It will be seen that the black clay, shale, or slate, has a constituent of aluminum of from 15.93 per cent., the lowest, to 32.60 per cent., the highest. Under every stratum of coal, and frequently mixed with it, are these under deposits that are rich in the metal. When exposed to the atmosphere, these shales yield a small deposit of alum. In the manufacture of alum near Glasgow the shale and slate clay from the old coal pits constitute the material used, and in France alum is manufactured directly from the clay.
Sufficient has been advanced to warrant the additional assertion that we are here everywhere surrounded by this incomparable mineral, that it is brought to the surface from its deposits deep in the earth by the natural process in mining, and is only exceeded in quantity by the coal itself. Taking a columnar section of our coal field, and computing the thickness of each shale stratum, we have from twenty-five to sixty feet in thickness of this metal-bearing substance, which averages over twenty-five per cent. of the whole in quantity in metal.
It is readily apparent that the only task now before us is the reduction of the ore and the extraction of the metal. Can this be done? We answer, it has been done. The egg has stood on end—the new world has been sighted. All that now remains is to repeat the operation and extend the process. Cheap aluminum will revolutionize industry, travel, comfort, and indulgence, transforming the present into an even greater civilization. Let us see.
We have seen the discovery of the mere chemical existence of the metal, we have stood by the birth of the first white globule or bead by Wohler, in 1846, and witnesssed its introduction as a manufactured product in 1855, since which time, by the alteration and cheapening of one process after another, it has fallen in price from thirty-two dollars per pound in 1855 to fifteen dollars per pound in 1885. Thirty years of persistent labor at smelting have increased the quantity over a thousandfold and reduced the cost upward of fifty per cent.
All these processes involve the application of heat—a mere question of the appliances. The electric currents of Berzelius and Oersted, the crucible of Wohler, the closed furnaces and the hydrogen gas of the French manufacturers and the Bessemer converter apparatus of Thompson, all indicate one direction. This metal can be made to abandon its bed in the earth and the rock at the will of man. During the past year, the Messrs. Cowles, of Cleveland, by their electric smelting process, claim to have made it possible to reduce the price of the metal to below four dollars per pound; and there is now erecting at Lockport, New York, a plant involving one million of capital for the purpose.
Turning from the employment of the expensive reducing agents to the simple and sole application of heat, we are unwilling to believe that we do not here possess in eminence both the mineral and the medium of its reduction. Whether the electric or the reverberatory or the converter furnace system be employed, it is surely possible to produce the result.
To enter into consideration of the details of these constructions would involve more time than is permitted us on this occasion. They are very interesting. We come again naturally to the limitless consideration of powdered fuel, concerning which certain conclusions have been reached. In the dissociation of water into its hydrogen and oxygen, with the mingled carbon in a powdered state, we undoubtedly possess the elements of combustion that are unexcelled on earth, a heat-producing combination that in both activity and power leaves little to be desired this side of the production of the electric force and heat directly from the carbon without the intermediary of boilers, engines, dynamos, and furnaces.
In the hope of stimulating thought to this infinite question of proper fuel combustion, with its attendant possibilities for man's gratification and ambition, this advanced step is presented. The discussion of processes will require an amount of time which I hope this Board will not grudgingly devote to the subject, but which is impossible at present. Do not forget that there is no single spot on the face of the globe where nature has lavished more freely her choicest gifts. Let us be active in the pursuit of the treasure and grateful for the distinguished consideration.
* * * * *
THE ORIGIN OF METEORITES.
On January 9, Professor Dewar delivered the sixth and last of his series of lectures at the Royal Institution on "The Story of a Meteorite." [For the preceding lectures, see SUPPLEMENTS 529 and 580.] He said that cosmic dust is found on Arctic snows and upon the bottom of the ocean; all over the world, in fact, at some time or other, there has been a large deposit of this meteoric dust, containing little round nodules found also in meteorites. In Greenland some time ago numbers of what were supposed to be meteoric stones were found; they contained iron, and this iron, on being analyzed at Copenhagen, was found to be rich in nickel. The Esquimaux once made knives from iron containing nickel; and as any such alloy they must have found and not manufactured, it was supposed to be of meteoric origin. Some young physicists visited the basaltic coast in Greenland from which some of the supposed meteoric stones had been brought, and in the middle of the rock large nodules were found composed of iron and nickel; it, therefore, became evident that the earth might produce masses not unlike such as come to us as meteorites. The lecturer here exhibited a section of the Greenland rock containing the iron, and nickel alloy, mixed with stony crystals, and its resemblance to a section of a meteorite was obvious. It was 21/2 times denser than water, yet the whole earth is 51/2 times denser than water, so that if we could go deep enough, it is not improbable that our own globe might be found to contain something like meteoric iron. He then called attention to the following tables:
Elementary Substances found in Meteorites.
Hydrogen. Chromium. Arsenic. Lithium. Manganese. Vanadium? Sodium. Iron. Phosphorus. Potassium. Nickel. Sulphur. Magnesium. Cobalt. Oxygen. Calcium. Copper. Silicon. Aluminum. Tin. Carbon. Titanium. Antimony. Chlorine.
Density of Meteorites.
