Chapter I (4)

Dr. R. Zeiss some time ago furnished me with a microphotograph of a frustule magnified 4500 diameters that seemed to confirm Mr. T. F. Smith’s view of the structure of these valves. Dr. Van Heurck has also made a study of this diatom, and concludes that the valves consist of two membranes of thin films, and of an intermediate layer, the outer being pierced with openings. The outer membrane is, he believes, “so delicate that it is easily destroyed by acid or by friction, and the several processes employed in cleaning and preparing it for microscopical examination. When the openings or apertures of the internal portion are arranged in alternate rows they assume the hexagonal form; when in straight rows, the openings are seen to be square or oblong.” A description hardly in accord with Fig. 294.

Movements of Diatoms.

The late Professor Smith, in his “Synopsis of Diatoms,” refers to their movements in the following terms: “I am constrained to believe that the movements observed in the Diatomaceæ are due to forces operating within the frustule, and are probably connected with the endosmotic and exosmotic action of the cells. The fluids which are concerned in these actions must enter, and be emitted through the minute foramina at the extremities of the silicious valves.” Schultze’s researches, which were made at a later date, carried this debatable question somewhat further. He is of opinion “that a sarcode (protoplasmic) substance envelops the external surface of the diatoms, and its movements are due to this agent exclusively.” His investigations were mainly confined to _P. angulatum_, and to the larger _P. attenuatum_ (Fig. 293, 1 and 2), as the transverse markings on the frustule do not impede to so great an extent the observation of what is going on within. The living specimen of _P. angulatum_ under the microscope usually has its broad side turned to view, with one long curved “raphe” uppermost, and the other in contact with the glass cover (Fig. 293). Within the frustule the yellow colouring matter, “endochrome,” fills the cavity more or less completely. In the broader part of the frustule these bands of endochrome describe one or two complicated windings. It is only possible in those specimens in which the bands are narrow to properly trace their foldings, and determine their number. The next objects which strike the eye on examining a freshly-gathered Pleurosigma are numerous highly refractive oil-globules. These are not, however, all in the same place, and one globule appears nearer the observer than the other; their relative position is best seen when a view of the narrow side of the frustule can be obtained, so that one raphe is to the left and the other to the right. The blue-black colour which is assumed by these globules after treating with acid demonstrates their oleaginous nature. The middle of the cavity of the frustule is occupied, in the larger navicula, by two large oil-globules (seen in the diagrammatic Fig. 295), and by a colourless finely granular mass, whose position in the body is not so clearly seen in the flat view as in the side view. Besides the central mass, the conical cavities at either end of the frustule are seen to enclose granular substance, and two linear extensions from each of three masses are developed, closely underlying the raphæ. In the side view, therefore, they appear attached to the right and left edges of the interior of the frustule. This colourless granular substance carries in its centre, near the middle part of the diatom, an imperfectly developed nucleus which is not very easy to see, but may be demonstrated by the application of an acid. The colourless substance is protoplasm, and encloses numerous small refractive particles; this, on adding a drop of a one per cent. solution of osmic acid, is coloured blue-black, and proves to be fat. It is, however, exceedingly difficult to determine the exact limitations of the protoplasm, on account of the highly refractive character of the silicious skeleton, and the obstruction to the light presented by the endochrome.

At a short distance the protoplasm reappears, contracted into a considerable mass, within the terminal ends of the frustule. Schultze observed in this part of the protoplasm a rapid molecular movement, “cyclosis,” such as occurs in Closterium, and also a current of the granules of the protoplasm along the raphe. “_Pleurosigma angulatum ‘crawls,’_ as do all diatoms possessing a raphe, along this line of suture. To crawl along, it must have a fixed support.” “There is obviously,” adds Schultze, “but one explanation; it is clear that there must be a band of protoplasm lying along the raphe, which causes the particles of colouring matter to adhere, and gives rise to a gliding movement. For there is but one phenomenon which can be compared with the gliding motion of foreign bodies on the Diatomaceæ, and that is, the clinging to and casting off of particles by the pseudopodia of the rhizopod, as observed, for instance, on placing a living Gromia or Miliolina in still water with finely-powdered carmine. The nature of the adhesion and of the motion is in both cases the same. And since, with diatoms as unicellular organisms, protoplasm forms a large part of the cell (in many cases two distinctly moving protoplasms), this implies that the external movements are referable to the movements of the protoplasm.” It is quite evident to those who have studied the movements of diatoms that they are surrounded by a sarcode structure of a more pellucid character than that of Amœba. Six years before Schultze’s observations were published, I wrote in a third edition of my book, page 307, “The act of progression favours the notion of contractile tentacular filaments--_pseudopodia_--as the organs of locomotion and prehension.”

