seed) or gonid´ia (λονἠ, seed, εἶδος, resemblance), escape from the cells either by their rupture or through a papillary orifice, and swim about in the water, ultimately losing their cilia and growing into cells resembling those of the parent plant. In the second method, which occurs, for instance, in Conferva bombycina, certain of the joints enlarge so as to become rounded or inflated; their endochrome then becomes coated with a new cell-wall, and so forms a spore, which subsequently escapes from the cell and germinates.

Chætophora´ceæ.—Draparnal´dia glomeráta forms small green jelly-like masses, adhering to sticks and stones in water. These consist of branched filaments (Pl. V. fig. 10), prolonged at the ends into colourless hair-like points, and composed of single rows of cells, the green endochrome forming a band across the middle of each cell, the ends being colourless.

In Coleochæ´te scutáta (Pl. V. fig. 14) the cells are closely united, so as to form a minute flat green disk. In the natural state, this beautiful little object adheres to the submerged leaves and stems of water-plants, and is therefore difficult to be found. But if a few healthy water-plants be kept for some time in a glass jar, the little Coleochæte, which is about as large as a pin’s head, will often be found adhering to the side of the glass.

Bat´rachosper´meæ.—The members of this family resemble to the naked eye the little masses of Draparnaldia, and they are found in the same localities. They are of various colours, being green, brown, purple, or red. They consist, as in Bat´rachosper´mum monilifor´me (Pl. V. fig. 8), of branched filaments, which have a knotty appearance under a low power. The larger filaments are composed of cells arranged end to end, the knots consisting of numerous smaller whorled filaments, i. e. filaments arising from around them at the same level (fig. 7). The cells composing the whorled filaments are beaded or moniliform, and are prolonged into colourless hair-like points. The globules seen among the branches (fig. 7) consist of groups of spores.

Zygnema´ceæ.—The members of this family resemble the Confervaceæ in consisting of simple cellular filaments (Pl. V. figs. 11, 13), but differ from them in the elegant arrangement of the endochrome: this forms beautiful spiral bands, as in Spirog´yra quini´na (fig. 11), or star-shaped masses, as in Zygne´ma crucia´ta (fig. 13). A remarkably curious phenomenon met with in them is the manner in which the spores are formed, and which is known as conjugation. In this process the opposite cells of two distinct filaments, lying near together, push out protrusions of the cell-walls, which meet and open into each other, forming cross tubes, as in Spirog´yra nit´ida (Pl. V. fig. 12). The contents of the opposite cells of the filaments then unite, forming large spores, which remain either in the cells of one of the filaments or in the cross tubes.

The three species figured are common in clear pools.

Desmidia´ceæ.—The Desmidiaceæ are truly microscopic, few of them being even perceptible to the naked eye without the very closest examination. They are very beautiful, on account of their bright green colour and often elegant forms. Many of them are very common, existing in every pond or ditch; but they abound most in clear open boggy pools on heaths. On placing some water containing them in a glass jar and exposing it to the light, they will often be found adhering to the glass, or forming a layer on the surface of the muddy sediment.

The Desmidiaceæ consist mostly of single cells (Pl. V. figs. 9, 16); and these consist of two equal halves or segments, as indicated either by a paleness of the endochrome or a deep constriction at the line of junction, which is called the suture. The cells are often elegantly lobed and cut, or spiny; and in many the surface exhibits minute markings, consisting of little protrusions of the cell-wall outwards, or inflations, as they are called.

Their reproduction is effected by division and conjugation. In the process of division the cells gradually separate at the suture, and a new half-cell is formed upon each old half, which grows until it attains the size and form of the original half of the parent-cell. The conjugation is effected by two cells approximating so that their sutures are near together, the cells then open at the sutures, and the effused contents become united to form a spore or sporange, from which one or more individuals are formed. These spores are often elegantly spinous on the surface.

Among the species selected for illustration is Clostérium acerósum (Pl. V. fig. 9), in which the cells are single, elongate, very slightly curved or lunate; the endochrome forms long bands, often containing numerous globules or transparent vesicles. At each end of the cells is a round transparent vesicle, containing exceedingly minute granules, which exhibit a trembling kind of motion. Between the cell-wall and the cell-contents very fine currents may also be detected, forming a circulation resembling that in the hairs of Tradescantia.

