16. Among the Desmidew, conjugation has been more frequently observed and described, and was known to take place at an earlier period in Closterium, than in any other genus. The earliestdescription is that of Morren,which is to be found in the Bulletins of the Aca demy of Brussels, for 1836, and is among the most accurate that we possess. The crescent shaped cell forming the frond of Closterium is, as in the Desmidex, composed of two similar halves, to the plane of junction of which its long axis is perpendicular. It differs from other genera in the absence of a median constriction, the junction being only indicated by a faint line in the external mem brane.* When two fronds are about to con jugate, they place themselves parallel and op posite to each other, with their concave sur faces facing. We next remark that the cell membrane partiallygives way at the line above mentioned, the two halves of each Closterium separating slightly on the side opposite its fellow, but remaining in contact on the other side. The openings are soon observed to be occupied by cushion-like projections of the in ternal membrane, which squeeze out between the valves. From the fact that the cavity of the internal membrane is double, or rather that each hal fof the Closterium possesses an independent primordial membrane, it follows that each of the projections above mentioned consists of two distinct sacculi. Soon the two double cushions come in contact ; they are at first perfectly colourless, but shortly afterwards become filled with green granular matter, and press so closely together as to be no longer distinguishable. It is next observed that from the junction of the four sacculi, two canals have resulted, each of which soon swells out in a hemispherical form, corresponding to that of a mass of green granular matter which now occupies its cavity. This mass is soon in vested by a delicate membrane, which, in the progress of development, thickens and pre sents an uneven surface. The two bodies which thus take their origin are the germ cells. They soon become free from the struc ture in which they were formed, and, according to Marren, display for about fifteen minutes after their escape an active motion. After this period, the motion ceases, and they attach themselves to a foreign body. Morren has observed their germination. The spherical germ lengthens first at one, then at the oppo site extremity, so as to assume the charac teristic crescentic form of the plant. Its green contents divide into two masses, each of which is invested by a separate primordial membrane, and occupies one of the future seg ments of the frond. In a short time the young Closterium completely resembles the adult. It is worthy of remark, that in the abnormal cases in which only one germ results from the conjugation of two individuals, only one of the halves of each empties itself, the other remaining unaltered.* In other families of Desmidew, the pro cess of conjugation, although variously modi fied as to its less important details, is essentially the same as that which occurs in Closterium.-I 17. In the Zygnemacew, confervoid plants, which seem to have a close relation with the Desmidem, the phenomena of conjugation have been long known. The frond consists of a series of cylindrical cells, which lengthens indefinitely by repeated division of its ele rnents. Here, as in the Desmidem, it is the last-produced cells in the filament which take part in the process of conjugation. In Spirogyra the union of two cells belonging to opposite filaments takes place by the ex pansion of one side of each, so as to form a papilla, or short tube with a rounded end. The ends of the two projections then come into contact, become slightly flattened as they are pressed ag,ainst each other, and unite. The double wall formed by their union, dissolves, or is broken through, so that a free passage is es tablished between the two cell cavities. Upon this, the whole of the chlorophylle previously arranged round the inside of each of the cells, becomes a confused mass, which soon forms itself either in the cavity of one of them, or in the connecting canal, into a globular or oval smooth spore, invested with a colourless cellulose membrane. Having arrived at this condition, it remains several months — from the autumn of one year to the spring of the following,—without undergoing any change of form.t During this period two new membranes are produced within the first by the secretion of cellulose on the surface of the primordial utri cle. Of these two, the external is of consider able thickness, and of a yellow colour. The internal, which may be considered as the proper membrane of the spore, is delicate and transparent. Germination consists in the growing out of this membrane at one end of the spore into a many-celled filament, which escapes through a lacerated opening in_the ex ternal membranes, and gradually assumes the character, and appearance of the parent plant. At the same time a tubular elongation of the same membrane of limited growth is formed in the opposite direction, which is the rudi ment of a root.* 18. In a species of Palinellem (Palmoglea macrococca) in which the whole individual consists of a single ovoid cell containing green granular matter, and usually multiplying itself by successive division, the phenomena of con jugation present themselves in a somewhat different and very remarkable form. Here two cells, probably the result of a series of divisions, undergo a complete union, affecting not only their contents, but also their mem branes. They coalesce as completely at their
points of contact, as two contiguous drops of water, the result of their union being a cell which differs in no-respect from its predeces sors, except in the greater thickness of its vvalls, and in the complete conversion of the chlorophylle of its contents into oily globules. Like the spore of the Zygnemace, it is des tined to a long period of inactivity, after which, by the successive division of its con tents, it gives rise to a new series of individuals, similar to those that preceded it.