Since my former observations on the movements of diatoms, I have given much attention to two forms, _P. angulatum_ and _Pinnularia_. The powers used were Hartnack’s No. 8, and Gunlack’s 1/16-inch immersion; Gillett’s condenser illumination, with lamp flame edge turned to mirror and bull’s-eye lens; a perforated slide with a square of thin glass ·006 cemented to it, and a cover-glass of ·005. So far as I could satisfy myself, no terminal space, as in the Closteria, could be seen, otherwise the course of the gemmules is as freely traced as in that form. They are more minute than the _Closterium lunula_ granules, more steadily or slowly seen to pass up and down one half the frustule towards the extremity, one half of the current seeming to turn round upon its axis and descending towards the other. The granules were thickly scattered at the apex, but gradually became fewer, and the ascending and descending current tapered away towards the central nodule, which became more filled up or closed in.

This beautiful sight was not confined to one frustule, but was exhibited in all that were in a healthy condition. I examined several, and watched them for a long time. The phenomenon described depends much upon the healthy condition of the frustule at the time; as the movements of the diatoms became sluggish, the circulation gradually slackens and then ceases altogether. I also saw a somewhat similar action in the more active specimens of _P. hippocampus_ and _Navicula cuspidata_, but the coarser markings and thickness of the wall of these diatoms seemed to place greater difficulties in the way of observation than the finer valves of the _P. angulatum_. One thing I believe is certain, that the circulation described is precisely similar to that seen in the Closteria, or, on a much larger scale, in Chara and the leaf of the Anacharis, bearing in mind also that in the Closterium the cell is divided by a transverse suture, and in _P. angulatum_ by a longitudinal one (Plate II., Nos. 38-40). About the same time some very lively specimens of the Pinnulariæ were sent to me, and the movements of these frustules were more closely observed. One or two of the more active would attack a body relatively larger than itself, it would also force its way into a mass of granular matter, and then recede from it with a jerky motion. In more than one instance a cell of Palmoglæa was seized and carried away by the Pinnularia, the former at the time being actively engaged in the process of cell division. Other diatoms present among my specimens were also in an active condition, and the circulation of granular matter in all was distinctly visible. In the Pinnulariæ two large colourless vesicles were seen on either side of the median nodule, each having a central nucleus, as represented in the accompanying sketch, made while under observation in two positions. The central portion of each frustule was closely packed with a rich yellowish-brown coloured endochrome, interspersed with a few fat globules. The phenomenon of cyclosis was not seen in any of these diatoms, but I have satisfied myself, by staining, of the presence of a delicately fine external protoplasmic covering in many diatoms. That their movements resemble the gliding movements exhibited by the Amœba can scarcely be doubted. Numerous forms of Diatomaceæ are found growing on or attached to water-plants or pieces of detached stalks, which, although generally simple, are sometimes compound, dividing and subdividing in a beautiful ramous manner. Pinnulariæ, Nitzschia, &c., are seen adherent by one extremity, about which they turn or bend themselves as on a hinge. By the process of cell-division, groups of Synedræ become attached by a point, in a fan-like form. The fan-like collection of frustules is said to be flabellate, or radiate. In Licmophora, Achnanthes and other species (Plate II., Nos. 29-33) the double condition of union of frustules and of attachment by a pedicle are illustrated. When a stipe branches it does so normally in a dichotomous manner, each new individual being produced by a secondary pedicle. This regular dichotomy is seen in several genera: Cocconema and Gomphonema, the latter more perfectly in Fig. 296, from a microphotograph, in which a branching, or rather longitudinal, rupture of the pedicle takes place at intervals, and the entire organism presents a more or less complete flabella, or fan-like cluster, on the summit of the branches, and imperfect or single frustules irregularly scattered throughout the whole length of the pedicle.

