Page:EB1911 - Volume 21.djvu/803

Rh In the Cyanophyceae the contents of the cell are differentiated into a central colourless region, and a peripheral layer containing the chlorophyll and other colouring matters together with granules of a reserve substance called cyanophycin. Chromatin is contained in the central part together with granules known as volutin, the function of which is unknown. The central body probably plays the part of a nucleus and some observers consider that it has the characters of a typical nucleus with mitotic division. But this is very doubtful. The central body seems to consist merely of a spongy mass of slightly stainable substance, more or less impregnated with chromatin, which divides by constriction. At a certain stage in the division figures are produced resembling a mitotic phase (fig. 8, 1), which are not, in the opinion of the writer, to be interpreted as a true mitosis. It is interesting to note that in many species the formation of new cell-walls is initiated before any indication of nuclear division is to be seen.

The bacteria, in most cases, have no definite nucleus or central body. The chromatin is distributed throughout the cytoplasm in the form of granules which may be regarded as a distributed nucleus corresponding to what Hertwig has designated, in protozoa, chromidia.

In the yeast cell the nucleus is represented by a homogeneous granule, probably of a nucleolar nature, surrounded and perhaps to some extent impregnated by chromatin and closely connected with a vacuole which often has chromatin at its periphery, and contains one or more volutin granules which appear to consist of nucleic acid in combination with an unknown base. Some observers consider that the yeast nucleus possesses a typical nuclear structure, and exhibits division by mitosis, but the evidence for this is not very satisfactory.

Tissues.—The component parts of the tissues of which plants are composed may consist of but slightly modified cells with copious protoplasmic contents, or of cells which have been modified in various ways to perform their several functions. In some the protoplasmic contents may persist, in others they disappear. The formation of the conducting tubes or secretory sacs which occur in all parts of the higher plants is due either to the elongation of single cells or to the fusion of cells together in rows by the absorption of the cell-walls separating them. Such cell-fusions may be partial or complete. Cases of complete fusion occur in the formation of laticiferous vessels, and in the spiral, annular and reticulated vessels of the xylem. Incomplete fusion occurs in sieve tubes. Tubes formed by the elongation of single cells are found in bast fibres, tracheides, and especially in laticiferous cells.

Laticiferous Tissue.—The laticiferous tissue consists of a network of branching or anastomosing tubes which contain a coagulable fluid known as latex. These tubes penetrate to all parts of the plant and occur in all parts of the root, stem and leaves. A protoplasmic lining is found on their walls which contains nuclei. The

walls are pitted, and protoplasmic connexions between the laticiferous tubes and neighbouring parenchyma-cells have been seen. There are two types of laticiferous tissue—non-articulate and articulate. The non-articulate tissue which occurs in Euphorbiaceae, Apocynaceae, Urticaceae, Asclepiadaceae, consists of long tubes, equivalent to single multinucleate cells, which ramify in all directions throughout the plant. Laticiferous vessels arise by the coalescence of originally distinct cells. The cells not only fuse together in longitudinal and transverse rows, but put out transverse projections, which fuse with others of a similar nature, and thus form an anastomosing network of tubes which extends to all parts of the plant. They are found in the Compositae (Cichoriaceae), Campanulaceae, Papaveraceae, Lobeliaceae, Papayaceae, in some Aroideae and Musaceae, and in Euphorbiaceae (Manihot, Hevea). The nuclei of the original cells persist in the protoplasmic membrane. The rows of cells from which the laticiferous vessels are formed can be distinguished in many cases in the young embryo while still in the dry seed (Scott), but the latex vessels in process of formation are more easily seen when germination has begun. In the process of cell-fusion the cell-wall swells slightly and then begins to dissolve gradually at some one point. The opening, which is at first very small, increases in size, and before the cross-wall has entirely disappeared the contents of the two cells become continuous (Scott). The absorption of the cell-walls takes place very early in the germinating seedling.

Sieve Tubes.—The sieve tubes consist of partially fused rows of cells, the transverse or lateral walls being perforated by minute openings, through which the contents of the cells are connected with each other, and which after a certain time become closed by the formation of callus on the sieve plates. The sieve tubes contain a thin lining layer of protoplasm on their walls, but no nuclei, and the cell sap contains albuminous substances which are coagulable by heat. Starch grains are sometimes present. In close contact with the segments of the sieve tubes are companion cells which communicate with the sieve tubes by delicate protoplasmic strands; they can be distinguished from ordinary parenchymatous cells by their small size and dense protoplasm. Companion cells are not found in the Pteridophyta and Gymnos erms. In the latter their place is taken by certain cells of the medullary rays and bast parenchyma. The companion cells are cut off from the same cells as those which unite to form the sieve tube. The mode of formation of the sieve plate is not certainly known; but from the fact that delicate connecting threads of protoplasm are present between the cells from their first development it is probable that it is a special case of the normal protoplasmic continuity, the sieve pores being produced by a secondary enlargement of the minute openings through which these delicate strands pass. According to Lecomte, the young wall consists partly of cellulose and partly of a substance which is not cellulose, the latter existing in the form of slight depressions, which mark the position of the future pores. As the sieve plate grows these non-cellulose regions swell and gradually become converted into the same kind of mucous substance as that contained in the tube; the two cells are thus placed in open communication. If this is correct it is easy to see that the changes which take place may be initiated by the original delicate protoplasmic strands which pass through the cell-wall (For further information regarding tissues, see the section on Anatomy above)

Protoplasmic Continuity.—Except in the unicellular plants the cell is not an independent unit. Apart from their dependence in various ways upon neighbouring cells, the protoplasts of all plants are probably connected together by fine strands of protoplasm which pass through the cell-wall (Tangl, Russow, Gardiner, Kienitz-Gerloff and others) (fig. 9). In Pinus the presence of connecting threads has recently been demonstrated throughout all the tissues of the plant. These protoplasmic strands are, except in the case of sieve tubes, so delicate that special methods have to be employed to make them visible. The basis of these methods consists in causing a swelling of the cell-wall by means of sulphuric acid or zinc chloride, and subsequent staining with Hoffmann's blue or other aniline dyes. The results so far obtained show that the connecting threads may be either “pit-threads” which traverse the closing membrane of the pits in the cell-walls (fig. 9, B), or “wall-threads” which are present in the wall of the cell (fig. 9, A). Both