On Molecular and Microscopic Science, Volume 1 (of 2)

The principal objects in the study of plant-life are the organs by means of which they obtain and assimilate substances that are essential for their nourishment and growth, and those by which the perpetuity of their race is maintained and their type transmitted from age to age. In the lowest group of plants, represented by the Algæ, which come first into consideration, the two properties are combined; in the highest they are distinctly different, but the progress from one to the other may be traced through an ascending series of vegetable structure. In the simple grades of vegetables, the primordial cell frequently constitutes the whole plant; it appears first, and then envelopes itself with a coat either of cellulose or of a gelatinous substance.

Many instances of this are to be found amongst the Algæ, which are all aquatic plants, and are found growing either attached to other bodies, or floating independently, and live, some species in fresh water, and others in the sea and its estuaries. The Algæ absorb carbonic acid and give out oxygen, under the influence of sun-light, exactly as do the flowering plants; and the quantity of oxygen disengaged by them is said to be enormous.

The slime that covers damp walls or stones, and moist cliffs or rocks in the sea, also the slime or mucus that sometimes swims on the surface of water, are said by M. Bory de St. Vincent to be provisional creations waiting to be organized. Of this the conferva, Palmoglœa macrococca (fig. 6), is an example. It is a green slime covering damp places, consisting of microscopic primordial cells, each of which is surrounded by a gelatinous envelope, and filled with green granular matter occasionally concentrated into a nucleus. This singular plant is propagated in two different ways. The endochrome or green matter within the cell spontaneously divides into two equal parts, the thin coat of the cell bends round the two ends, separates them, then each half takes a globular or ovoid form, and secretes a gelatinous substance round itself which completes the separation, so that they form two distinct and independent plants, in every respect similar to that from whence they were derived. After a little time, each of these plants undergoes a similar bisection, so that four new plants are formed with their gelatinous envelopes; by the same process eight are produced and so on indefinitely, the organ of nutrition being the same with that of reproduction. Again, the membrane or film that covers each of these primordial cells is so thin and soft, that occasionally two adjacent cells of the series unite into one mass by a fusion of their sides and internal matter, which is then coated by a membrane, and after various internal changes becomes a spore which terminates a generation. By and by the spore germinates, produces a green primordial cell which secretes a gelatinous coat, and becomes by the process of bisection the parent of a new generation, which terminates by the union of two adjacent cells to produce a spore, a cycle of alternate modes of reproduction that may be continued till ended by some external circumstance, as the cold of winter.

When the matter in two adjacent cells joins to form a spore, it becomes granular, and mixed with minute particles of oil, which unite in a drop; and the spore, which is at first green, gradually assumes a yellow brown colour; conversely when the spore begins to grow, the oil disappears, and the green matter takes its place. This is a frequent occurrence during the formation of spores in this class of plants, for the endochrome or internal matter,—which consists of a small variety of elements probably in a state of unstable equilibrium or change,—is easily decomposed and recombined into new substances by chemical action, but the bisection of the cells of the Palmoglœa so as to form new individuals is probably owing to heat alone. There is no apparent difference between the cells selected to produce spores by their union, and the others. It seems that in every plant certain cells are reserved for certain purposes. Professor Karsten conceives the nucleated cells to be reserved for reproduction, while those destitute of nuclei are designed for secretion.

The Protococcus pluvialis (fig. 7), one of the unicellular Confervæ, is frequently met with in rain-water cisterns. The spore of the plant (fig. 7 A) is a globular primordial cell invested with a double coat of cellulose, sometimes separated by an aqueous fluid, sometimes not. The cell is filled with protoplasm, a colourless watery liquid in which red and green particles are scattered. When this spore begins to grow, the endochrome, or solid matter in the primordial cell, divides spontaneously into two similar and equal parts, round at one end, and tapering to a point or beak at the other, each being coated by a very thin film of the transparent colourless protoplasm.