_Isthmia enervis_ (Fig. 297).--The unicellular frustule of this species is extremely difficult to define, owing to the large areolations of the valves; it has a remarkable internal structure. The olive-brown cell contents are found collected, for the most part, into a central mass, from which radiating, branched, granular threads extend to and unite with the periphery. When viewed by a magnifying power of 600 or 700 diameters, these prolongations are seen to be composed of aggregations of ovate or spindle-shaped corpuscles, held together by protoplasmic matter. These bodies are sometimes quiescent, but more often travel slowly to and fro from the central mass. The general aspect under these conditions so nearly corresponds to the characteristic circulation in the frustules of unicellular plants and of certain rhizopoda, that it is difficult to realise that the object when under examination is an elegant marine diatom.

There is a large section of diatoms in which the frustules are diffused throughout a muco-gelatinous envelope in a definite manner. Histologically this is homologous with the pedicles and connecting nodules thrown out during the act of self-division, and in some species (Cocconeis, Fragillaria, &c.) it often persists after that act is complete.

Diatomaceæ, Recent and Fossil.

1, Achnanthidium; 2, _Diatoma vulgare_, side view and front view; 3, Biddulphia; 4, 5, 6, 7, _Amphitetias antediluviana_, front view, with globular and oval forms; _Gomphonema elongatum_ and _capitatum_.]

_Fossilised Diatomaceæ._--Dr. Gregory was of opinion that a large number of diatoms separated into species are only transition forms, and more extended observations have proved that form and outline are not always to be trusted in this matter. Species-making is a modern invention, and can hardly apply to the indestructible fossilised forms of the frustules of Diatomaceæ, with their beautiful sculpturings and geometrical constructions, which have not been materially changed since they were first deposited. Startling and almost incredible as the assertion may appear to some, it is none the less a fact established beyond all question, that some of the most gigantic mountain-ranges, as the mighty Andes, towering into space 25,250 feet above the level of the sea, their base occupying vast areas of land; as also massive limestone rocks; the sand that covers boundless deserts; and the soil of many wide-extended plains, are each and all principally composed of Diatomaceæ. And, as Dr. Buckland once observed: “The remains of such minute animals have added much more to the mass of materials which compose the exterior crust of the globe than the bones of elephants, hippopotami, and whales.”

In 1841 the late Mr. Sollitt, of Hull, discovered the beautiful longitudinal and transverse _striæ_ (markings) on the _Pleurosigma hippocampus_. A curved graceful line runs down the shell, in the centre of which is an expanded oval opening. Near to the central opening the dots elongate crossways, presenting the appearance of small short bands.

In the vicinity of this town many interesting varieties of Diatomaceæ have been found, the beauty of the varied forms of which are constantly under investigation; at the same time some of them are highly useful, as forming that class of _test objects_ which are better calculated than many others for determining the excellence and powers of certain objectives. Mr. Sollitt carefully measured the markings on some of the frustules and found they ranged between the 1/30000th and 1/130000th of an inch; the _Pleurosigma strigilis_ having the strongest markings, and the _Pleurosigma acus_ the finest.

Mr. J. D. Sollitt not only first proposed their use, but he also furnished the measurements of the lines of the several members of this family, as follows:--

Amphipleura pellucida, or Acus, 130,000 in the inch, cross lines.
" sigmoidea, 70,000 in the inch.
Navicula rhomboides, 111,000 in the inch, cross lines.
Pleurosigma fasciola, fine shell, 86,000 in the inch, cross lines.
" " strong shell, 64,000 in the inch, cross lines.
" strigosum, 72,000 in the inch, diagonal lines.
" angulatum, 51,000 in the inch, diagonal lines.
" quadratum, 50,000 in the inch, diagonal lines.
" Spencerii, 50,000 in the inch, cross lines.
" attenuatum, 42,000 in the inch, cross lines.
" Balticum, 40,000 in the inch, cross lines.
" formosum, 32,000 in the inch, diagonal lines.
" strigilis, 30,000 in the inch, cross lines.