Carbonaceous (Orgueil, etc.) 1.9 to 3 Aluminous (Java) 3.0 " 3.2 Peridotes (Chassigny, etc.) 3.5 " — Ordinary type (Saint Mes) 3.1 " 3.8 Rich in iron (Sierra de Chuco) 6.5 " 7.0 Iron with stone (Krasnoyarsk) 7.1 " 7.8 True irons (Caille) 7.0 " 8.0
Interior of the Earth
Parts of the radius. Density. 0.0 11.0 0.1 10.3 0.2 9.6 0.3 8.9 0.4 8.3 0.5 7.8 0.6 7.4 0.7 7.1 0.8 6.2 0.9 5.0 1.0 2.6
Twice a year, said Professor Dewar, what are called "falling stars" maybe plentifully seen; the times of their appearance are in August and November. Although thousands upon thousands of such small meteors have passed through our atmosphere, there is no distinct record of one having ever fallen to the earth during these annual displays. One was said to have fallen recently at Naples, but on investigation it turned out to be a myth. These annual meteors in the upper air are supposed to be only small ones, and to be dissipated into dust and vapor at the time of their sudden heating; so numerous are they that 40,000 have been counted in one evening, and an exceptionally great display comes about once in 331/4 years. The inference from their periodicity is, that they are small bodies moving round the sun in orbits of their own, and that whenever the earth crosses their orbits, thereby getting into their path, a splendid display of meteors results. A second display, a year later, usually follows the exceptionally great display just mentioned, consequently the train of meteors is of great length. Some of these meteors just enter the atmosphere of the earth, then pass out again forever, with their direction of motion altered by the influence of the attraction of the earth. He here called attention to the accompanying diagram of the orbits of meteors.
The lecturer next invited attention to a hollow globe of linen or some light material; it was about 2 ft. or 2 ft. 6 in. in diameter, and contained hidden within it the great electro-magnet, weighing 2 cwt., so often used by Faraday in his experiments. He also exhibited a ball made partly of thin iron; the globe represented the earth, for the purposes of the experiment, and the ball a meteorite of somewhat large relative size. The ball was then discharged at the globe from a little catapult; sometimes the globe attracted the ball to its surface, and held it there, sometimes it missed it, but altered its curve of motion through the air. So was it, said the lecturer, with meteorites when they neared the earth. Photographs from drawings, by Professor A. Herschel, of the paths of meteors as seen by night were projected on the screen; they all seemed to emanate from one radiant point, which, said the lecturer, is a proof that their motions are parallel to each other; the parallel lines seem to draw to a point at the greatest distance, for the same reason that the rails of a straight line of railway seem to come from a distant central point. The most interesting thing about the path of a company of meteors is, that a comet is known to move in the same orbit; the comet heads the procession, the meteors follow, and they are therefore, in all probability, parts of comets, although everything about these difficult matters cannot as yet be entirely explained; enough, however, is known to give foundation for the assumption that meteorites and comets are not very dissimilar.
The light of a meteorite is not seen until it enters the atmosphere of the earth, but falling meteorites can be vaporized by electricity, and the light emitted by their constituents be then examined with the spectroscope. The light of comets can be directly examined, and it reveals the presence in those bodies of sodium, carbon, and a few other well-known substances. He would put a piece of meteorite in the electric arc to see what light it would give; he had never tried the experiment before. The lights of the theater were then turned down, and the discourse was continued in darkness; among the most prominent lines visible in the spectrum of the meteorite, Professor Dewar specified magnesium, sodium, and lithium. "Where do meteorites come from?" said the lecturer. It might be, he continued, that they were portions of exploded planets, or had been ejected from planets. In this relation, he should like to explain the modern idea of the possible method of construction of our own earth. He then set forth the nebular hypothesis that at some long past time our sun and all his planets existed but as a volume of gas, which in contracting and cooling formed a hot volume of rotating liquid, and that as this further contracted and cooled, the planets, and moons, and planetary rings fell off from it and gradually solidified, the sun being left as the solitary comparatively uncooled portion of the original nebula. In partial illustration of this, he caused a little globe of oil, suspended in an aqueous liquid of nearly its own specific gravity, to rotate, and as it rotated it was seen, by means of its magnified image upon the screen, to throw off from its outer circumference rings and little globes.
* * * * *
CANDELABRA CACTUS AND CALIFORNIA WOODPECKER.
By C.F. HOLDER.
One of the most picturesque objects that meet the eye of the traveler over the great plains of the southern portion of California and New Mexico is the candelabra cactus. Systematically it belongs to the Cereus family, in which the notable Night-blooming Cereus also is naturally included. In tropical or semi-tropical countries these plants thrive, and grow to enormous size. For example, the Cereus that bears those great flowers, and blooms at night, exhaling powerful perfume, as we see them in hothouses in our cold climate, are even in the semi-tropical region of Key West, on the Florida Reef, seen to grow enormously in length.
We cultivated several species of the more interesting forms during a residence on the reef. Our brick house, two stories in height, was entirely covered on a broad gable end, the branches more than gaining the top. There is a regular monthly growth, and this is indicated by a joint between each two lengths. Should the stalk be allowed to grow without support, it will continue growing without division, and exhibit stalks five or six feet in length, when they droop, and fall upon the ground.
