Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/20

10 10 HISTOLOGY [VEGETABLE. cell with others. At any rate the fibres are to be looked upon as outgrowths or prolongations of nerve-cells. But some of the outgrowths of the nerve-cells, instead of passing into nerve-fibres, become ramified, and eventually break up into fine twigs, each of which is occupied by a nerve-fibril, and these form by their interlacement a network which joins that of the branched processes of the ciliated epi thelium. With the exception of the formation of the medullary substance in the sheath, the nervous tissue of the in vertebrate Metazoa agrees precisely, so far as the minute structure is concerned, with that of vertebrates. The lowest forms in which nervous structures have been found are the Medusae. In these the tissue exhibits itself under two modifications. The first of these is a so-called nerve epithelium (fig. 24, A), consisting of a portion of the ciliated ectoderm, the cells of which are prolonged at their attached ends into fine ramified fibres which interlace with one another and form a network of nerve-fibrils underneath the epithelium. This form seems to correspond with the ciliated epithelium of the vertebrate neural canal. The second modification occurs in certain cells of the ectoderm, which have become sunken singly here and there below the general epithelium of the surface, and between it and the muscular layer of the sub-umbrella. These cells become enlarged, and their nucleus takes on the characteristic appearance of the nucleus of a nerve-cell. Then generally from opposite ends of the cell (fig. 25) two processes grow out into long fibres, FIG 25. Nerve cell and fibre of a Jelly-fish (Aurelta). which exhibit all the features of the nerve-fibres of higher animals, and may even possess a nucleated sheath. These fibres, which may be branched or unbranched, seem to be applied to the substance of the muscular fibres, and in all probability serve to convey impulses to the muscle. There can be no doubt of the correspondence of these cells and fibres with the nerve-cells and nerve-fibre prolongations of the Vertebrata. In other invertebrates the nervous tissue is not only more localized than in the Ccdenterata, but the original ectodermal epithelium cells from which it is derived become much more extensively developed into well-charac terized nerve-cells and nerve-fibres, and tend moreover to be completely separated from the rest of the ectoderm and embedded in the mesoderm. But they are never originally developed in common with connective-tissue cells, as are the cells which form the muscular tissue. (E. A. s.) IE. VEGETABLE HISTOLOGY. By Vegetable Histology is meant the study by means of the microscope of the texture, web, or tissue of which plants are composed. It may be considered as synonymous with the minute anatomy of plants, and embraces the study of all those points of structure and development requiring the use of the microscope for their elucidation. Histology is, therefore, a modern science of observation and experiment, and it dates its origin from the time when magnifying glasses were first applied to the scrutiny of the organs of plants. All advances in histology have been preceded by some important improvement either in the construction of lenses and microscopes, or by the invention of some new method of research and application of new reagents. In order to prosecute the study of vege table histology, it is necessary to understand thoroughly the construction and use of the microscope, to be able to manipulate well and dexterously employ the various cutting and other instruments required, and, lastly, to be able to use the numerous reagents now so important in assisting to unravel the more difficult tissues. Nature of the Vegetable Cell. If a small portion of the Natur contents of the fertilized embryo-sac of the Phaseolus multi- the ce florus (scarlet-runner, or French bean) be examined in a drop of water on a slide, it is seen to consist of protoplasm with a number of small free cells, in different stages of development, floating in it. These cells consist of little rounded masses of protoplasm with a single contour line ; they have the protoplasm more or less granular; and each contains a rounded solid body, the nucleus, usually with a small spot, the nucleolus. Other cells in the preparation have a distinct wall with a double contour line, these being older and more fully developed. In examining the cells it is usually best not to employ pure water, but to use instead a solution of sugar or gum (1 part to 50 or 100 of water). Strasburger recommends, for examining the contents of the embryo-sac in phanerogams, a 3 per cent, solution of sugar, to which is added on the slide one drop of a 1 per cent, solution of osmic acid. Absolute alcohol may also be used for fixing the pro toplasm in a nearly unaltered state. A longitudinal section of the growing end of the root of Fritillaria imperialis will exhibit the different stages of development of tissue cells. Near the apex the cells are more or less hexagonal in shape, and have a marked wall with a more or less distinct double contour. Inside the cell-wall, and in close contact with it, is the protoplasm, a densely granular soft inelastic mass, consisting of a mixture of albuminoids, and having in the centre a round and relatively very large solid nucleus, with one or two nucleoli. In both Fhaseolus and Fritillaria, as the cell enlarges, clear spaces, called vacuoles, but filled with cell-sap, that is, water with substances in solution, appear in the protoplasm of the cell. In some algse con tractile vacuoles are met with. The ordinary vacuoles rapidly increase in number and enlarge, separating the pro toplasm into two parts one in close contact with the wall of the cell, the other forming strings of varying size and thickness separating the vacuoles. Presently the vacuoles all coalesce and form a central cell-sap cavity, the protoplasm forming a completely closed sac inside the cell-wall. The nucleus remains imbedded in the protoplasm, and is pushed to one side, appearing as if in contact with the wall. The vacuoled condition of the protoplasm may be considered as representing the cell at its state of greatest activity : the central cell-sap cavity is usually seen in tissue-cells, as in Fritillaria, and may he taken to indicate a condition of diminished activity. Further changes take place in tissue cells. The protoplasm with its nucleus may disappear and the cell-sap remain, or even the cell-sap itself may disappear sooner or later, and the dry cell-walls, as those of cork, be left. The conditions here described in Fritillaria may be taken as typical of all young tissue cells. The protoplasm is the essential part of the cell, and by it Proto- all the other parts are formed, as well as all the substances, plasm. such as chlorophyll and starch, that are contained in cells. When the cell contains protoplasm it can grow, multiply, and elaborate new chemical compounds ; when the proto plasm disappears it ceases to perform any of these functions, and passively acts as a protection to deeper cells, or permits certain physical processes to take place, as the transport of water through the walls. The substance of the protoplasm seems to consist of a mixture of various albuminoids, and probably of other nitrogenous compounds. It is a more or less granular, soft, inelastic substance, never a true fluid, but varying in consistence in accordance with the quantity of water it contains. The chemical reactions of protoplasm arc those of albumen. It Chemi&amp;lt; contracts when substances are applied to it which remove some of rcactio the water, as glycerin and alcohol. It contracts when heat is applied, a temperature of between 50 and 60 C. completely altering the texture of protoplasm containing a normal quantity of