them artificially in Aquaria, it is requisite to protect them from an excess of light; since otherwise they become unhealthy. The nature of the fructification of the Rhodosperme (or Floride) is less perfectly understood than that of the Fucoid Algæ. It is certain, however, that antheridia exist among them; these being developed in individuals that do not produce spores, and in pretty much the same situations. The products of these antheridia, however, do not exhibit the spontaneous motion of ordinary antherozoids. Of the spores there are two kinds, of which one set are probably "gemma," whilst the other are "germ-cells;" but it is not yet determined to which of the two these characters respectively belong. The "tetraspores," which are peculiarly characteristic of the group, being found in every one of its subdivisions, are usually imbedded in the general substance of the frond, though they sometimes congregate in particular parts, or are restricted to a special branch. Each group (Fig. 117, B) seems

Arrangement of tretraspores, in Carpocaulon mediterraneum :-A, entire plant; B, longitudinal section of branch. (N. B. Where only three tetraspores are seen, it is merely because the fourth did not happen to be so placed as to be seen at the same view.)

to be evolved within one of the ordinary cells of the frond, which undergoes a duplicative subdivision; the four secondary cells, however, remain enclosed within their primary cell until the period of maturity, a new envelope, the "perispore," being formed around them. In the Corallines, which are sea-weeds whose tissue is consolidated by calcareous deposit, the tetraspores are included within hollow conceptacles; but generally speaking, it is the simple spores only which are thus specially

protected. These are never scattered through the frond, like the tetraspores; and are commonly developed within a ceramidium, which is an urn-shaped case, furnished with a pore at its summit, and containing a tuft of pear-shaped spores arising from the base of its cavity. The resemblance of these bodies in position to the "octospores" of Fuci, would seem to justify the conclusion that they are the true generative spores, whilst the tetraspores are gemmæ, as Harvey and Thwaites consider them; but a different view is taken by Decaisne, Agardh, and other eminent Algologists, who regard the tetraspores as the true generative spores, and consider the simple spores to be gemmæ. It is, therefore, a point of much interest to determine by careful observation and experiment which is the right view; and Microscopists who have the opportunity of studying these plants, either in their native haunts, or in artificial Aquaria, can scarcely apply themselves to a better subject of investigation.

207. The class of Lichens, which consists of plants that closely correspond with Alge in simplicity of organization, but differ from them widely in habit, does not present so many objects of attractive interest to the Microscopist; and the peculiar density which usually characterizes their structure, renders a minute examination of it more than ordinarily difficult. Lichens are commonly found growing upon the trunks or branches of trees, upon rocks or stones, upon hard earth, or in other situations in which they are sparingly supplied with moisture, but are freely exposed to light and air. In the simpler forms of this group, the primordial cell gives origin, by the ordinary process of subdivision, to a single layer of cells, which may spread itself over the surface to which it is attached, in a more or less circular form; and one or more additional layers being afterwards developed upon its free surface, a thallus is formed, which has no very defined limit, and which, in consequence of the very slight adhesion of its component cells, is said to be "pulverulent." Sometimes, however, the cells of the thallus are rather arranged in the form of filaments, which penetrate the superficial layers of the bark whereon such Lichens grow, and which are sometimes also so interwoven at the outer surface, as to form a sort of cuticle. Interposed among the ordinary cells of the thallus, we very commonly find certain green globular cells, arranged in single beadlike filaments; these, which are termed gonidia, being found to be capable of reproducing the plant when detached, must be considered as gemma. In the higher tribes of Lichens, we find the interlacing filaments forming a tough cortical envelope to both surfaces; whilst in the interior of the firm "crustaceous" thallus, the gonidial cells are found in regular layers. Sometimes these increase in particular spots, and make their way through the upper cortical layer, so as to appear on the surface as little masses of dust, which are called soredia. Besides these, Lichens contain proper Generative organs, by which a true sexual

reproduction seems to be effected. In addition to the "fructification" which is commonly recognized by its projection from the surface of the thallus, the researches of M. Tulasne have detected a set of peculiar organs of much smaller size, not unlike the male receptacles of Fuci (§ 205), to which he has given the appellation of spermogonia. From the exterior of the cellular filaments which line these cavities, a vast number of minute oval bodies termed spermatia are budded off, which, when mature, escape in great numbers from the orifices of the spermogonia. They differ from ordinary antherozoids in being destitute of any power of spontaneous movement; but in this respect they are paralleled by the spermatoid bodies of the Floridea (§ 206). As their participation in the production of fertile spores has not yet been demonstrated, we cannot yet indubitably assign to them the character of "sperm-cells;" although various considerations concur to render their possession of this attribute highly probable. The female portion of the generative apparatus, though sometimes dispersed through the thallus, is usually collected into special aggregations, which form projections of various shapes; these, although they have received a variety of designations according to their particular conformation, may all be included under the general term apothecia. When divided by a vertical section, these bodies at their maturity are found to contain a number of asci or spore-cases, arranged vertically in the midst of straight elongated cells or filaments, which are termed paraphyses. Each of the asci contains a definite number of spores (usually eight, but always a multiple of two), which are projected from the apothecia with some force, the emission being kept up continuously for some time; this discharge seems to be due to the different effect of moisture upon the different layers of the apothecium. When and how the act of fecundation is accomplished, is a matter still hidden in obscurity; and the problem is one which, owing to the difficulties arising out of the dense structure of the organs, will only be resolved by a combination of sagacity, manipulative skill, and perseverance, on the part of Microscopic observers who may devote themselves to the study. 208. In the simplest forms of Fungi, we again return to the lowest type of Vegetable existence, namely, the single cell; and such, if perfect plants, would properly take rank among the lowest Protophytes. But there is good reason for regarding many -perhaps all-of those which seem most simple, as the imperfectly developed states of other Plants, which, if they attained their full evolution, would present a much more complex structure. This is the case, for example, with the Torula cerevisia or Yeast-plant, which so abounds in Yeast, that this substance may be said to be almost entirely made up of it. When a small quantity of yeast is placed under the Microscope, and is magnified 300 or 400 diameters, it is found to be full of globules, which are clearly cells; and these cells vegetate, when placed in a fer