After various changes, the green matter with several red spots is condensed into the thick round half, while the tapering beaked part is left transparent, being only filled with the watery liquid. Both bodies are then coated with cellulose, and two vibratile filaments called cilia, from their resemblance to eye-lashes, proceed from a point near the beak. The whole of these changes take place while the two bodies are still within the common cellulose covering; the moment they come out of it, by a rupture in the cell-wall, they swim about with the greatest velocity by means of their cilia, which lash the water so rapidly that they are invisible even with a microscope. The activity of these zoospores, as they are called, continues for about an hour and a half; the motion then becomes gradually less rapid; the cilia may now be seen, and soon fall off; then the bodies acquire a firmer coat of cellulose, and sink to the bottom of the water, where they remain at rest as still, or winter spores. There is great variety in the Protococcus, for the matter in the primordial cell sometimes divides not only into two equal and similar parts, but into 4, 8, 16, 32 equal and similar parts consecutively; each brood is developed into zoospores, which ultimately become resting spores.

When a spore is to be formed in a primordial cell, the starch and green matter condense into a nucleus in its centre, and a membrane envelopes the liquid and the nucleus within it, so that a spore in its first stage is a free and independent cell containing azotized matter swimming in a formative liquid. If the spore is to be motile it remains of a green colour, and gets cilia; but if it is to be a winter spore, the internal matter forms into granules, mixed with particles of red oil, which coalesce into a drop, and it generally undergoes the same transformations as those which take place after the conjugation or union of two adjacent cells into one as already described. The zoospores may lose their cilia, fall to the bottom of the water as green spores, and reproduce a facsimile of the parent plant as buds do, or they may acquire a cellulose coat, undergo the transformations and change of colour mentioned, and sink to the bottom of the water as red winter spores.

Under certain circumstances which do not seem to be perfectly known, it happens that during the formation of some of the zoospores the green matter is gradually changed into a red oily substance; they lose their cilia, acquire by secretion their cell-walls and a mucous envelope, and float on the water as winter spores. Should they be left dry, they may remain in that state for an indefinite length of time without losing their vitality, and as they are extremely small, they are carried by currents of air into the atmosphere, from whence they are brought down in the rain, and having fallen occasionally in places where they were never seen before, have given rise to the idea of spontaneous generation.

Many cycles may be accomplished from the still cell to the zoospores, and back again, producing numerous generations from the same plant before it returns to the red thick-walled cell, which may again be dormant for an unlimited time. These cycles, however, do not finish the history of the plant, for there can be little doubt that, in some stage of its existence, a conjugation of two cells occurs, as in the Palmoglœa.

Sometimes when the division of the endochrome of the spore of the Protococcus is successively divided into sixteen parts, or even sooner, the new cells thus produced get two long cilia, as in fig. 7 H, and are liberated before they acquire their cellulose coat. This motile primordial cell soon acquires a bag-like investment (fig. 7 I, K, L,) of cellulose, through which the cilia pass, and thread-like extensions of the protoplasm are not unfrequently seen to radiate from the primordial cell to the surrounding bag, as in fig. 7 I, showing that the transparent space is only occupied by a watery liquid. The varieties of this plant are very numerous, and all related to one another. Sometimes the whole of the matter within the primordial cell of the spore divides at once into 4, 8, or 16 parts, giving rise to as many minute primordial cells.

The cilia are extensions of the colourless transparent film which covers the zoospores, and their vibrations are generally believed to be a consequence of the vital contractibility of that film, and intimately related to the changes taking place in the cell on which they are borne. The persistence of their motions after a cell is detached from a compound body covered with them, being like the persistence of the contractibility of muscle fibre after being detached from a living animal, proves that we must look to a contractile energy in the film of protoplasm for the maintenance of these curious operations.

It appears that a cell cannot perform two functions at the same time, and that one must either precede or follow the other. Thus, the zoospores have two distinct periods of action; the first is that of mechanical motion alone, which is followed by one of growth and multiplication, manifestations which, though very dissimilar, are really modes of action of the same vital energy that formed these bodies while they were yet in their parent cell. In fact, it seems to be a general law, that each cell is endowed, altogether or for a time, with its own mode of action, and is incapable of any other.

