14 HISTOLOGY [VEGETABLE. tissue is formed, the cells divide in the ordinary way, and at length give rise to a true tissue, as different modes of cytogenesis may occur in the same plant, either in different parts or at different times. The second mode of formation of a false tissue is seen in the fungi and lichens, in the peculiar hyph* tissue so characteristic of these plants. The cells form long narrow rows or filaments, which branch and interlace, producing a network of interlacing fibres, but without the walls becoming fused firmly together as in Pediastrum. In some cases the hyphse cells swell up and come into such close contact by mutual pressure that they form a tissue so like ordinary cell-tissue that it has been denominated pseudo-parenchyma. This variety of tissue occurs commonly in the higher fungi, as in the mushroom. Mode of Cells are united in various ways, the modes of union being often union of very characteristic of certain of the lower groups of plants, although cells. the same modes of union repeat themselves in the higher plants. The following are the chief varieties. (1) Cell-rows have the cells united by their ends to form a long filament, formed by the repeated division of the cells. Examples of cell-rows are seen in Spirogyra, Conferva, CEdogonium, the hyphse of fungi, the mouili- form hairs in Tradcscaniia, and in many others ; not unfrequently these cells branch in various ways. Cell-fusions or vessels are cell- rows occurring in the higher plants, but having the transverse walls separating the original cells either partially or completely absorbed. They occur in the fibro-vascular bundles of plants, both in the wood and in the bast. Laticiferous vessels are examples of branching and anastomosing cell-rows. (2) Cell-surfaces have the cells united to form a single layer, and are thus in contact by the ends and sides, having an upper and under (rarely only one) free surface. Examples are afforded by some of the sea-weeds, as Ulva, and by the leaves of liverworts. In the higher plants cell-surfaces occur not uufrequently, as in the epidermis, a layer of distinct cells, free on one surface, but in contact with other cells below. Many flat, scale-like hairs are also cell-surfaces, as well as the thin plates of cells separating tlie remarkable air spaces in the petioles of Nuphar and Musa. (3) Cell- bundles are bands or bundles of similar cells either occurring separately or running through the other tissues of plants, and when doing so easily recognized in a transverse section of the part, as the bast- bundles in the stem of flax. Other examples occur among the red sea-weeds, and in the bundles of sclerenchyma in the stems of ferns. (4) Cell-groups are small masses of similar cells, either forming the families or colonies (ccenobia) of many thallophytes, as Chroocoecus, Glceocajisa, Pandorina, &c., or forming the curious groups of sclereuchyma producing the gritty particles in the pulp of the pear or the hard masses in cork. (5) Cell-masses are formed when the cells are united in all directions of space, the whole not having necessarily any definite external shape. Examples are numerous, but wo may cite the tissues of large fungi, the ground tissue of the higher plants, and the pulp and hard endocarps of fleshy fruits. (6) Lastly, separate cells occur, either distinguished from the cells in the neighbourhood by their peculiar form and development (idioblasts), or the originally united cells separate themselves, as in pollen-grains and spores, and form eremoblasts. Prosen- By the form and connexions of the cells aggregations of cells chymat- ma y b e described as parenchymatous tissue and prosenchy- parenchy- matous tissue, both these forms occurring very commonly niatous in plants, and usually shortly designated by botanists rells. parenchyma and prosenchyma. Parenchymatous cells are usually thin-walled, and have a correspondingly large cavity; their length is generally not very much greater than their breadth, the form frequently being rounded or polyhedral ; the _ walls are broad and flat, the cells, if elongated, not having pointed and overlapping ends, At the places where neighbouring cell-walls meet triangular or quadrangular intercellular spaces are formed, by splitting of the wall during rapid growth. Sometimes these spaces are very minute, in other cases they are largely developed, and if irregular growth of the wall occurs a very loose form of parenchyma may be produced, as in the pith of Juncus. In other cases tolerably large intercellular spaces occur, as in the spongy parenchyma of the mesophyll of leaves. In prosenchyma the individual cells are greatly elongated and fibre-like, the walls are very thick, and the cavity small or even nearly obliterated ; the ends of the cells are elongated, pointed, and overlapping those above and below; and lastly, no intercellular spaces are developed. Wood-fibres and
- >ast-fibres are examples of prosenchyma ; the young cells
of stem or root, and the tissues of pith, leaves, flowers, and many fruits, of parenchyma. By the power possessed by the cells in a tissue of divid- Meris , ing and forming new cells such a tissue is distinguished ancl ! as meristsm or the formative tissue of plants, all the other ane tissues being permanent tissues, or incapable of further nlargement by the formation of new cells. Meristem is observed forming the whole of the tissue of the young embryo plant, as also the whole tissue at the apex of a stem and root. All the other tissues of the plant are formed by the gradual differentiation of the originally similar cells of the rneristem. Generally meristem tissue differentiates into special layers, each capable of forming cells which will ultimately form some definite portion of permanent tissue, not necessarily of the same value, however, in different groups of plants. The meristem of the embryo and of young stems and roots is distinguished as primary meristem, because occasionally a zone of cells forms in the permanent tissue having the characters of meristem, and secondary meristem, which either originates from the permanent tissue or is partly connected with the primary meristem. The cork-cambium or phellogen in the cortical tissues of dicoty ledons is a layer of secondary meristem, while the cambium layer between the wood and bast portion of the bundle is partly (the fascicular cambium) derived from the primary meristem (the procambium) of the fibro-vascular bundle. Walls of Tissue-Cells. The cell-wall separating the con- Walls tiguous cavities of two young cells appears as a simple tissue homogeneous plate or lamella of pure cellulose, giving the cells - usual reaction with Schultz s solution and iodine and sulphuric acid. As the tissue grows older and the wall thickens, it apparently separates into distinct layers having different chemical and physical properties, so that in some cases it appears as if each cavity had its own special wall separated from the neighbouring wall by a thin or thick layer of material, to which the older botanists gave the name of intercellular substance. The thickening layers usually exhibit a well-marked stratification, the strata often differ ing in chemical composition, as in pine-wood, in the bast of laburnum, or in the epidermis of Viscum, Ephedra, Nerium Oleander, &c. The application of Schultz s solution usually brings out the differences very well. In a few instances the middle lamella becomes gelatinous, and swells up enormously in water. Examples are afforded by the stems of many algae, and by the endosperm of Ceratonia, where the so- called intercellular substance separates the cell-cavities widely one from the other. The middle lamella or inter cellular substance and the thickening layers in the stratified cell-wall vary much in composition, but generally it is found that the incrusting layers are soluble in sulphuric acid, while the middle lamella is dissolved by nitric acid and chlorate of potash. These two substances, just mentioned under the name of Schultz s maceration process, are con stantly employed to separate cells from their connexions, as the markings in the thickening layers are not injured by the solution of the middle lamella in the chlorate of potash and nitric acid. Classification of Tissues. In classifying vegetable tissues it is necessary first to distinguish the different kinds of tissue depending on the characters of the individual elements composing it, and, secondly, to consider the various group ing of these kinds or species into systems more or less homogeneous and obeying certain common laws of growth. It is necessary to distinguish the kinds of tissue, because different kinds may occur in the same system, and it is further necessary to distinguish the systems, because tho same form of cell may be repeated in different systems or in different parts of the same system and yet be of very
different morphological and physiological value:. In classi-