Where there is a convenient resting place on which it can spread out and attach itself, the stalk throws out feelers and rootlets, which fasten securely to the wall or brickwork; then, this being a normal growth, there is a separation at intervals of about a foot. That is, the stalk grows in one month about twelve inches, and if it has support, the middle woody stalk continues to grow about an inch further, but has no green, succulent portion, in fact, looks like a stem; then the other monthly growth takes place, and ends with a stem, and so on indefinitely. Our house was entirely covered by the stems of such a plant, and the flowers were gorgeous in the extreme. The perfume, however, was so potent that it became a nuisance. Such is the Night-blooming Cereus in the warm climates, and similarly the Candelabra Cereus grows in stalks, but architecturally erect, fluted like columns. The flowers are large, and resemble those of the night-blooming variety. Some columns remain single, and are amazingly artificial appearing; others throw off shoots, as seen in the picture. There are some smaller varieties that have even more of a candelabra look, there being clusters of side shoots, the latter putting out from the trunk regularly, and standing up parallel to each other. The enormous size these attain is well shown in the picture.
Whenever the great stalks of these cacti die, the succulent portion is dried, and nothing is left but the woody fiber. They are hollow in places, and easily penetrated. A species of woodpecker, Melanerpes formicivorus, is found to have adopted the use of these dry stalks for storing the winter's stock of provisions. There are several round apertures seen on the stems in the pictures, which were pecked by this bird. This species of woodpecker is about the size of our common robin or migratory thrush, and has a bill stout and sharp. The holes are pecked for the purpose of storing away acorns or other nuts; they are just large enough to admit the fruit, while the cup or larger end remains outside. The nuts are forced in, so that it requires considerable wrenching to dislodge them. In many instances the nuts are so numerous, the stalk has the appearance of being studded with bullets. This appearance is more pronounced in cases where the dead trunk of an oak is used. There are some specimens of the latter now owned by the American Museum of Natural History, which were originally sent to the Centennial Exhibition at Philadelphia. They were placed in the department contributed by the Pacific Railroad Company, and at that time were regarded as some of the wonders of that newly explored region through which the railroad was then penetrating. Some portions of the surface of these logs are nearly entirely occupied by the holes with acorns in them. The acorns are driven in very tightly in these examples; much more so than in the cactus plants, as the oak is nearly round, and the holes were pecked in solid though dead wood. One of the most remarkable circumstances connected with this habit of the woodpecker is the length of flight required and accomplished. At Mount Pizarro, where such storehouses are found, the nearest oak trees are in the Cordilleras, thirty miles distant; thus the birds are obliged to make a journey of sixty miles to accomplish the storing of one acorn. At first it seemed strange that a bird should spend so much labor to place those bits of food, and so far away. De Saussure, a Swiss naturalist, published in the Bibliotheque Universelle, of Geneva, entertaining accounts of the Mexican Colaptes, a variety of the familiar "high hold," or golden winged woodpecker. They were seen to store acorns in the dead stalks of the maguey (Agave Americana). Sumichrast, who accompanied him to Central America, records the same facts. These travelers saw great numbers of the woodpeckers in a region on the slope of a range of volcanic mountains. There was little else of vegetation than the Agave, whose barren, dead stems were studded with acorns placed there by the woodpeckers.
The maguey throws up a stalk about fifteen feet in height yearly, which, after flowering, grows stalky and brittle, and remains an unsightly thing. The interior is pithy, but after the death of the stalk the pith contracts, and leaves the greater portion of the interior hollow, as we have seen in the case of the cactus branches. How the birds found that these stalks were hollow is a problem not yet solved, but, nevertheless, they take the trouble to peck away at the hard bark, and once penetrated, they commence to fill the interior; when one space is full, the bird pecks a little higher up, and so continues.
Dr. Heerman, of California, describes the California Melanerpes as one of the most abundant of the woodpeckers; and remarks that it catches insects on the wing like a flycatcher. It is well determined that it also eats the acorns that it takes so much pains to transport.
It seems that these birds also store the pine trees, as well as the oaks. It is not quite apparent why these birds exhibit such variation in habits; they at times select the more solid trees, where the storing cannot go on without each nut is separately set in a hole of its own. There seems an instinct prompting them to do this work, though there may not be any of the nuts touched again by the birds. Curiously enough, there are many instances of the birds placing pebbles instead of nuts in holes they have purposely pecked for them. Serious trouble has been experienced by these pebbles suddenly coming in contact with the saw of the mill through which the tree is running. The stone having been placed in a living tree, as is often the case, its exterior had been lost to sight during growth.
Some doubt has been entertained about the purpose of the bird in storing the nuts in this manner. De Saussure tells us he has witnessed the birds eating the acorns after they had been placed in holes in trees, and expresses his conviction that the insignificant grub which is only seen in a small proportion of nuts is not the food they are in search of.
C.W. Plass, Esq., of Napa City, California, had an interesting example of the habits of the California Melanerpes displayed in his own house. The birds had deposited numbers of acorns in the gable end. A considerable number of shells were found dropped underneath the eaves, while some were found in place under the gable, and these were perfect, having no grubs in them.