Of the Volvocineæ, by some regarded as fresh-water microscopic plants, the Stephanosphæra pluvialis may be taken as a type. This plant consists of a colourless transparent globe not more than the ⁴⁸⁄₁₀₀₀th of an inch in diameter, containing eight green primordial cells arranged in a circle in its equator. Each primordial cell is furnished with a pair of cilia; these 16 cilia pierce through the hyaline globe, and by their vibrations, they make it rotate about an axis perpendicular to the plane of its equator, and move actively through the water. Each of the primordial cells, which are green with a spot of red in the centre, secretes a cellular covering, and they swim about in the interior of the globe as free cells. Eventually they escape either by fissure of the globe, or by its gradual dissolution. After swimming about for a short time they become motionless, lose their cilia, and sink to the bottom as green still spores.

If, after being dried, water be poured on one of these green still spores, it takes up the water, its contents become closely granular, and fill the whole membrane of the spore. Then it divides, first into halves, then into quadrants or heart-shaped segments, meeting in a point in the centre of the membrane. These quadrants are ultimately divided into 8 wedge-shaped segments, whose contour lines, like the spokes of a wheel, meet in the centre, and each gets a pair of cilia. The coloured matter is driven back in each individual towards the thick end of the wedge as if by centrifugal force, and a colourless plasm remains in the points or beak. These disappear, a cavity is formed in the centre of the disc, the eight bodies assume the form of a wreath in close contact, and the original cilia, which continue to vibrate, cause the rotatory and progressive motion of the whole organism.

Sometimes the eight globular bodies have been seen to divide into a number of extremely minute motile cells, while yet within the parent globe. These gonidia, as they are called, are, with a few exceptions which may reproduce the plant, believed to perish when they come into the water.

The division of the primordial cell of this plant is confined to a certain time of day; it begins towards evening, and is completed the following morning, and according to Mr. F. Currey, the exact time is the same in Lapland, where there is no night, and at Berlin in spring when the day and night are almost equal. The fertility is enormous. It is calculated that in eight days, under favourable circumstances, 16,777,216 families of the Stephanosphæra pluvialis may be formed from one resting spore.

The transmutation of chlorophyll in the Protococcus and Volvocineæ, from green to red and vice versâ, which so frequently occurs in the lowest class of plants, shows that its molecules must be united by very feeble affinities, and easily converted into new combinations either by direct chemical action, or by other substances also in a state of change.

The Volvocineæ consist of various species according as the internal matter of the primordial cell divides into 2, 4, 8, 32, or a greater number of equal parts, forming respectively as many free cells which ultimately become ciliated spores, by means of which the globe either rotates on the spot, or in straight lines. The Volvox globator found in fresh-water pools is one of the most remarkable of these, both for its peculiarity and beauty of structure and for its comparatively large size, since in some lights it is visible to the naked eye while swimming in a drop of water. When viewed with a microscope, it is a pellucid sphere whose surface is studded with green spots, often connected by green threads; each of the spots has two cilia, so that the surface is bristled with these filaments, whose vibrations give the sphere either a rolling or smooth motion, or make it spin like a top in the same place.

In the interior of the sphere there are from two to twenty dark green globes of different sizes; the smaller are attached to the internal surface, while the larger rotate freely by their cilia in the internal cavity. After a time the sphere bursts open and its inhabitants swim forth, and soon assume the form and character of that which gave them birth.

The growth and development of the Volvox globator are peculiar, for in the primordial cell the red and green endochrome breaks up into numerous angular masses, and a central globe rather larger than the rest. The angular masses are connected by green threads, the interstices between all the bodies are filled with a hyaline substance secreted from their surfaces, and the whole is enclosed in a distinctly membranous globular envelope.

As this young Volvox increases gradually in size, the hyaline matter is increased, the green threads lengthen, and the angular masses assume the form of a flask the ¹⁄₃₀₀₀th of an inch in diameter exactly as in the Protococcus; for the green matter with a few red spots is collected in the thick end, while the hyaline beak is turned towards the circumference of the sphere, which is pierced by their long cilia. Each of them is invested with a pellucid envelope of considerable thickness, the borders of which are flattened against those of similar envelopes. While these ciliated bodies are approaching maturity their endochrome exhibits vacuoles or apparently empty cavities of a spherical form about one-third of its own diameter. Mr. G. Busk discovered that these vacuoles expand and contract at regular intervals of about forty seconds. The contraction, which almost obliterates the cavity of the vacuole, is rapid and sudden; the dilatation is slow and gradual. This action ceases when the body comes to maturity.

