338
CYTOLOGY
threads and wall-threads may occur in the same cell, but more often the threads are limited to the pits. The pitthreads are larger and stain more readily than the wall-threads. The threads vary in size in different plants. They are very thick in Viscum album, and are well seen in Phaseolus multijlorus and Lilium Alartagon. In epidermal cells (Tamus communes, Lihum Mavtagon) the external walls are penetrated by threads which extend to the cuticle. In dead cells the threads appear to be converted into mucilage. These connecting threads probably consist of ectoplasm. They are present from the beginning of the development of the cell-wall, and arise from the spindle fibres, all of which may be continued as connecting threads (Endosperm of Tamus communis), or part of them may be overlaid by cellulose lamellae (Endosperm of Lilium Martagon), or they may be all overlaid as in pollen mother-cells and pollen grains of Helleborus foetidus. The presence of these threads between all the cells of the plant shows that the plant body must be regarded as a connected whole; the threads themselves probably play an important part in the growth of the cell-wall, the conduction of food and water, the process of secretion, and the transmission of impulses. In certain cells of the rootapices of plants, however, there exist, according to Nemec, longitudinal protoplasmic fibrils which are especially concerned in the conduction of stimuli. 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 reticulate 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. 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 ous^'fssue Par^s 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. The fluid sap contains various substances, either in solution or suspended in the form of minute globules, such as sugar, tannin, malic acid in combination with lime, starch-grains, and small globules composed of caoutchouc or resin, which give to the fluid its milky appearance. There are two types of laticiferous tissue—non-articulate and articulate. The non-articulate tissue which occurs in Euphorbiacese, Apocyneae, Urticaceae, Asclepiadeae, consists of long tubes, equivalent to single multinucleate cells, which .ramify in all directions throughout the plant. The development of laticiferous cells takes place quite early in
- the embryo, from a group or groups of cells which form a
single layer in the pericycle around the central cylinder of the embryo at the cotyledonary node. The number of these initial cells appears to be constant for each species. In some cases a second layer is found just external to the central cylinder, in the cortex, from which a separate cortical system is developed. In other cases the single layer of initial cells gives off branches which penetrate the cortex, central cylinder, and pith (Chauveaud). 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 Composite {Cichoriacece), Campanulaceae, Papaveraceas, Lobeliacese, Papayacese, in some Aroideae and Musaceae, and in Euphorbiaceae (Manihot, Hevea). The nuclei of the original cells persist in the protoplasmic membrane. Kny has shown that in the Cichoriaceae hairs containing latex are commonly present, connected to the laticiferous network by means of a narrow opening at the base. 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. 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 Sjeve are connected with each other, and which after 0/^,es> 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 Gymnosperms. 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 Anatomy of Plants.) The formation of the zygote or egg-cell takes place usually by the fusion of the contents of two cells, and always includes, as an essential feature, the fusion pertlj}zg. of two germ nuclei. In many of the lower plants the fusing cells—gametes—are precisely similar so far as size and general appearance are concerned ; and the whole contents of the two cells fuse together, cytoplasm with cytoplasm, nucleus with nucleus, nucleolus with nucleolus, and plastid with plastid. The gametes