MICRO-PHOTOGRAPH OF TEST DIATOMS.]

Lichenaceæ.

The lichens are a family of autonomous plants, an intermediary group of algals or cellular cryptogams, drawing their nourishment from the air through their whole surface medium, and propagating by spores usually enclosed in asci, and always having green gonidia in their thallus. Their gonidia, bright coloured globular cells, form layers under the cortical covering of the thallus, and generally develop in the form of incrustations, which cover stones, wood, and the bark of trees, or penetrate into the lamellæ of the epidermis of woody plants. The gonidia of lichens partake of both the character of vegetative and reproductive cells.

The thallus in the fructicose group attaches itself by a narrow base, growing in the form of a miniature shrub. Another group is met with in a slimy condition--the gelatinous lichens. These species, for the most part, furnished dyes before the discovery of the coal-tar dyes. In many of the _Palmella cruenta_, commonly found growing on the walls and roofs of houses, a colourless acid liquid is found, which, on being treated with alkali, produces a bright yellow colour; and another, _Avernia vulpina_, furnishes a brown dye; the _Rocella fuciformis_ and _R. tinctoria_ yield the purple dye substance known as orchil, or archil, from which the useful blue paper of the chemist for testing acidity is manufactured. Usnic acid, combined with green and yellow resins, seems to be more or less a constituent of many lichens.

A vertical section of _Palmella stellata_ is given in Plate I., No. 26, in which the emission of the ripe spores of the lichens is seen to be not unlike that which takes place in some of the fungi, Pezizæ, Sphæriæ, &c. If a portion of the thallus be moistened and placed in a common phial, with the apotheca turned toward one side, in a few hours the opposite surface of the glass will be found covered with patches of spores, easily perceptible by their colour; or if placed on a moistened surface, and one of the usual glass slips laid over it, the latter will be covered in a short time. As to the powers of dissemination of these lowly organised plants, an observation led to the conclusion that the gonidia of lichens have greater powers in this direction than was formerly supposed. It is found that by placing a clean sheet of glass in the open air during a fall of snow, and receiving the melting water in a tube or bottle, quantities of what has been looked upon as a “unicellular plant” can be taken, the cells of which may be kept in a dormant condition for a long time during cold weather, but upon the return of spring warmth and moisture they begin to increase, by a process of subdivision, into two, four or eight portions; these soon assume a rounded form, and burst the parent cell-wall open; these secondary cells then begin to divide and subdivide again, and the process may go on without much variation for a long time. The phenomena described may be watched by taking a portion of the bark of a tree on which Chlorococcus has been deposited, and placing it under a glass to keep it in a moderately moist atmosphere; the only difference being a change in colour, caused by the growth of the fibres, as may be seen on microscopical examination. “And this,” says Dr. Hicks, who first observed this phenomenon in plant life, “is an instructive point, because it will be found that the colour varies notably according to the lichen prevalent in its neighbourhood.”[59] He believes there can be no doubt that what has been called Chlorococcus is nothing more than the gonidia of a lichen; and that under suitable conditions, chiefly drought and warmth, the gonidium often throws out from its external envelope a small fibre, which, adhering and branching, forms a “soridium.” “The soridia remain dormant for a very long time, and do not develop into thalli unless in a favourable situation, in some cases it may be for years. It will be perceived that the soridium contains all the elements of a thallus in miniature; in fact, a thallus does frequently arise from one alone, and the fibres of neighbouring soridia interlace; thus a thallus is matured very rapidly. This is one of the causes of the variation of appearance so common in many species of lichens, more readily seen towards the centre of the parent thallus. When the gonidia remain attached to the parent thallus, the circumstances are, of course, more favourable, and they develop into secondary thalli, attached more or less to the older one, which, in many instances, decays beneath them. This process being continued year after year gives an apparent thickness and spongy appearance to the lichen, and is the principal cause of the various modifications in the external aspect of the lichens which caused them formerly to be misunderstood and wrongly classified.”[60]