When this mass of zoospores connected by green threads is immature and begins to expand into a hollow sphere, then the central globe is continually bisected so as to form 4, 8, 16, 32, 64, or a greater number of equal and similar parts, each of which is ultimately developed into a zoospore exactly the same with the matured green zoospores on the surface of the primary sphere, so that the ‘Volvox globator is a composite fabric made up of a repetition of organisms in all respects similar to each other,’ which Professor Ehrenberg the first to discover, though he did not investigate the development of the plant.

It appears that certain spheres of the Volvox are monœcious, that is, each sphere contains male and female cells, though the greater number of cells are neutral. The germ or female cells are larger and of a deeper green than the others; the male cells resemble them, but the endochrome within them breaks up symmetrically into a multitude of linear particles aggregated into discoid bundles beset with vibratile cilia, which move about within their cells and soon become decomposed into their component corpuscles. Each of these corpuscles has a linear body, thicker at its posterior end, and furnished with two long cilia. The female cell, when fertilized, gets a smooth envelope, and then a thicker one, beset with conical-pointed processes, and the contained chlorophyll gives place, as in Palmoglœa, to starch and a red or orange coloured oil. It appears that the Volvox stellatus and V. aureus are only phases of the Volvox globator.

The Desmidiaceæ are minute green algæ inhabiting fresh-water pools or slow running streams, never those that are muddy. They are free unicellular plants, sometimes triangular, sometimes cylindrical, crescent or bow-shaped, smooth or spined. So varied are their microscopic forms that a description would be tedious. In plants of such extreme minuteness, the only means of ascertaining the nature of their component materials is by chemical tests. A solution of iodine turns starch blue, and cellulose brown, and thus it is found that the interior of the Desmidiaceæ is occupied by a mass of starch granules, covered with chlorophyll, and mixed with a formative fluid. This mass, enclosed in a delicate membrane, constitutes the primordial cell; it has an exterior coat of firm cellulose, and the whole is more or less enveloped in a gelatinous substance. Like other plants, when in bright sunshine, the Desmidiaceæ decompose carbonic acid gas, give off the oxygen, and assimilate the carbon into chlorophyll.

These plants are frequently distinguished by projections from their cellulose coat above their surface, these being sometimes short and conspicuous, but often projected in spines, which form a beautiful symmetrical hyaline border round the green internal cell, as shown in fig. 9. Another peculiarity of the Desmidiaceæ is the appearance of their being divided into two symmetrical parts by a satural line, as the name implies, though there is no real division.

Many of the Desmidiaceæ, but more especially the genus Closterium, are remarkable for having a double circulation of the internal fluid in opposite directions, maintained by a vital contractile energy. One current flows between the cellulose horny coat, and the thin film covering the chlorophyll, while the other spreads in a broad stream in the contrary direction between the thin film and the chlorophyll mass, carrying from the latter some of its coloured particles to the extremities of the frond, where there seems to be a connection between the two streams.

The type of the Desmidiaceæ is continued by various modes of bisection, depending upon the genus and species of the plant. In the Closterium Lunula, which has an elongated crescent shape, as in fig. 10 A, the endochrome or internal matter divides into two equal parts, which retreat from one another at the middle line; and a constriction of the cellulose coat takes place between them, which increases till it closes entirely round the extremities, as in fig. 10 D; then one of the halves remains at rest while the other moves from side to side, and finally detaches itself from the other with a jerk. In each of these halves a constriction of the endochrome may be seen, dividing it into an obtuse and an elongated part, and for some time the circulating fluid flows round the obtuse end, but the latter gradually assumes the form of the elongated end, the regular circulation of the fluid is established, and in five or six hours after the separation, two young desmids are formed precisely similar to their parent, the Closterium Lunula.