The picture shows a very common scene in New Mexico. The columns, straight and angular, are often sixty feet in height. It is called torch cactus in some places. There are many varieties, and as many different shapes. Some lie on the ground; others, attached to trunks of trees as parasites, hang from branches like great serpents; but none is so majestic as the species called systematically Cereus giganteus, most appropriately. The species growing pretty abundantly on the island of Key West is called candle cactus. It reaches some ten or twelve feet, and is about three inches in diameter. The angles are not so prominent, which gives the cylinders a roundish appearance. They form a pretty, rather picturesque feature in the otherwise barren undergrowth of shrubbery and small trees. Accompanied by a few flowering cocoa palms, the view is not unpleasing. The fiber of these plants is utilized in some coarse manufactures. The maguey, or Agave, is used in the manufacture of fine roping. Manila hemp is made from a species. The species whose dried stalks are used by the woodpeckers for their winter storage was cultivated at Key West, Florida, during several years before 1858. Extensive fields of the Agave stood unappropriated at that period. Considerable funds were dissipated on this venture. Extensive works were established, and much confidence was entertained that the scheme would prove a paying one, but the "hemp" rope which this was intended to rival could be made cheaper than this. The great Agave plants, with their long stalks, stand now, increasing every year, until a portion of the island is overrun with them.
This wonderful cactus, its colossal proportions, and weird, yet grand, appearance in the rocky regions of Mexico and California, where it is found in abundance, have been made known to us only through books of travel, no large plants of it having as yet appeared in cultivation in this country. It is questionable if ever the natural desire to see such a vegetable curiosity represented by a large specimen in gardens like Kew can be realized, owing to the difficulty of importing large stems in a living condition; and even if successfully brought here, they survive only a very short time. To grow young plants to a large size seems equally beyond our power, as plants 6 inches high and carefully managed are quite ten years old. When young, the stem is globose, afterward becoming club-shaped or cylindrical. It flowers at the height of 12 feet, but grows up to four or five times that height, when it develops lateral branches, which curve upward and present the appearance of an immense candelabrum, the base of the stem being as thick as a man's body. The flower, of which a figure is given here, is about 5 inches long and wide, the petals cream colored, the sepals greenish white. Large clusters of flowers are developed together near the top of the stem. A richly colored edible fruit like a large fig succeeds each flower, and this is gathered by the natives and used as food under the name of saguarro. A specimen of this cactus 3 feet high may be seen in the succulent house at Kew.—B., The Garden.
* * * * *
HOW PLANTS ARE REPRODUCED.
[Footnote: Read at a meeting of the Chemists' Assistants' Association. December 16, 1885.]
By C.E. STUART, B.Sc.
In two previous papers read before this Association I have tried to condense into as small a space as I could the processes of the nutrition and of the growth of plants; in the present paper I want to set before you the broad lines of the methods by which plants are reproduced.
Although in the great trees of the conifers and the dicotyledons we have apparently provision for growth for any number of years, or even centuries, yet accident or decay, or one of the many ills that plants are heirs to, will sooner or later put an end to the life of every individual plant.
Hence the most important act of a plant—not for itself perhaps, but for its race—is the act by which it, as we say, "reproduces itself," that is, the act which results in the giving of life to a second individual of the same form, structure, and nature as the original plant.
The methods by which it is secured that the second generation of the plant shall be as well or even better fitted for the struggle of life than the parent generation are so numerous and complicated that I cannot in this paper do more than allude to them; they are most completely seen in cross fertilization, and the adaptation of plant structures to that end.
What I want to point out at present are the principles and not so much the details of reproduction, and I wish you to notice, as I proceed, what is true not only of reproduction in plants but also of all processes in nature, namely, the paucity of typical methods of attaining the given end, and the multiplicity of special variation from those typical methods. When we see the wonderfully varied forms of plant life, and yet learn that, so to speak, each edifice is built with the same kind of brick, called a cell, modified in form and function; when we see the smallest and simplest equally with the largest and most complicated plant increasing in size subject to the laws of growth by intussusception and cell division, which are universal in the organic world; we should not be surprised if all the methods by which plants are reproduced can be reduced to a very small number of types.
The first great generalization is into—
1. The vegetative type of reproduction, in which one or more ordinary cells separate from the parent plant and become an independent plant; and—
2. The special-cell type of reproduction, in which either one special cell reproduces the plant, or two special cells by their union form the origin of the new plant; these two modifications of the process are known respectively as asexual and sexual.
The third modification is a combination of the two others, namely, the asexual special cell does not directly reproduce its parent form, but gives rise to a structure in which sexual special cells are developed, from whose coalescence springs again the likeness of the original plant. This is termed alternation of generations.
The sexual special cell is termed the spore.
The sexual special cells are of one kind or of two kinds.
Those which are of one kind may be termed, from their habit of yoking themselves together, zygoblasts, or conjugating cells.
Those which are of two kinds are, first, a generally aggressive and motile fertilizing or so-called "male cell," called in its typical form an antherozoid; and, second, a passive and motionless receptive or so-called "female cell," called an oosphere.
The product of the union of two zygoblasts is termed a zygospore.
The product of the union of an antherozoid and an oosphere is termed an oospore.
In many cases the differentiation of the sexual cells does not proceed so far as the formation of antherozoids or of distinct oospheres; these cases I shall investigate with the others in detail presently.
First, then, I will point out some of the modes of vegetative reproduction.
The commonest of these is cell division, as seen in unicellular plants, such as protococcus, where the one cell which composes the plant simply divides into two, and each newly formed cell is then a complete plant.