mentable fluid containing some form of albuminous matter in addition to sugar, in the manner represented in Fig. 118. Each

Torula Cerevisiæ, or Yeast-Plant, as developed during the process of fermentation:-a, b, c, d, suc cessive stages of Cell-multiplication.

cell puts forth one or two projections, which seem to be young cells developed as buds or offsets from their predecessors; these, in the course of a short time, become complete cells, and again perform the same process; and in this manner the single cells of yeast develope themselves, in the course of a few hours, into rows of four, five, or six, which remain in continuity with each other whilst the plant is still growing, but which separate if the fermenting process be checked, and return to the isolated condition of those which originally constituted the yeast. Thus it is, that the quantity of yeast first introduced into the fermentable fluid, is multiplied six times or more, during the changes in which it takes part. The full development of the Plant, however, and the evolution of its apparatus of fructification, only occur, when the fermenting process is allowed to go on without check; and it seems capable of producing a considerable variety of forms, whose precise relationship to each other has not yet been made clear. In fact, with regard to the Fungi generally, it has been made apparent by recent observations, that different individuals of the very same species may not only develope themselves according to a great number of very dissimilar modes of growth, but that they may even bear very dissimilar types of fructification; and further, that even the same individual may put forth, at different periods of its life, those two kinds of fructification,-the basidio-sporous, in which the spores are developed by out-growth from free points (basidia), and the theca-sporous, in which they are developed in the interior of cases (thecæ or asci, Fig. 125),—which had been previously considered as separately characterizing the two principal groups, into which the class is primarily divided.

209. Many of the simpler forms of Fungi are inhabitants of the interior of the bodies of other animals, and are only known as living in these situations. Among these may first be mentioned the Sarcina ventriculi (Fig. 119), which is most frequently found in the matters vomited by persons suffering under disorder of the stomach, but has also been met with in other diseased parts of the body. The plant has been detected in the contents of the stomach, however, under circumstances which seem to indicate that it is not an uncommon tenant of that organ even in health, and that it may accumulate there to a

considerable amount without producing any inconvenience; it seems probable, therefore, that its presence in disease is

FIG. 119.

Surcina ventriculi.

rather to be considered as favored by the changed state of the fluids which the disease induces (either an acid or a fermentable state of the contents of the stomach having been generally found to exist in the cases in which the plant has been most abundant), than to be itself the occasion of the disease, as some have supposed. The Sarcina presents itself in the form of clusters of adherent cells arranged in squares, each square containing from 4 to 64, and the number of cells being obviously multiplied by duplicative subdivision in directions transverse to each other. In fact, its general mode of growth would indicate a near relationship to Gonium, one of the Volvocineæ, which presents itself in similar quadripartite aggregations; and many Botanists, looking to this circumstance, and to the residence of the plant in liquid, regard it as belonging to the group of Algae. It agrees with the Fungi, however, in not living elsewhere than in liquids containing organic matter; and there can be little doubt that, as no fructification has yet been seen in it, only its earlier and simpler condition is yet known to us. Its true place cannot be determined, until its whole lifehistory shall have been followed out. There is a form of Fungous vegetation that is prone to develope itself within the living body, which is of great economic importance, as well as of scientific interest; this is the Botrytis bassiana (Fig. 120), a kind of "mould," the growth of which is the real source of a disease termed Muscardine, that sometimes carries off Silk-worms in large numbers, just when they are about to enter the chrysalis state, to the great injury of their breeders. The sporules of this fungus, floating in the air, enter the breathing-pores which open into the tracheal system of the silk-worm (Chap. XVII); they first develope themselves within the air-tubes, which are soon blocked up by their growth; and they then extend themselves through the fatty mass beneath the skin, occasioning the destruction of this tissue, which is very important as a reservoir of nutriment to the animal, when it is about to pass into a state of complete inactivity. The disease invariably occasions the death of the silk-worm which it attacks; but it seldom shows itself externally until afterwards, when it rapidly shoots forth from beneath the skin, especially at the junction of the rings of the body. Although it spontaneously attacks only the larva, yet it may be communi