Note on the above. By F. H. WENHAM. In inadvertently making use of the term "shown opaquely," I did not wish it to be inferred that I considered this method as strictly an opaque illumination, which is understood when the light is thrown only on the upper surface of the object. The truncated lens, or flat-topped parabola, first used by me in the way referred to above, gives such a brilliant luminosity to the object, on a jet-black field, that it has all the appearance of an opaque illumination, and perhaps on many objects the difference in apparent structure would not be material, and may be illustrated in this way: Suppose some semi-transparent body, such as a green grape, be let into a piece of black card; on holding this against a strong light, so that it enters sideways, the seeds and internal structure will be shown satisfactorily. If a side light is condensed down upon the object, the same internal structure will be seen, though not so perfectly on account of surface glare. When a side light is thrown into the body of an object either way, each dense particle that intercepts it serves to illuminate its neighbour, and so the rays are diffused in every possible direction, and if the structure contains particles actually impervious to light, they will not be seen like dark shadows as by direct light, but luminous, and in their natural colours. I consider this is the main principle-to send the light into the object in any or all directions beyond the angle at which rays from the source can enter the eye. Dr. Woodward has kindly sent me the photographs referred to in the above note. No. 2, which quite agrees with my description, is in places very sharp and distinct, showing the intercostal stria or bars plainly. Nos. 1 and 3 are somewhat blurred, and to my mind do not show structure satisfactorily. Knowing the difficulty of obtaining a photograph of an object of this character, merely from its own diffused light, I was much surprised at Dr. Woodward's remarkable skill in producing a perfect picture—a feat that I should have thought scarcely possible. III.-On Bog Mosses. By R. BRAITHWAITE, M.D., F.L.S. (Read before the ROYAL MICROSCOPICAL SOCIETY, Nov. 1, 1871.) Part II. BEFORE commencing the descriptive portion of our subject, it may be well to enter a little more into detail with respect to the histology of the interesting plants constituting the Sphagnaceae. The published materials of which I have availed myself in the study, are the following, and to Prof. Lindberg, of Helsingfors, I am also deeply indebted for beautiful specimens of some of the rarer species. 1. Dozy-Bijdrage tot de Anatomie en Phytographie der Sphagna. 1854. 2. Schimper-Entwickelungs-geschichte der Torfmoose. 1858. 3. Lindberg-Torfmossornas byggnad udbredning och systematiska uppstallning. 1862. 4. Russow-Beiträge zur Kenntniss der Torfmoose. 1865. The roots, which form only on the young plants, are chiefly of use in fixing them to floating objects, for as soon as branches shoot forth, a part of these forming each fascicle drop perpendicularly downward, and becoming appressed to the stem, are from their hygroscopic quality far more effective than true roots, in transmitting fluid to the other parts of the plants; while the dense masses formed by the aggregation of stems equally supersede the use of roots as fixing organs. As the stem increases in size, the simple flageller branches arise laterally from the uppermost leaves, and are crowded together into a head or capitulum, which supplies fascicles of branches to the stem below, by elongation of the internodes, and keeps up its stock of young branches by constant renewals from the growing point at the apex. The dichotomous ramification of a Sphagnum depends on the annual production of an innovation, which is a perfect repetition of the stem of the previous year, and derives its origin from one of the lateral branches of the capitulum, which rises upward and becomes elongated into a main axis. The number of branches in a fascicle seems tolerably constant in each species, a part of these we may call the divergent branches, which proceed at a right angle from the stem, then bend about the middle and arch gracefully downward; the rest we will term the pendent branches, and these are longer, more attenuated, and fall down from their point of origin in the fascicle, and lie close to the stem. A part of the divergent branches become condensed and club-shaped to form the catkins of male flowers, and a few others become fruit branches. The leaves of Bog mosses vary considerably on different parts of the plant: the stem leaves are distant from each other, and usually reflexed against the stem, probably pushed back by the descent of the pendent branches; at their basal angles we also frequently observe appendages or auricles formed of larger perforated cells. The areolation of the stem leaves is wider than that of the brauch leaves, and the prosenchym cells of the lower part are often altogether threadless, while one or more rows at the extreme base are small, hexagonal, vesicular, and coloured red or yellow. The branch leaves are small, more densely reticulated, closely imbricated over each other, and very variable in form and size; this variability, however, is greatest in the pendent branches, where both they and their component cells become extremely elongated. Russow, however, points out that the leaves on the centre of the divergent branches are very constant in form in the individual species, and that they all become narrower and more distant as they approach the apex of the branch. Moreover, the 3-5 lowest leaves at the base of the divergent branches are remarkably different in form from those which succeed them, and stand midway between them and the stem leaves; the typical form of these intermediate leaves is an irregular-sided, obtuse-angled triangle, and they are always much smaller than the succeeding branch leaves; the margin of narrow cells which borders these leaves is widest at the base of the longest side. The peduncular leaves, or those found at the base of the naked branch which bears the fruit, differ from the others both in form and structure, sufficiently to render their description necessary. As an aid to our examination of leaf structure, certain colouring agents are of advantage in enabling us to obtain a better definition of the delicate textures of which the leaves are composed. Iodine and sulphuric acid or a solution of biniodide of zinc, have been used for this purpose, the latter of these being most convenient, an immersion of the leaf for two to twenty-four hours being required. Transverse sections of the leaves are also necessary in order to determine the relative positions of the chlorophyllose and hyaline cells; these are best prepared by immersing a branch in thick mucilage of gum arabic, and when sufficiently dry, enclosing between two pieces of elder-pith, and slices of the whole cut and placed in water. Anatomy of the Leaf.