The particular kind of cell division termed "budding" here deserves mention. It is well seen in the yeast-plant, where the cell bulges at one side, and this bulge becomes larger until it is nipped off from the parent by contraction at the point of junction, and is then an independent plant.
Next, there is the process by which one plant becomes two by the dying off of some connecting portion between two growing parts.
Take, for instance, the case of the liverworts. In these there is a thallus which starts from a central point and continually divides in a forked or dichotomous manner. Now, if the central portion dies away, it is obvious that there will be as many plants as there were forkings, and the further the dying of the old end proceeds, the more young plants will there be.
Take again, among higher plants, the cases of suckers, runners, stolons, offsets, etc. Here, by a process of growth but little removed from the normal, portions of stems develop adventitious roots, and by the dying away of the connecting links may become independent plants.
Still another vegetative method of reproduction is that by bulbils or gemmae.
A bulbil is a bud which becomes an independent plant before it commences to elongate; it is generally fleshy, somewhat after the manner of a bulb, hence its name. Examples occur in the axillary buds of Lilium bulbiferum, in some Alliums, etc.
The gemma is found most frequently in the liverworts and mosses, and is highly characteristic of these plants, in which indeed vegetative reproduction maybe said to reach its fullest and most varied extent.
Gemmae are here formed in a sort of flat cup, by division of superficial cells of the thallus or of the stem, and they consist when mature of flattened masses of cells, which lie loose in the cup, so that wind or wet will carry them away on to soil or rock, when, either by direct growth from apical cells, as with those of the liverworts, or with previous emission of thread-like cells forming a "protonema," in the case of the mosses, the young plant is produced from them.
The lichens have a very peculiar method of gemmation. The lichen-thallus is composed of chains or groups of round chlorophyl-containing cells, called "gonidia," and masses of interwoven rows of elongated cells which constitute the hyphae. Under certain conditions single cells of the gonidia become surrounded with a dense felt of hyphae, these accumulate in numbers below the surface of the thallus, until at last they break out, are blown or washed away, and start germination by ordinary cell division, and thus at once reproduce a fresh lichen-thallus. These masses of cells are called soredia.
Artificial budding and grafting do not enter into the scope of this paper.
As in the general growth and the vegetative reproduction of plants cell-division is the chief method of cell formation, so in the reproduction of plants by special cells the great feature is the part played by cells which are produced not by the ordinary method of cell division, but by one or the other processes of cell formation, namely, free-cell formation or rejuvenescence.
If we broaden somewhat the definition of rejuvenescence and free-cell formation, and do not call the mother-cells of spores of mosses, higher cryptogams, and also the mother-cells of pollen-grains, reproductive cells, which strictly speaking they are not, but only producers of the spores or pollen-grains, then we may say that cell-division is confined to vegetative processes, rejuvenescence and free-cell formation are confined to reproductive processes.
Rejuvenescence may be defined as the rearrangement of the whole of the protoplasm of a cell into a new cell, which becomes free from the mother-cell, and may or may not secrete a cell-wall around it.
If instead of the whole protoplasm of the cell arranging itself into one mass, it divides into several, or if portions only of the protoplasm become marked out into new cells, in each case accompanied by rounding off and contraction, the new cells remaining free from one another, and usually each secreting a cell wall, then this process, whose relation to rejuvenescence is apparent, is called free-cell formation.
The only case of purely vegetative cell-formation which takes place by either of these processes is that of the formation of endosperm in Selaginella and phanerogams, which is a process of free-cell formation.
On the other hand, the universal contraction and rounding off of the protoplasm, and the formation by either rejuvenescence or free-cell formation, distinctly mark out the special or true reproductive cell.
Examples of reproductive cells formed by rejuvenescence are:
1. The swarm spores of many algae, as Stigeoclonium (figured in Sachs' "Botany"). Here the contents of the cell contract, rearrange themselves, and burst the side of the containing wall, becoming free as a reproductive cell.
2. The zygoblasts of conjugating algae, as in Spirogyra. Here the contents of a cell contract and rearrange themselves only after contact of the cell with one of another filament of the plant. This zygoblast only becomes free after the process of conjugation, as described below.
3. The oosphere of characeae, mosses and liverworts, and vascular cryptogams, where in special structures produced by cell-divisions there arise single primordial cells, which divide into two portions, of which the upper portion dissolves or becomes mucilaginous, while the lower contracts and rearranges itself to form the oosphere.
4. Spores of mosses and liverworts, of vascular cryptogams, and pollen cells of phanerogams, which are the analogue of the spores.
The type in all these cases is this: A mother-cell produces by cell-division four daughter-cells. This is so far vegetative. Each daughter-cell contracts and becomes more or less rounded, secretes a wall of its own, and by the bursting or absorption of the wall of its mother-cell becomes free. This is evidently a rejuvenescence.
Examples of reproductive cells formed by free-cell formation are:
1. The ascospores of fungi and algae.
2. The zoospores or mobile spores of many algae and fungi.
3. The germinal vesicles of phanerogams.
The next portion of my subject is the study of the methods by which these special cells reproduce the plant.
1st. Asexual methods.