The erratic lichens are found among the genus Palmella, some of which grow among boulders of the primary and metamorphic formations, curled up into a ball, and only fixed to their matrix by a slender thread. The globular _Lecanora esculenta_ will at times suddenly cover large tracts of country in Persia and Tartary, where it is eaten by the cattle. During a scarcity of food a shower of these lichens, Mr. Berkeley tells us, fell at Erzeroum, and saved the cattle from starvation.[61]

Another group of the Palmella, or Peltigeri, so named from the target-like discs on their surface, spread their foliaceous fronds over the ground, and as the fruit is marginal, it gives the thallus a digitate appearance. These are often spotted over by a little red fungus. The Lecidinei contains numerous species of the most varied habits, and always crustaceous, and so closely adherent to the hard rocks and stones on which they grow, that at length they disintegrate them. From this low species a higher form arises, with erect branching stems, and clothed with foliaceous, brightly-coloured scales.

The Coccocarpei is mainly distinguished by having orbicular discs entirely deprived of the cortical envelope called an excipulum. The discs spring at once from the medullary stratum, and contain asci and sporidia similar to those of the minute fungi Sphæriæ. Some of the lichens are themselves parasitic, and begin existence under the thick skin of the leaves of tropical plants, and spread encrusting thallus over their surface, the excipulum and perithecia being black; but in most cases these are beautifully sculptured, and are interesting objects for the microscope. Indeed, the sphere-bearing lichens, with upright stems bearing globular fruit at the extremity of their branches, are at first indicated by a swelling, but in time the outer layer bursts and exposes sporidia, which are beautiful objects under the microscope on account of their spherical form and more or less deep blue tint. Humble and lowly as lichens may appear to be, they have been divided into fifty-eight or more genera and 2,500 species. The brothers Tulasne, De Bary, the Rev. Mr. Berkeley, and others, devoted great attention to the peculiarities of their structure and natural history.

_Hepaticæ._--An intermediary group of much interest to the microscopist are the Hepaticæ (liverworts). These are found growing on damp rocks in the neighbourhood of springs and dripping banks. The scale-moss, the _Marchantia polymorphia_ (Fig. 299), may be taken as typical of this little group, with its gemmiparous conceptacles and lobed receptacles, bearing archegones on transparent glass-like fruit stalks, carrying on their summits either round shield-like discs or radiating bodies with a striking resemblance to a wheel without its tyre.

The liverworts are closely allied to the mosses, and as much difficulty was experienced in dividing the two, Hooker placed the whole under one genus, the Jungermannia. More recently, however, they have been divided into those with a stem and leaves confluent in a frond, Marchantia; those with stem and leaves distinct, Jungermannia; and those with a solitary capsule, filiform, bivalved, stalked, with a free central placentation, Anthocotaceæ. Some botanists have further divided them, but they are all extensively propagated by gemmæ.

The fronds carry the male organs, or _antherids_, and the disc, in the first instance, bears the female organs, or _archegones_, and after a time gives place to the _sporanges_, or spore cases. It is these bodies which are of so much interest to microscopists; if the plant is brought into a warm room, they suddenly burst open with some violence the moment a drop of water is applied to them, and the sporanges are dispersed in a small cloud of brownish dust. If this dust is examined under a medium power, it is seen to consist of a number of chain-like bodies, somewhat like the spring of a small watch; and if the process of bursting be closely watched, these minute springs will be found twisting and curling about in every direction. The structure of the frond itself will be seen to be interesting when cut in the vertical direction and placed under the microscope.

The gemmæ of _Marchantia polymorphia_ are produced in elegant membranous cups, with a toothed margin growing on the upper surface of the frond, especially in very damp courtyards between the stones, or near running water, where its lobed fronds are found covering extensive tracts of moist soil. At the period of fructification the fronds send up stalks, which carry at their summit round shield-like radiating discs, which bear upon their surface a number of little open basket-shaped “conceptacles.” These again expand into singularly graceful cups (as in Fig. 300), and are found in all stages of development. When mature, the basket contains a number of little green round or oblong discs, each composed of two or more layers of cells; the wall itself being surmounted by a glistening fringe of teeth, whose edges are themselves regularly fringed with minute outgrowths. The cup seems to be formed by a development of the superior epidermis, which is raised up, and finally bursts and spreads out, laying bare the seeds.