The Cosmarium, another Desmid, consists of a cell of two lobes united by a narrow isthmus. When about to multiply, the isthmus swells into two globular expansions, separated from each other and from the two lobes of the cell, by a narrow neck. These enlargements increase and assume the appearance of half segments of the original cell. In this state the plant consists of four segments lying end to end, the two old ones forming the extremes, with the two new ones in the middle. At last, each of the middle segments gets a new half, which soon acquires the full size and characteristics of the old one. This process, which is accomplished in twenty-four hours, is repeated ere long, and being continued indefinitely, the extreme lobes of the row are thrust farther and farther asunder, and the whole constricted thread or chain of Cosmaria is enclosed in a gelatinous sheath. The last two central lobes contain no portion of the original frond or plant, and may thus be considered to be entirely new individuals.

Many of the Desmidiaceæ multiply by the subdivision of their endochrome into a multitude of granular particles called gonidia, which are set free by the rupture of the cell wall, and of which every one may develop itself into a new cell. The gonidia may be zoospores with cilia and active locomotion, or they may be enclosed in a firm envelope, and become resting spores. The movement of the zoospores at first within the cavity of the cell which gave them their origin, and afterwards externally to it, has frequently been observed in the varied species of the genus Cosmarium, and has been described under the name of the ‘swarming of the granules,’ from the resemblance of the moving mass to a swarm of bees. Their subsequent history is unknown.

In the Pediastrum, a plant consisting of a cluster of cells, the zoospores are not emitted separately, but those formed by the subdivision of the endochrome of one cell into 4, 8, 16, 32, or 64 parts, escape from the parent plant still enclosed in the inner tunic of the cell, and it is within this that they develop themselves into a cluster resembling that in which they originated.

Mr. Thwaites discovered that the Desmidiaceæ are also propagated by conjugation, which would be impossible if the hard coat of the adjacent cells about to unite did not split open; then the whole endochrome in one cell passes into and blends with that in the other cell, so as to form one mass, which soon acquires a delicate membranaceous envelope. At first the mass consists of granular green matter, but when the membrane becomes thicker, it changes to brown or red. This body, which is called a sporangium, is sometimes smooth, sometimes granular, covered with tubercles or rough with spines, according to the nature of the original plants. The filamental species are propagated by conjugation, but the subsequent history of the produce is still obscure, though there is reason to believe that they give rise to plants of different forms, while all the other modes of increase only reproduce a facsimile of the parent.

Desmidiaceæ exist in America, but their distribution is little known. In Europe, their maximum seems to be in the south of England. They abound in small shallow pools that do not dry up in summer, and also on boggy moors. The larger kinds are spread out as a thin gelatinous stratum at the bottom of water, or collected in little tufts; others form a dirty cloud upon the stems and leaves of aquatic plants. They have been found in a fossil state in flint, their spores have been discovered in the grey chalk at Folkestone, and the cells of various species of Closterium and Euastrum are imbedded in the marls of the United States of North America.

The Diatomaceæ, or Brittleworts, are unicellular microscopic plants so numerous that there is hardly a spot on the face of the earth, from Spitzbergen to Victoria Land, where they may not be found. They abound in the ocean, in still and running fresh water, and even on the surface of the bare ground. They extend in latitude beyond the limits of all other plants, and can endure extremes of temperature, being able to exist in thermal springs, and in the pancake ice in the south polar latitudes. Though much too small to be visible to the naked eye, they occur in such countless myriads as to stain the berg and pancake ice wherever they are washed by the swell of the sea; and when enclosed in the congealing surface of the water they impart to the brash and pancake ice a pale ochreous colour.

Although the diatoms have a vast variety of forms, they all consist of a simple primordial cell whose external coat of cellulose is so deeply interpenetrated with silex that it is indestructible, a structure which constitutes the peculiar characteristic of the tribe. This primordial cell, as in other plants, contains organizable liquid or protoplasm, through which golden-brown granules are pretty regularly distributed, except in the centre, where they are collected into a nucleus. Round this nucleus they commonly form a ring from which radiating lines of granules diverge to the interior wall of the cell. In each of these there is a double current of granules, similar to the circulation in the Desmidiaceæ; it was discovered by Professor Smith in some of the comparatively large diatoms. At times oil globules are seen in the protoplasm. The golden-brown matter is supposed to be chlorophyll, whose green tint has been changed by the presence of iron, which is assimilated in this group. Such is the internal structure of a race of plants altogether invisible to the naked eye. Their external forms, reproduction and movements, are no less wonderful.