1. Rejuvenescence gives rise to a swarm-spore or zoospore. The whole of the protoplasm of a cell contracts, becomes rounded and rearranged, and escapes into the water, in which the plant floats as a mass of protoplasm, clear at one end and provided with cilia by which it is enabled to move, until after a time it comes to rest, and after secreting a wall forms a new plant by ordinary cell-division. Example: Oedogonium.
2. Free-cell formation forms swarm-spores which behave as above. Example: Achlya.
3. Free-cell formation forms the typical motionless spore of algae and fungi. For instance, in the asci of lichens there are formed from a portion of the protoplasm four or more small ascospores, which secrete a cell-wall and lie loose in the ascus. Occasionally these spores may consist of two or more cells. They are set free by the rupture of the ascus, and germinate by putting out through their walls one or more filaments which branch and form the thallus of a new individual. Various other spores formed in the same way are known as tetraspores, etc.
4. Cell-division with rejuvenescence forms the spores of mosses and higher cryptogams.
To take the example of moss spores:
Certain cells in the sporogonium of a moss are called mother-cells. The protoplasm of each one of these becomes divided into four parts. Each of these parts then secretes a cell-wall and becomes free as a spore by the rupture or absorption of the wall of the mother-cell. The germination of the spores I shall describe later.
5. A process of budding which in the yeast plant and in mosses is merely vegetatively reproductive, in fungi becomes truly reproductive, namely, the buds are special cells arising from other special cells of the hyphae.
For example, the so-called "gills" of the common mushroom have their surface composed of the ends of the threads of cells constituting the hyphae. Some of these terminal cells push out a little finger of protoplasm, which swells, thickens its wall, and becomes detached from the mother-cell as a spore, here called specially a basidiospore.
Also in the common gray mould of infusions and preserves, Penicillium, by a process which is perhaps intermediate between budding and cell-division, a cell at the end of a hypha constricts itself in several places, and the constricted portions become separate as conidiospores.
Teleutospores, uredospores, etc., are other names for spores similarly formed.
These conidiospores sometimes at once develop hyphae, and sometimes, as in the case of the potato fungus, they turn out their contents as a swarm-spore, which actively moves about and penetrates the potato leaves through the stomata before they come to rest and elongate into the hyphal form.
So far for asexual methods of reproduction.
I shall now consider the sexual methods.
The distinctive character of these methods is that the cell from which the new individual is derived is incapable of producing by division or otherwise that new individual without the aid of the protoplasm of another cell.
Why this should be we do not know; all that we can do is to guess that there is some physical or chemical want which is only supplied through the union of the two protoplasmic masses. The process is of benefit to the species to which the individuals belong, since it gives it a greater vigor and adaptability to varying conditions, for the separate peculiarities of two individuals due to climatic or other conditions are in the new generation combined in one individual.
The simplest of the sexual processes is conjugation. Here the two combining cells are apparently of precisely similar nature and structure. I say apparently, because if they are really alike it is difficult to see what is gained by the union.
Conjugation occurs in algae and fungi. A typical case is that of Spirogyra. This is an alga with its cells in long filaments. Two contiguous cells of two parallel filaments push each a little projection from its cell-wall toward the other. When these meet, the protoplasm of each of the two cells contracts, and assumes an elliptical form—it undergoes rejuvenescence. Next an opening forms where the two cells are in contact, and the contents of one cell pass over into the other, where the two protoplasmic bodies coalesce, contract, and develop a cell-wall. The zygospore thus formed germinates after a long period and forms a new filament of cells.
Another example of conjugation is that of Pandorina, an alga allied to the well-known volvox. Here the conjugating cells swim free in water; they have no cell-wall, and move actively by cilia. Two out of a number approach, coalesce, contract, and secrete a cell-wall. After a long period of rest, this zygospore allows the whole of its contents to escape as a swarm-spore, which after a time secretes a gelatinous wall, and by division reproduces the sixteen-celled family.
We now come to fertilization, where the uniting cells are of two kinds.
The simplest case is that of Vaucheria, an alga. Here the vegetative filament puts out two protuberances, which become shut off from the body of the filament by partitions. The protoplasm in one of these protuberances arranges itself into a round mass—the oosphere or female cell. The protoplasm of the other protuberance divides into many small masses, furnished with cilia, the spermatozoids or male cells. Each protuberance bursts, and some of the spermatozoids come in contact with and are absorbed by the oosphere, which then secretes a cell-wall, and after a time germinates.
The most advanced type of fertilization is that of angiosperms.
In them there are these differences from the above process: the contents of the male cell, represented by the pollen, are not differentiated into spermatozoids, and there is no actual contact between the contents of the pollen tube and the germinal vesicle, but according to Strashurger, there is a transference of the substance of the nucleus of the pollen cell to that of the germinal vesicle by osmose. The coalescence of the two nuclei within the substance of the germinal vesicle causes the latter to secrete a wall, and to form a new plant by division, being nourished the while by the mother plant, from whose tissues the young embryo plant contained in the seed only becomes free when it is in an advanced stage of differentiation.
Perhaps the most remarkable cases of fertilization occur in the Florideae or red seaweeds, to which class the well-known Irish moss belongs.
Here, instead of the cell which is fertilized by the rounded spermatozoid producing a new plant through the medium of spores, some other cell which is quite distinct from the primarily fertilized cell carries on the reproductive process.