The archegones of Marchantia are very curious bodies, while the elater and spores are even still more so. These are elongated cells, each containing a double spiral fibre coiled up in the interior. It is the elasticity of this which tears apart the cell-membrane, and sends forth the spores with a jerk, and thus assists in their dispersion. Marchantia is the type of the malloid Hepaticæ.

Musci, Bryophyta.

Mosses are a beautiful class of non-vascular cryptogams. Linnæus called them _servi_, servants or workmen, as they seem to labour to produce vegetation in places where soil is not already formed. The Bryophyta form three natural divisions: the Bryinæ, or true mosses; the Sphagnaceæ, or peat-mosses; and the Hepaticæ, or liverworts. The two first are commonly united. In these the sexual organs consist of antheridia and archegonia, but they are of simpler structure than will be found in ferns; and the first generation from the spore is asexual.

The common or wall screw-moss (Fig. 301) grows almost everywhere, and if examined closely, is seen to have springing from its base numerous very slender stems, each terminating in a dark brown case, which encloses antheroids. If a patch of the moss is gathered when in this state, and the green part of the base is put into water, the threads of the fringe will uncoil and disentangle themselves in a most curious and beautiful manner; from this circumstance the plant takes its popular name of screw-moss. The leaf usually consists of either a single or a double layer of cells, having flattened sides, by which they adhere one to another. The leaf-cells (Fig. 302) of the Sphagnum or bog-moss exhibit a curious departure from the ordinary type; they are large, polygonal, and elongated, and contain spiral fibres loosely coiled in their interior. The young leaf does not differ from the older; both are evolved by a gradual process of differentiation.

Mosses, like liverworts, possess both antheridia and pistillida, which are engaged in the process of fructification. The fertilized cell becomes gradually developed into a conical body elevated upon a footstalk, the walls of the flask-shaped body carrying the higher part upwards as a _calyptra_ or hood upon its summit, while the lower part remains to form a kind of collar round the base. These spore-capsules are closed on their summit by _opercula_ or lids, and their mouths when laid open are surrounded by a beautiful toothed fringe, termed the _peristome_. This fringe is shown in Fig. 303, in the centre of a capsule of Funaria, with its peristome _in situ_. The fringes of teeth are variously constructed, and are of great service in discriminating the genera. In _Neckera antipyretica_ the peristome is double, the inner being composed of teeth united by cross bars, forming a very pretty trellis. The seed spores are contained in the upper part of the capsule, where they are clustered round the central pillar, termed the _columella_; and at the time of maturity, the interior of the capsule is almost entirely occupied by spores.

The undulating hair-moss, _Polytrichum undulatum_ (Fig. 304), is found on moist, shady banks of pools and rivulets. The seed-vessel has a curious shaggy cap; but in its construction it is very similar to that of the screw-moss, except that the fringe around its opening is not twisted. The reproductive organs of mosses are of two kinds; the capsule containing minute spores, _archegonia_, and the _antheridia_, or male efflorescence. The capsule, _theca_, or sporangium, is lateral or terminal, sessile, or on a fruit stalk (_seta_) of various shapes, indehiscent, or bursting by four valves at the sides, or more commonly by a deciduous cup, _operculum_. When this falls the mouth of the capsule becomes exposed. The rim is crowned with tooth-like or cilia-like appendages in sets of four or multiples of that number--_peristome_. These are often brightly coloured and hydroscopic. By simply breathing upon them they suddenly fly open, and are endowed with motion, that is, if they contain spores. The spores on germination produce a green confervoid-like mass of threads, from which the young plant arises.