In Micrastérias rotáta (Pl. V. fig. 16) the cells, which are single, are deeply cleft into two segments at the suture, the segments being again regularly cut into five lobes, which are toothed or dentate.

In Hyalothéca dissil´iens (Pl. V. fig. 21) the cells are united into a cylindrical filament, and are surrounded by a very delicate gelatinous sheath. In Ankistrodes´mus falcátus (Pl. V. fig. 19) the cells resemble those of Closterium in shape, but are aggregated into faggot-like bundles, and are very much smaller. In the beautiful little Pedias´trum boryánum (Pl. V. fig. 20) the cells are aggregated into a disk, the marginal cells being bidentate or having each two points, so that the whole resembles a star. The species of Pediastrum are reproduced by the contents of each cell subdividing into numerous ciliated segments or zoospores, which subsequently escape in a mass from the cell, ultimately losing their cilia, and reuniting to form a new individual.

In Scenedes´mus quadricau´da (Pl. V. fig. 25) the oblong cells are united, side by side, the outermost cells being furnished with a bristle at each end. The division of these cells takes place obliquely, so that in the divided groups the cells are situated in two alternate rows.

The spores of many of the Desmidiaceæ are spinous, and they are often found fossil in flint (Pl. V. fig. 15). To detect them in this substance, thin slips of flint may be examined under a half-inch power; or the chips of flint may be cemented to a slide with balsam, and ground down on a hone.

The Desmidiaceæ must be mounted in the moist state: the smaller ones will keep well in chloride of calcium; but the larger ones are injured both by that liquid and by glycerine. The remarks made upon mounting, at page 15, are especially applicable to these delicate organisms.

Diatoma´ceæ, or Siliceous Algæ.—The members of this family are singly very minute; but when existing in large numbers, as they are often found at the bottom of ditches and ponds, on the submerged stems of water-plants, or upon damp ground, they form yellowish-brown evident masses or strata. They occur both in sea-and in fresh water. They usually consist, like the Desmidiaceæ, of single cells, which are called frustules. But they are especially characterized by the cell-walls being imbued with silica or flint, so that if the frustules be heated to redness upon the point of a knife or a slip of platinum-foil, which destroys the organic part of the cells, the coat of silica remains, exhibiting the perfect form of the original cells or frustules. The form of the frustules is very different in the various genera and species, as represented in Pl. V. figs. 22, 23, 27, 30, 31, and Pl. VI. figs. 16, 17, 23; and it will be noticed that, in the figures, two views are given of each frustule, f indicating the front view, and s the side view. In all the front views, as in Pl. V. fig. 22, one or more lines will be observed running longitudinally down the middle of the frustules, and corresponding to the indications of division existing in the cells of the Desmidiaceæ. Each half of a frustule is called a valve, and the line at which these valves meet is called the suture. That side or aspect of the frustule in which the suture lies (fig. 22 f) is the front view; and the other aspect of the frustule (fig. 22 s) is the side view. The frustules are mostly four-sided—the main breadths of the two opposite valves forming two sides, and the bent margins of the valves, with the back and front of the hoop, forming the two other sides; so that the view presented by the side of a frustule is the same as that of a single valve. The suture is the line at which the division of the frustules takes place in the formation of new individuals. In this process the cell-contents divide into two parts, as in ordinary endogenous cell-formation,—the two new surfaces thus produced becoming coated with a new portion of cell-wall or valve, so that two frustules now occupy the place of the original one. At the same time a siliceous band, encircling the frustules at the line of suture, is formed to fill up the interval between the edges of the parent valves; this is the hoop (Pl. V. fig. 22 f; Pl. VI. fig. 10 f), and beneath it lie the two newly formed valves. In many cases I believe that each half-frustule becomes coated with a new entire cell-wall, with its siliceous valves.