-Hedwig, in his Fundam. Hist. Nat. Musc.,' i.; p. 25 (1782), evidently noticed the composite character of the Sphagnum leaf, for he mentions the large areolæ, void of chlorophyll, traversed by very fine vessels, running double, which he thinks may possibly correspond to the ducts of flowering plants, and these anastomosing vessels containing parenchyma. Moldenhawer first pointed out the true nature of the two kinds of cells, and the presence of threads and pores in the vesicular cells, and Von Mohl afterwards confirmed his views and elaborated the whole organization of the Sphagna. A Sphagnum leaf consists of a single stratum of cells, the framework of which is constituted by network of extremely slender coloured or chlorophyll cells, into each of the meshes of which we might fancy one of the vesicular cells had been dropped. By section we see that the relative position of these two kinds of cells to each other may vary, for the chlorophyll cells may lie midway between the anterior and posterior surface of the leaf, and their section shows us that they are lenticularly compressed, or they may take part in forming the anterior or posterior surface of the leaf, their transverse section being triangular, so that they resemble a wedge pushed in between each pair of hyaline cells: minute as this structure is, we must admit its importance, since it originates in the fundamental formation of the leaf. The hyaline cells are more or less united by their adjacent walls, and nearly always contain threads attached to their internal walls; these threads may form complete spirals, composed of one or several fibrils, or they may be broken up into rings and spiral fragments, and sometimes run across diagonally so as to unite two spirals. Threads, however, are not always present in all the leaves, for in S. fimbriatum they are wanting in both the stem and peduncular leaves, and others have them in one part of the leaf while they are absent from the rest; in S. (Isocladus) macrophyllum no threads are seen except those forming a ring round the orifices of the pores. The threads are firm and intimately united to the inner wall of the cells, so that in S. subsecundum, the walls of the hyaline cells are strongly contracted by them. The apertures or pores are most abundant on the back of the leaf, and stand near the adjoining cell-walls; they vary in size and number according to the species, and no doubt originate by the resorption of the delicate cell-wall, within the boundary of a small thread-ring. Besides these, Russow calls attention to larger openings which become visible after treatment with iodine, and indicating more extensive resorption of the cell-membrane. Thus in the lower part of a branch leaf of S. fimbriatum so treated, these large apertures reach across the whole width of the cell, and stand between each pair of thread spirals; in the corresponding leaves of the nearly allied S. Girgensohnii this resorption appearance does not occur. In leaves from the pendent branches of S. intermedium, a hole is always seen at the apical end of each cell. In S. Lindbergii, fimbriatum, and Girgensohnii, whose stem leaves are fringed at the apex, this appearance is due to complete resorption of the membrane of the hyaline cells, and consequent projection of the intermediate parenchym cells. The chlorophyll cells of peduncular leaves usually have deficiencies in the thickening layers of their walls, and these standing opposite to each other, resemble imperforate dots, not unlike the dotted pleurenchyma of coniferous wood; a similar condition is observable in the walls of young axile cells of the Sphagnum stem.* In most peduncular leaves the hyaline cells are less evident than in those from other parts of the plant, and are often confined to the upper third of the leaf. Development of the Plant.-To the investigations of Nägeli and Hofmeister are we principally indebted for an account of this interesting process. It not unfrequently happens that in floating Sphagnum plants, whose capsules are submerged, that the spores germinate in the capsule, where their delicate pro-embryos are so closely packed, that the whole contents are caked together into a solid mass, which first becomes free by the breaking up of the capsular wall, and in this condition swims about until the individual plantlets have separated from one another to establish themselves on some floating object and undergo further evolution. The spores of capsules maturing out of the water, germinate on damp earth in two to three months. Prof. Schimper rarely noticed the pro-embryonal cell break through the exospore in less than five weeks. In water the pro-embryonal cell elongates and ramifies as confervoid filaments formed of nearly globose cells, and the terminal or some other cell becomes the mother cell of the young plant, while the rest ramify and put forth brood-gemmæ, which develope into young plants, the radicles being always distinguishable by the oblique commissural walls of the cells. The spores germinating on damp earth behave in quite a different manner, the pro-embryonal cell goes on subdividing in a horizontal plane, so that an expansion results resembling the prothallium of Equisetum, or the perfect plant of Blasia or Anthoceros. This hepaticine frond throws out radicles from the under surface and margins, and from these again brood-gemmæ sprout out, from which arise prothallia precisely resembling the first. The first commencement of the young plant originates in a tuberculoid aggregation of cells, some of which develope downward into hair-like radicles, while the upper cell elongates and subdivides to form the young stemlet, some of the cells, laterally becoming free, form the first rudiment of leaves. The young stem, at first transparent, soon acquires minute chlorophyll granules, and a differentiation into medullary, ligneous, and cortical layers is early set up. When it has reached a height of about 5 mm., it begins to throw off at the sides single flagellar branches, which arise laterally from the uppermost leaves, and are crowded together at the top of the stem. The branches come off at every fourth leaf, as an obtuse bud of few cells, on * Hofmeister, 'Higher Cryptogamia,' pl. xvii., fig. 96. |