If the allied group of the Coleochaeteae is considered together with the Florideae, we find a transition between the ordinary case of Coleochaete and that of Dudresnaya. In Coleochaete, the male cell is a round spermatozoid, and the female cell an oosphere contained in the base of a cell which is elongated into an open and hair-like tube called the trichogyne. The spermatozoid coalesces with the oosphere, which secretes a wall, becomes surrounded with a covering of cells called a cystocarp, which springs from cells below the trichogyne, and after the whole structure falls from the parent plant, spores are developed from the oospore, and from them arises a new generation.
In Dudresnaya, on the other hand, the spermatozoid coalesces indeed with the trichogyne, but this does not develop further. From below the trichogyne, however, spring several branches, which run to the ends of adjacent branches, with the apical cells of which they conjugate, and the result of this conjugation is the development of a cystocarp similar to that of Coleochaete. The remarkable point here is the way in which the effect of the fertilizing process is carried from one cell to another entirely distinct from it.
Thus I have endeavored to sum up the processes of asexual and of sexual reproduction. But it is a peculiar characteristic of most classes of plants that the cycle of their existence is not complete until both methods of reproduction have been called into play, and that the structure produced by one method is entirely different from that produced by the other method.
Indeed, it is only in some algae and fungi that the reproductive cells of one generation produce a generation similar to the parent; in all other plants a generation A produces are unlike generation B, which may either go on to produce another generation, C, and then back to A, or it may go on producing B's until one of these reproduces A, or again it may directly reproduce; A. Thus we have the three types:
1. A-B-C.—A-B-C.—A..................... etc. 2. A-B-B.—B-B...................B—A ... etc. 3. A B A B A............................. etc.
The first case is not common, the usual number of generations being two only; but a typical example of the occurrence of three generations is in such fungi as Puccinia Graminis. Here the first generation grows on barberry leaves, and produces a kind of spore called an aecidium spore. These aecidium spores germinate only on a grass stem or leaf, and a distinct generation is produced, having a particular kind of spore called an uredospore. The uredospore forms fresh generations of the same kind until the close of the summer, when the third generation with another kind of spore, called a teleutospore, is produced.
The teleutospores only germinate on barberry leaves, and there reproduce the original aecidium generation.
Thus we have the series A.B.B.B ... BCA
In this instance all the generations are asexual, but the most common case is for the sexual and the asexual generations to alternate. I will describe as examples the reproduction of a moss, a fern, and a dicotyledon.
In such a typical moss as Funaria, we have the following cycle of developments: The sexual generation is a dioecious leafy structure, having a central elongated axis, with leaves arranged regularly around and along it. At the top of the axis in the male plant rise the antheridia, surrounded by an envelope of modified leaves called the perigonium. The antheridia are stalked sacs, with a single wall of cells, and the spiral antherozoids arise by free-cell formation from the cells of the interior. They are discharged by the bursting of the antheridium, together with a mucilage formed of the degraded walls of their mother cells.
In the female plant there arise at the apex of the stem, surrounded by an envelope of ordinary leaves, several archegonia. These are of the ordinary type of those organs, namely, a broad lower portion, containing a naked oosphere and a long narrow neck with a central canal leading to the oosphere. Down this canal pass one or more antherozoids, which become absorbed into the oosphere, and this then secretes a wall, and from it grows the second or asexual generation. The peculiarity of this asexual or spore-bearing plant is that it is parasitic on the sexual plant; the two generations, although not organically connected, yet remain in close contact, and the spore-bearing generation is at all events for a time nourished by the leafy sexual generation.
The spore-bearing generation consists of a long stalk, closely held below by the cells of the base of the archegonium; this supports a broadened portion which contains the spores, and the top is covered with the remains of the neck of the archegonium forming the calyptra.
The spores arise from special or mother-cells by a process of division, or it may be even termed free-cell formation, the protoplasm of each mother-cell dividing into four parts, each of which contracts, secretes a wall, and thus by rejuvenescence becomes a spore, and by the absorption of the mother-cells the spores lie loose in the spore sac. The spores are set free by the bursting of their chamber, and each germinates, putting out a branched thread of cells called a protonema, which may perhaps properly be termed a third generation in the cycle of the plant; for it is only from buds developed on this protonema that the leafy sexual plant arises.
The characteristics, then, of the mosses are, that the sexual generation is leafy, the one or two asexual generations are thalloid, and that the spore-bearing generation is in parasitic connection with the sexual generation.
In the case of the fern, these conditions are very different.
The sexual generation is a small green thalloid structure called a prothallium, which bears antheridia and archegonia, each archegonium having a neck-canal and oosphere, which is fertilized just as in the moss.
But the asexual generation derived from the oospore only for a short while remains in connection with the prothallium, which, of course, answers to the leafy portion of the moss. What is generally known as the fern is this asexual generation, a great contrast to the small leafless moss fruit or sporogonium as it is called, to which it is morphologically equivalent. On the leaves of this generation arise the sporangia which contain the spores. The spores are formed in a manner very similar to those of the mosses, and are set free by rupture of the sporangium.
The spore produces the small green prothallium by cell-division in the usual way, and this completes the cycle of fern life.
The alternation of generations, which is perhaps most clear and typical in the case of the fern, becomes less distinctly marked in the plants of higher organization and type.