The frustules of the Diatomaceæ are constantly undergoing division when in vigorous growth. After the frustules have divided, the new ones either separate entirely, as is perhaps most commonly the case; or they remain united, sometimes completely, so as to constitute a filament (Pl. V. fig. 23), while at others the frustules cohere only at the angles (Pl VI. fig. 23), so as to form a zigzag chain.

In some species, the frustules are attached to foreign bodies by means of a gelatinous cushion (Pl. V. fig. 5; Pl. VI. fig. 7); while in others they are situated upon a simple or branched gelatinous stalk (Pl. V. fig. 17) or stipes (stipes, a stem).

When the frustules are examined in the living state, the cell-contents resemble those of ordinary vegetable cells, excepting in regard to the colour, and exhibit granules and globules, and sometimes a nucleus is visible. It will also be noticed that many of the free frustules move slowly across the field of the microscope; but the cause of the motion is unknown.

When the frustules have been properly prepared, the surface of the valves exhibits a number of coarser or finer markings, consisting of dots, lines (striæ), flutings, or networks, &c., arranged with great regularity and symmetry, often of extreme minuteness, and rendering them exquisite objects under the microscope. The exhibition of these markings requires not only that the valves shall be properly prepared and mounted, but that the object-glasses be of good quality, and that the management of the light be thoroughly understood; so that to a beginner, their examination is often a matter of great difficulty; for only the very coarsest or largest of these markings can be perceived in the natural frustules.

The appearance of these markings, and even their apparent absence or presence, frequently depends upon the kind of illumination used: thus, under one kind of illumination the valves may appear simply white or coloured, while under another they appear covered with lines, and under a third with dots. It will often be observed, also, that the colour of the valves varies according to the illumination and the power used—the same valve appearing white, yellow, brown, blue, &c.; and the wet or dry state of the frustules often cause a decided difference in their appearance as regards colour.

To illustrate the forms and markings of the frustules and valves, we may select the following species taking first those which occur in fresh water.

In Epithémia tur´gida (Pl. V. fig. 30), the side-view or valve (s) exhibits transverse or slightly radiating lines, with intermediate rows of dots—these markings being continued over the margins of the valves so as to appear also in the front view (fig. 30 f), ceasing at the hoop. The frustules are curved or arcuate (ar´cus, a bow) in the side view, oblong and narrowed at the ends in the front view.

In Fragilária capucína (Pl. V. fig. 23), which is extremely common in fresh-water pools, &c., the frustules are united side by side into long filaments, which are often twisted. In the separate and prepared frustule, the front view (Pl. V. fig. 24 f) is rectangular, the valves (s) being narrowly lance-shaped or lanceolate. The valves under ordinary illumination appear colourless and without markings, but, by proper management of the light, very fine transverse striæ are seen upon them, consisting of rows of very minute dots. Fig. 24 s* represents the valve of Fragilaria vires´cens, a nearly allied species.

Diat´oma elongátum (Pl. V. fig. 31) is often found with the above. Its frustules are coherent at the angles. The front view (fig. 32 f) is rectangular, often slightly narrowed in the middle; and the valves are narrowly linear, and capitate at the ends; they are also transversely striated.

In Synédra splen´dens (Pl. V. fig. 5) the frustules radiate from a soft gelatinous cushion. They are linear in the front view (fig. 4 f), the valves (fig. 4 s) being gradually narrowed or attenuated from the middle to the ends, and exhibit transverse striæ interrupted opposite a middle longitudinal line.

In Campylodis´cus costátus (Pl. V. fig. 27) the frustules are disk-shaped and curved, so as somewhat to resemble a saddle. The markings consist of central dots, with radiating coarse flutings.

Nitzsch´ia minutis´sima (Pl. V. fig. 28) has oblique valves, i. e. the front half of the suture is not opposite the back portion; the valves (fig. 29) are constricted in the middle, and the ends narrowed and prolonged. The markings consist of a row of oblong dots or puncta (punctum, a point). This species often forms yellowish layers upon damp paths, &c.

In the next group, the valves have a longitudinal line running down the middle of the valves, with a little knob or nodule in its centre (Pl. V. fig. 22 s), both consisting of internal thickened portions of the valves.