The silicious envelope of the simple cell of a Diatom or frustule, as a single plant is usually called, consists of two valves or plates, commonly of the most perfect symmetry, closely applied to each other along a line of junction like the two valves of a bivalve shell, and each valve being more or less concavo-convex, a cavity is left between the two which is occupied by the golden-brown cell described above. The form of the cavity differs greatly, for sometimes each valve is hemispherical, so that the cavity is globular; sometimes it is a small segment of a sphere, resembling a watch-glass, so that the cavity is lenticular; in short, the form of the cavity depends upon that of the valves, which may be heart-shaped, or much elongated, square, triangular, boat-shaped, or furnished with outgrowths, which, however, is rare.

The diatom or frustule is considered to present its front view when the joint or suture of the valves is turned to the eye, as in fig. 11 B, b, whilst the side view is seen when the centre of either valve is directly beneath the eye, as in fig. 11 A, a. When the diatoms are young the valves are in close contact, but as they increase in size by a secretion round their edges, the valves separate from one another, and the cell membrane which is left exposed is immediately consolidated by silex, and forms a kind of hoop between the valves, as in fig. 12. This hoop increases in breadth as the cell increases in length. When the two valves are circular discs, they are separated by a circular hoop, round the edges of which water is admitted to nourish the plant; but when the diatom has an elongated form, the water enters through depressed points in its extremities which are free from silex.

Numerous as these plants are, the valves of each genus have their own peculiar ornaments, consisting of the most beautiful and symmetrical designs, which are impressed upon the young valves when they are in a plastic state. The genus Navicula and others are marked with the finest striæ, some diagonally, others transversely. Rows of round or oval spots disposed in parallel lines are peculiar to some; the valves of others are covered with hexagonal forms of the most perfect structure, as those of the Pleurosigma angulatum, fig. 13, where A is the magnified diatom, and B and C its hexagonal areolations, seen under higher and higher microscopic powers; but the figures on the discoid genera are the most beautiful of all. There is generally a small ornamented circular space in the centre of the valves, from whence rays extend to the circumference, dividing the surface of the valves into eight, ten, or more equal parts, the alternate segments being differently and highly ornamented, as in the Actinocyclus undulatus (fig. 14), where A is the side view, and B is the front view. The Arachnoidiscus Ehrenbergii takes its name from the likeness of the figures on its circular valves to a spider’s web. According to the observations of Mr. Shadbolt, each valve is formed of two superposed layers; on the uppermost of these, which is a thin horny transparent substance, the spider’s web is engraven; and the undermost silicious layer, which forms the supporting frame-work, is like a circular Gothic window. The genus Triceratium, nearly allied to the preceding in general characters, though differing in having a triangular shape, has many species in a fossil state, while others are still existing in the ocean, and in tidal rivers. The Triceratium favus, one of the largest and most beautifully marked, occurs in the mud of the Thames, and that of the estuaries of other rivers on our coasts; it is also frequently found on the surface of uncleaned shells.^[32] From the few examples given, a faint idea only can be formed of the variety and beauty of the engravings on the diatoms. It had long been doubted whether those on the valves of Coscinodiscus, Triceratium and others, were elevations or depressions, but Professor Rood of New York, United States, has proved them to be depressions by an optical arrangement which will be useful for the investigation of microscopic forms.

Diatoms increase by spontaneous bisection, by conjugation, and by the resolution of their endochrome into minute spores, called gonidia. When bisection is about to take place the cell elongates, the hoop increases in breadth, the endochrome divides into two equal parts, and the coating of the cell bends in between them, which gives the diatom the appearance of an hour-glass. At last they separate, and upon each of the new surfaces a new silicious half is formed, usually the exact counterpart of the old one, so that there are two diatoms instead of one; and the process may be continued indefinitely. In most cases, the new diatoms thus produced are free and independent. Sometimes, however, they adhere to one another by a fragment or connecting membrane, and if they happen to be slender and rectangular, and attached side by side, they form a slender filament, or if attached by alternate angles they form a zigzag chain, as in fig. 11.