Thus in the Rhizocarpae there are two kinds of spores, microspores and macrospores, producing prothallia which bear respectively antheridia and archegonia; in the Lycopodiaceae, the two kinds of spores produce very rudimentary prothallia; in the cycads and conifers, the microspore or pollen grain only divides once or twice, just indicating a prothallium, and no antheridia or antherozoids are formed. The macrospore or embryo-sac produces a prothallium called the endosperm, in which archegonia or corpuscula are formed; and lastly, in typical dicotyledons it is only lately that any trace of a prothallium from the microspore or pollen cell has been discovered, while the macrospore or embryo-sac produces only two or three prothallium cells, known as antipodal cells, and two or three oospheres, known as germinal vesicles.
This description of the analogies of the pollen and embryo-sac of dicotyledons assumes that the general vegetative structure of this class of plants is equivalent to the asexual generation of the higher cryptogams. In describing their cycle of reproduction I will endeavor to show grounds for this assumption.
We start with the embryo as contained in the seed. This embryo is the product of fertilization of a germinal vesicle by a pollen tube. Hence, by analogy with the product of fertilization of rhizocarp's, ferns, and mosses, it should develop into a spore bearing plant. It does develop into a plant in which on certain modified leaves are produced masses of tissue in which two kinds of special reproductive cells are formed. This is precisely analogous to the case of gymnosperms, lycopods, etc., where on leaf structures are formed macro and micro sporangia.
To deal first with the microsporangium or pollen-sac. The pollen cells are formed from mother cells by a process of cell division and subsequent setting free of the daughter cells or pollen cells by rejuvenescence, which is distinctly comparable with that of the formation of the microspores of Lycopodiaceae, etc. The subsequent behavior of the pollen cell, its division and its fertilization of the germinal vesicle or oosphere, leave no doubt as to its analogy with the microspore of vascular cryptogams.
Secondly, the nucleus of the ovule corresponds with the macrosporangium of Selaginella, through the connecting link of the conifers, where the ovule is of similar origin and position to the macrosporangium of the Lycopodiaceae. But the formation of the macrospore or embryo-sac is simpler than the corresponding process in cryptogams. It arises by a simple enlargement of one cell of the nucleus instead of by the division of one cell into four, each thus becoming a macrospore. At the top of this macrospore or embryo-sac two or three germinal vesicles are formed by free cell formation, and also two or three cells called antipodal cells, since they travel to the other end of the embryo-sac; these latter represent a rudimentary prothallium. This formation of germinal vesicles and prothallium seems very different from the formation of archegonia and prothallium in Selaginella, for instance; but the link which connects the two is in the gymnosperms, where distinct archegonia in a prothallium are formed.
Thus we see that the flowering plant is essentially the equivalent of the asexual fern, and of the sporogonium of the moss, and the pollen cell and the embryo-sac represent the two spores of the higher cryptogams, and the pollen tube and the germinal vesicles and antipodal cells are all that remain of the sexual generation, seen in the moss as a leafy plant, and in the fern as a prothallium. Indeed, when a plant has monoecious or dioecious flowers, the distinction between the asexual and the sexual generation has practically been lost, and the spore-bearing generation has become identified with the sexual generation.
Having now described the formation of the pollen and the germinal vesicles, it only remains to show how they form the embryo. The pollen cell forms two or three divisions, which are either permanent or soon absorbed; this, as before stated, is the rudimentary male prothallium. Then when it lies on the stigma it develops a long tube, which passes down the style and through the micropyle of the ovule to the germinal vesicles, one of which is fertilized by what is probably an osmotic transference of nuclear matter. The germinal vesicle now secretes a wall, divides into two parts, and while the rest of the embyro-sac fills with endosperm cells, it produces by cell division from the upper half a short row of cells termed a suspensor, and from the lower half a mass of cells constituting the embryo. Thus while in the moss the asexual generation or sporogonium is nourished by the sexual generation or leafy plant, and while in the fern each generation is an independent structure, here in the dicotyledon, on the other hand, the asexual generation or embryo is again for a time nourished in the interior of the embryo-sac representing the sexual generation, and this again derives its nourishment from the previous asexual generation, so that as in the moss, there is again a partial parasitism of one generation on the other.
To sum up the methods of plant reproduction: They resolve themselves into two classes.
1st. Purely vegetative.
2d. Truly reproductive by special cells.
In the second class, if we count conjugation as a simple form of fertilization, there are only two types of reproductive methods.
1st. Reproduction from an asexual spore.
2d. Reproduction from an oospore formed by the combination of two sexual cells.
In the vast majority of plant species these two types are used by the individuals alternately.
The extraordinary similarity of the reproductive process, as shown in the examples I have given, Achlya, Spirogyra, and Vaucheria among algae, the moss, the fern, and the flowering plant, a similarity which becomes the more marked the more the details of each case and of the cases of plants which form links between these great classes are studied, points to a community of origin of all plants in some few or one primeval ancestor. And to this inference the study of plant structure and morphology, together with the evidence of palaeobotany among other circumstances, lends confirmatory evidence, and all modern discoveries, as for instance that of the rudimentary prothallium formed by the pollen of angiosperms, tend to the smoothing of the path by which the descent of the higher plants from simpler types will, as I think, be eventually shown.
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