In Cocconéis placen´tula (Pl. V. fig. 33) the valves are oval, and the markings consist of longitudinal rows of minute dots, with a marginal row of puncta; these markings are invisible under ordinary illumination.

In Gomphonéma acuminátum (Pl. V. fig. 17) the frustules are attached to a branched stalk (stipes); they are wedge-shaped or cúneate (cúneus, a wedge) and transversely striate (fig. 18), the striæ consisting of dots.

In Pinnulária vir´idis (Pl. V. fig. 22), which is very often seen slowly traversing the field of the microscope when a drop of pond-water is examined, the frustules in the front view are linear, the valves being elliptic oblong, and transversely striated, the striæ consisting of furrows.

In Gyrosig´ma (Pleurosig´ma) attenuátum (Pl. VI. fig. 16) the valves are sigmoid, or somewhat resemble a Greek ς (sigma) in outline, and the markings consist of rectangularly crossed rows of very fine dots; in the front view, the frustules are linear-oblong with truncate ends.

Tabellária flocculósa (Pl. VI. fig. 23) has the frustules adherent only at the angles, as in Diátoma. They are rectangular, and in the front view exhibit a row of longitudinal dark lines interrupted in the middle; these have been compared to the vittæ of the fruit of the Umbelliferæ, and have received the same name.

Among the marine species may be mentioned Melosíra nummuloídes (Pl. VI. fig. 9), in which the frustules are united into a chain or cylindrical filament. This is very common among sea-weeds, &c.; and it illustrates well the process of division of the frustules (fig. 10 f). The valves are covered with fine dots, and near each end of the frustules is a projecting rim encircling it, and appearing as a curved line extending beyond the margin of the frustule in the front view. In Actinocy´clus undulátus (Pl. VI. fig. 5) the frustules are separate, disk-shaped, and the valves are divided into six equal parts by six rays, each alternate portion of the surface of the valves being situated on a lower level than those adjacent, so that an alteration in the focus is required to bring into view the dots on any two adjacent divisions of the valve. The surface of the valves is covered with easily recognized dots. The form of the surface is best seen in the front view (fig. 5 f) when the frustule is placed on its edge.

Rhabdonéma arcuátum (Pl. VI. fig. 7), which is very commonly found attached to sea-weeds, resembles Tabellária in the frustules having the vittæ (Pl. VI. fig. 8 f). The frustules form short filaments, attached by a little gelatinous cushion. The valves have transverse striæ, interrupted in the middle (fig. 8 s).

Gyrosig´ma (Pleurosig´ma) angulátum (Pl. VI. fig. 17) resembles G. attenuátum in the sigmoid form; but the markings consist of lines crossing each other obliquely; and these are resolvable into rows of dots (fig. 17 a) under suitable illumination.

In Coscinodis´cus radiátus (Pl. VI. fig. 3) the frustules are disk-shaped, the valves being elegantly sculptured with easily recognized cell-like markings or dots, so as to resemble a piece of vegetable cellular tissue. But in some other species the dots are very minute, and difficult to be shown satisfactorily. These markings consist of depressions or pits in the surface of the valves. That this is the case may easily be seen by examining a fragment of the valve, when the shadings of the broken ends of the netted thicker portions, which project like teeth, strongly contrast with the difficultly distinguishable portions of the thin interspaces. The fossil forms from the Bermuda deposit are best for the investigation of this structure; many of these are extremely beautiful microscopic objects, their markings resembling those on the engine-turned back of a watch.

The detection of the finer markings of the Diatomaceæ, which, according to my view, consist of depressions like those upon the valves of Coscinodiscus, is a matter of great difficulty to those who are unaccustomed to the use of the microscope, and who have not a complete set of apparatus. The main point to be attended to in bringing them into view, is to use one-sided oblique light, i. e. to turn the mirror by its stem as much as possible to one side, and then to incline it so as to throw the light upon the object. In this way the valves of the species of Gyrosigma, for instance, appear covered with lines (Pl. VI. figs. 16 and 17); but the lines are spurious, i. e. they are the optical expression of rows of minute dots (figs. 16 a, 17 a); and when oblique light is thrown upon the valves from all sides, by means of a special achromatic condenser, in which the central rays are excluded, the dots become distinct, and the markings resemble those on the valves of Coscinodiscus. To show the finer dots clearly, a valve should be crushed, so as to obtain a fragment as flat as possible; for the surface of the valves is curved more or less in all the species. The valves of G. angulatum are generally used to test the quality of the object-glasses of the microscope, and also for practice in “making out” the lines and dots; there are, however, many Diatomaceæ more difficult.