The Meridion circulare (fig. 15) is a diatom of exquisite beauty, millions and millions of which cover every submerged stone, twig, or blade of grass, and even form the mud at the bottom of the streams at West Point, in the United States of North America. Its frustule or single diatom is long, slender, and rectilinear, but being broader at one end than at the other, by continued bisection and adhering to one another they form a circular, spiral, or flattened helical screw of several turns. The individual frustules of some marine diatoms have a precisely similar form, being rectilinear and broader at one end than the other, but each frustule is attached by its narrow end to the extremity of branching cellulose stems fixed to sea-weeds or stones, and by a continuous subdivision of which the stem does not partake, they are spread out at their free ends like a fan.

By continual bisection a diatom is propagated through many generations, but at some stage or other, owing to an unknown cause, propagation by conjugation takes place. When two frustules are near to each other, two little swellings arise in one, which meet two little swellings in the other opposite to it. These soon unite and elongate, the septum or division between them is absorbed so that they form two tubes in which the endochrome of the two frustules becomes mixed, and a spore is formed in each of the two connecting tubes, which increase in size and change in form till they resemble in every respect the parent, except in being much larger. As these young diatoms swell, they split the two parent frustules, become free, and lay the foundation of a twin series of generations. In the Fragillaria only a single spore is formed.^[33]

In Surirella and Epithemia the manner of conjugation is somewhat different. In the former the valves of two free adjacent frustules separate from each other at the suture or line of junction and the two endochromes are discharged; they coalesce and form a single mass, which becomes enclosed in a gelatinous envelope, and in time this mass shapes itself into a frustule resembling that of its parent, but larger. In Epithemia, however, the endochrome of each of the conjugating frustules divides at the time of its discharge into two halves; each half of the one coalesces with each half of the other, and two frustules are formed which become invested with a gelatinous envelope and gradually assume the form and markings of the parent frustules, but grow to a much larger size, for the spore masses have the power of self-increase up to the time that their envelopes are consolidated. This double conjugation seems to be the ordinary type of the process among the diatoms.^[34] But these plants multiply also by gonidia. It is thought probable that as long as the vegetative processes are in full activity diatoms multiply by self-bisection, but when a deficiency of warmth, of moisture, or of some other condition, gives a check to these, that they increase by gonidia, some of which becoming encysted, possess a greater power of resisting unfavourable circumstances, and thus the species is maintained in a dormant state till a change enables them to germinate. It is even thought they may be the origin of distinct species.

A peculiar spontaneous locomotion is exhibited by some diatoms of a long narrow form, as the Naviculæ, which by a succession of jerks in the direction of their length, go to a certain distance, and then return nearly by the same path. The motion of the Bacillaria cursoria is still more unprecedented. The frustules, which are narrow, lanceolate, and acute, are joined end to end in a long line by some highly elastic invisible medium. One of the terminal frustules remains at rest while all the others slide over it till the line is so much stretched that they are nearly detached from one another; then they all slide back again in the same manner, and this alternate motion is continued indefinitely at regular intervals of time. The velocity of the diatoms at the free end of the row is very considerable; in the Bacillaria paradoxa it is ¹⁄₂₀₀th of an inch in a second; the impetus of one has been observed to upset and even to push aside a plant as much as three times its size which obstructed its path. If the frustule at the free end gets entangled, the fixed frustule takes the lead and continues the motion till the other is free. Minute particles in the vicinity are sometimes attracted and dragged after the frustules, sometimes they are repelled, possibly by some invisible organs; but the whole motion of the diatoms themselves may perhaps be attributed to the action of light and heat upon the highly contractile substance, whatever it may be, which connects their frustules, since their motion is exactly in proportion to the quantity of light and heat received, for it ceases during darkness, and is renewed on the return of light; ultimately it may disperse the individual frustules, which are not more than between the ²⁸⁄_10,000th and the ⁸⁴⁄_10,000th of an inch in length and the ⁴⁄_10,000th of an inch in breadth.