I 9. We have still to consider the most remark able condition under which conjugation takes place among the Alg. The evolution of the aggregate zoospore of Vaucheria has been already. described. In the plant which results from its germination, Karsten has observed that along the course of those filaments which come in contact with the atmosphere are formed organs of a peculiar structure. They originate like the ordinary branches, as nipple shaped buddings out of the cell-wall, which are distributed in pairs along the whole course of the older filaments. In every pair of organs, one elongates so as to form a closed tube, which curves round into a spiral form, like the leaves of a Pilularia, while its fellow soon ceases to grow in length, but swells out into a globular or oval form, about three times as wide as the other. At first both contain chlorophylle, which, in the tubular organ, is soon replaced by colourless globules. In the meantime its fellow, which resembles a dark green-coloured globe, supported on a short pedicle, alters in forrn, its cell-wall extending into a nipple-shaped projection on the side next the tubular organ, with which it finally comes in contact. This condition lasts for some time, but it does not appear determined with sufficient distinctness by Karsten's vations, that an actual interchange of the contents takes place. All that we learn as certain is, that after the completion of what he calls the act of fructification, a newly formed cell appears in the cavity of the globular organ, which shortly after separates from the mother plant. In this instance, as in those above described, conjugation is pre ceded by the conversion of the green gra nular contents of the conjugating cells into oil globules. The germ thus produced re tains its power of development for several months, and gives rise to a new plant re sembling its parent in structure.* 20. In Saprolegnia, which is morphologically so closely related to Vaucheria, and like it, in its earliest state of existence, produces zoospores, we obtain, by the germination of these zoospores, plants which produce repro ductive organs of an entirely different cha racter. These, when completely formed, consist of spheroidal cells, each supported on a cylindrical pedicle. Each contains in its interior a number of round spores (from five or six to forty), which differ from the zoo spores, not only in their external form, but in possessing a distinct investing membrane. This complication of structure corresponds with the capability of retaining their vitality for a long period. They may be found in an unaltered condition in the water in which the parent plant has grown for many tnonths after the total destruction of the latter ; and it is to them, doubtless, that we must attri bute the extraordinary facility with which the Saprolegnia makes its appearance whenever the peculiar conditions it requires present themselves.* On the filaments which produce the above-described spore-cases, there are developed among them, and at the same time with them, slender, worm-like branchlets. These, as they reach the spore-cases, attach themselves firmly to them, and even some times wind round them in a regular manner. An actual interchange of contents, however, has not yet been observed.-1 21. Reproductive organs of the red Algce or Floridece.—In this group of plants weunfortu nately know too little of the origin and deve lopment of the germ-producing organs, to compare them with the forms which prevail in other groups. It is altogether beyond the limits of the present article to describe in detail all the perplexing varieties of structures to be found in the Floridem which may be supposed to have some relation with the re productive function. It will be sufficient to mention the three leading forms that are met with, and which may at all times be easily identified, in spite of the innumerable subor dinate modifications that they undergo. The first form, to , which the term polyspore is usually applied, is that of a gelatinous or membranous pericarp or conceptacle, in which an indefinite number of sporidia are contained. This organ may be placed either at the summit or in the axil of a branch, or it may be con cealed in or below the cortical layer of the stem. In other cases a number of sporidium bearing filaments emanate from a kind of placenta at the base of a spheroidal, cellular perisporangium, by the rupture of which the sporidia which are formed from the endo chromes of the filaments, make their escape. Other forms, which it does not seem neces sary to mention, are observed : they all agree in one particular, viz. that the sporidium is developed in the interior of a cell, the wall of which forms its perispore, and the internal protoplasmic membrane (endochrome), the .sporidium itself, for the escape of which the perispore ruptures at its apex.