As the nature of these markings is a disputed point, the discussion of which is not adapted for an elementary work, I must refer for further details to the ‘Micrographic Dictionary;’ it may be remarked, however, that some observers have regarded them as cells, and others as elevations or tubercles on the surface of the valves.

The preparation of the valves for showing the markings should be effected by burning the frustules, or the mass containing them, on a strip of platinum-foil over a spirit-lamp. The incinerated mass should then be transferred to a slide, and the valves separated with the greatest care by a bristle mounted in a hair-pencil stick under a low power of the microscope.

This is, however, a substitute for the proper method, which is dangerous in the hands of one unpractised in chemical manipulation. It is this:—The mass of Diatomaceæ (the water containing it having been carefully poured off as far as possible) is put into a Florence oil-flask, and strong nitric acid (aquafortis) gently added, more than sufficient to cover it. The mixture is then carefully boiled over a spirit-lamp for some time. When it is cold, distilled water is added, the whole shaken, and allowed to settle. The watery part is then gently poured off, more water added, and this poured off after settling, and the process repeated until a drop of the water evaporated to dryness on a slide leaves no residue. The Diatomaceæ then form a white sediment at the bottom of the water, and can be transferred to a slide with a dipping-tube. The drop is then dried with a gentle heat, and the valves mounted as dry transparent objects (p. 12).

If the valves have coarse markings, they may be mounted in balsam; but if the markings are fine, balsam makes them much more difficult of detection.

Many of the most beautiful Diatomaceæ are found in the fossil state; and specimens of these are sold already mounted. I would advise those unacquainted with them to purchase a slide of the “Bermuda” or “Richmond” earth, which abounds in the species of Coscinodiscus; and of the “San Fiore deposit,” which contains many species of Epithemia, Navicula, Pinnularia, &c. These may be procured from Mr. Norman, 178 City Road, or from the microscope-makers.

Volvocin´eæ.—The Volvocineæ are inhabitants of clear fresh-water pools, on heaths and bogs. They are very minute, of a rounded or plate-like (tabular) form, of a green colour, and are pretty readily distinguished from most of the other Algæ by their free motion; for they swim about in the water like animals, as which they were formerly considered. They consist usually of groups of thick-walled soft cells, each being furnished with one or two cilia, by means of which the movement of the compound bodies is produced.

In the beautiful Volvox globátor (which is not uncommon) the cells form a hollow sphere (Pl. VI. fig. 18), studded with exceedingly minute green spots or zoospore-like bodies, representing the green endochrome of the component cells, and from each of which very fine radiating lines extend, so as to give the surface a netted appearance; the lines consisting of delicate processes of the endochrome, which may be compared with those existing in the cells of the hairs of Tradescantia. In the interior of the parent globes are often seen several young organisms, usually eight, of a deep green colour; these escape by the rupture of the parent, so as to form independent beings. Sometimes they are found of a yellow colour, and furnished with a thick transparent coat; these are called “resting spores,” as they remain for some time before undergoing their full development.

The cilia of Volvox, of which there are two to each of the component cells, are difficult to detect; they are best seen when the organism is dried without a cover, or after moistening them with a little solution of iodine, which dyes them brown.

Synúra volvox (Pl. VI. fig. 13) is a still more minute member of this family, and is often found rolling along among Confervæ. The greenish zoospore-like bodies of this Alga have one cilium only, and arise from a common centre by a narrowing of the base (fig. 14).

In Gónium pectorále (fig. 11 a) the green bodies, which are sixteen in number, and furnished each with two cilia, are grouped into a flat square plate; and in the very minute Gonium tranquil´lum (fig. 11 c) these bodies are also sixteen in number, and arranged in a tabular form, but are without cilia.