The Sphagnaceæ, or “bog mosses,” have been separated from true mosses from the marked differences they present. The stem is more widely differentiated, and throughout its structure a rapid passage of fluid takes place. It has the power of absorbing moisture from the atmosphere, so that if a plant be placed dry in a glass of water with its rosette of leaves hanging over the edge, it acts like a syphon, and the water will drop from it until the glass is emptied. As may be supposed, the leaf is composed of large open cells, and it absorbs more water than the root. The antherids or male organs of Sphagnaceæ resemble those of liverworts, rather than those of mosses, both in form and arrangement; they are grouped in “catkins” at the tips of the lateral branches, each of the imbricated perigonal leaves enclosing a single globose antherid on a slender foot-stalk, and surrounded by long branched paraphyses of cobweb-like tenuity. The female organs, or archegones, do not differ materially in structure from those of mosses; they are grouped together in a sheath of deep green leaves at the end of the shorter lateral branchlets at the side of the rosette or terminal crown of leaves. The sporange is very uniform in all the species, and the spores are in groups of fours, as in mosses, around a hemispherical columella. These plants grow so rapidly that they soon cover a pool with thin matted bundles of branches, and as they decay they fall to the bottom, and become the foundation of the future bog or peat moss.

=Felices.=--Of all the spore-bearing families the ferns are the more universally known. They constitute an exceedingly numerous genera and species, and vary from low herbaceous plants of an inch in height to that of tree ferns, which attain a height of fifty or more feet, terminating in a graceful coronet of fronds or leaves. Of whatever size a fern may be, its spores are, for the most part, microscopic, produced within the sporangium by cell division, and are therefore free and variously shaped.

The true mode of development of ferns from their spores was that furnished by Nägeli, who announced the existence of antheridia. On the spore starting into life it sends out from the cell-wall of its outer coat a white tubular projection, or root fibre (Fig. 305, A, B, and C), which passes through the cell-wall of its outer coat. This attracts sufficient moisture to burst open the outer, and then it begins to increase by the subdivision of its cells, until the primary green prothallus D is formed. This falls to the ground, and, being furnished on its under side with thread-like fibres, fixes itself to the earth, and thus is developed the rhizome, or root of the future plant. In each of the antheridia, which are numerous, a cell is formed, chiefly filled with albuminous matter and free spores, each having attached a flat ribbon-like filament, or stermatoid, curled in a spiral manner. These are ultimately set free by the rupture of the cell-wall, and commence revolving rapidly by the agency of the whip-like appendage at the larger end.

A. Spores; B, C. Early stages of development; D. Prothallus with radial fibres; _a_, _a_ and _a_, _b_ are stermatoids; and _h_, _h_. Enclosed antheridia.]

The sporangia, or spore-cases, are, for the most part, globular in form, and are nearly or quite surrounded by a strong elastic ring, which in some cases is continued to form a stalk. When the spores are ripe, this ring, by its elastic force, tears open the sporangia and gives exit to a quantity of microscopic filaments, curled in corkscrew-like fashion (Figs. 305 and 307). The ring assumes various forms; in one group it passes vertically up the back of the sporangium, and is continued to a point termed the stomata, where the horizontal bursting takes place. This form is seen in Fig. 306, _a_, _b_. In other groups it is vertical, as in _c_, _c_; in others transverse, as in _d_; or apical, as at _e_; and in a few instances it is obsolete, as in _f_. These are the true ferns, and their systematic arrangement is chiefly founded on the peculiarity of the sori and sporangia, characters which become quite intelligible by the aid of the microscope.

_a_, _b_. Polypodiaceæ; _c._ Cyantheineæ; _d._ Gleichenineæ; _e._ Schizeineæ; _f._ Osmundineæ.]

The beautiful ringed sporangium of the fern (Fig. 307) when ruptured gives exit to the dust-like spores; these, examined under a moderate power, are seen to be sub-globose and pyramidal, the outer coat or exospore being a coloured hyaline cell with nuclei similar to the spores of mosses, but in which chlorophyll soon begins to form, and from this little green embryonic growth the organs of reproduction are formed.

In all ferns the pistillidia or archegonia are analogous to the ovules or nascent seeds of flowering plants, and contain, like them, a germinal vesicle, which becomes fertilized through the agency of the spiral filaments, and then gradually develops into an embryo plant possessing a terminal bud. This bud begins at once to unfold and push out leaves with a circinate vernation, of a very simple form at first, and growing up beneath the prothallium, coming out at the notch; single fibrous roots are at the same time sent down into the earth, the delicate expanded prothallium withers away, and the foundation of the perfect fern plant is laid. When a fern acquires a considerable stem, as in a tree fern, it consists of cellular tissue and an external cortical portion forming fibro-vascular bundles, scalariform ducts, and woody fibre. Fig. 308, _b_, shows an oblique section of the footstalk of a fern leaf with its bundle of scalariform ducts.

These observations on ferns have acquired increased interest from subsequent investigations made on the allied Cryptogams, and on the processes occurring in the impregnation of the Conifers. Not only have later researches furnished a satisfactory interpretation of the archegonia and antheridia of the mosses and liverworts, but they have made known and co-ordinated the existence of analogous phenomena in the Equisetaceæ, Lycopodiaceæ, and Rhizocarpeæ, and prove, moreover, that the bodies described by Dr. Brown in the Conifers under the name of “corpuscles” are analogous to the _archegonia_ of the Cryptogams; so that a link is hereby formed between these groups and the higher flowering plants.

_Equisetaceæ._--The development of _Horse-tails_ (Fig. 309), the name by which they are commonly known, corresponds in some respects with that of ferns. They comprise a little group, and the whole of their structure is composed in an extraordinary degree by silex, so that even when the organic portion has been destroyed by prolonged maceration in strong acid, a consistent skeleton still remains. It is this flinty material that constitutes their chief interest for microscopists. A portion of their silicious particles is distributed in two lines, arranged parallel to the axis of the plant, others are grouped into oval forms, and connected by a chain as in a necklace. The form and arrangement of the crystals are better seen under polarised light. Plate VIII., No. 170, a portion of the epidermis, forms an extremely beautiful object. Sir David Brewster pointed out that each silicious particle has a regular axis of double refraction. What is usually said to be the fructification of the Equisetaceæ forms a cone or spike-like extremity to the top of the stem (Fig. 309), the whole resembling a series of spike-like branches (the real stem being a horizontal rhizome), and a cluster of shield-like discs, each of which carries a circle of sporanges that open by longitudinal slits to set free the spores which are attached to it in two pairs of elastic filaments (shown in Fig. 291, F, G), _elaters_; these are at first coiled up around the spore in the manner represented at G, but on their liberation they extend themselves as shown at F. The slightest moisture will close them up again, and their purpose having been served in the distribution of the spores, they are no longer required. If a number of spores be spread out on a glass-slip under the microscope and, while watching, a bystander breathes upon them, they immediately respond, are set in motion, presenting a curious appearance, but as soon as the hydroscopic effect has passed off they return to their previous condition. These spores can be mounted in a cell with a movable cover, and made to exhibit the same effect over and over again.

_a._ Fragment of stem showing mode of branching out; _b._ Cone or spike of fructification; _c._ Scale detached from cone; _d._ Spore with elastic filaments; _e._ Vertical section of stem; _f._ Transverse section showing hexagonal cells.]

The vascular tissue of the Equisetaceæ (Fig. 309, _e_, _f_) shows them to be of a higher grade than the ferns. More recently discovered Horse-tails, of Brazil, grow to a gigantic size, but even these are comparatively small when compared with the Calamites, and other fossil Equisetaceæ of the coal measures and new red sandstone. They all require a calcareous flinty soil for growth. A spring water-course making its way to the sea, as in the Chines of the Isle of Wight, is very favourable, the author having gathered them more than once in Bramble Chine.

Nearly allied to ferns is a little group of small aquatic plants, the Rhizocarpeæ (pepperworts), which either float on the water or creep along shallow bottoms. These are chiefly curious from having two kinds of spores produced from separate sporanges; smaller and larger “microspores” undergoing progressive sub-division without the formation of a distinct prothallium; each cell giving origin to an antherozoid, a generative process said to belong exclusively to flowering plants, corresponding indeed to the pollen grains of higher plants.

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.

_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.

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.

_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.

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.

_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.