Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/26

16 HISTOLOGY [VEGETABLE. many others. Such cells arc rare in monocotyledons, and the typical form (like the cells in the pear) does not occur in cryptogams. A variety of the short cells is described under the name of stegmata. The long sclerenchymatous fibres are pointed, with overlapping ends, and occur commonly in dicotyledons. They are either simple or branched. The best examples arc the bast fibres of the fibro-vascu- lar bundles, and the libriform fibres of the secondary wood. The wall of the sclerenchym fibre often exhibits peculiar split-like pitted markings (Pteris aquilina). Not unfrequently the sclerenchym fibres have numerous small crystals of calcic oxalato imbedded in the wall, a very beautiful example of which is ailbrded by Wdwitschia mirabilis. Sometimes the cavity in the interior of the fibre is divided by transverse partitions forming chambered fibres, as seen in the bast of the vine, Platanus, Tamarix, &c. 3. Gland Cells. Gum, resin, ethereal oils, balsam, and gum-resins are usually met with in peculiar elongated cells, which develop from special meristem-cells. Sometimes they are isolated, in other cases they occur in rows; they are considered by De Bary as a special kind of tissue, the cells being at once distinguished from the others in the neighbourhood by their contents. In many ways these gland-cells exhibit intermediate transition forms to laticiferous tubes on the one hand, and to intercellular spaces containing special secretions on the other. De Bary distinguishes four varieties, (a. ) Cells with the cavity nearly filled with raphides or with single crystals or groups of crystals, e.g., Aloe, Scilla, and many monocotyledons, as also many dicotyledons. Groups of crystals occur in petiole of aroids, pith of Ricinus, and others, large crystals in leaves of Citrus, and in the bast of Acer, Robinia, Ulmus, Bcrberis, &c. (b.) Cells with the cavity filled with mucilaginous and gummy substances, root of Symphy- tum, Orchis tubers, and in the parenchyma of Malvaceae, Tiliacccc, Ulmaccoe, &c. (c. ) Cells containing resin or gum-resin. Two modifi cations occur, the cells being either short or long, and frequently, as Zacharias (Bot. Zcit., 1879, p. 167) has pointed out, the walls are corky. Short cells occur in Acorus, Canclla, Zingibcraccce, &c., while long ones occur with milk-like juice, the laticiferous cells of some authors. As examples De Bary gives Allium, aroids, Musacece, Convolvulacccc, Sapotaccce, Sambucus, Acer, and doubtfully Sanguinaria, Glaucium, &c. (d. ) Cells containing tannin, occurring in rhizocarps, ferns, monocotyledons, and dicotyledons. 4. Trachcal Tissue. Under this head De Bary distinguishes all those cells which become more or less lignified, and in which the thickening of the Avail assumes the form of spirals, rings, reticula tions, or pits, and which as soon as these markings are formed either lose their contents completely and become filled with air, or con tain clear watery iluid. Usiially these form long cell-fusions, the vessels of plants, or else they form elongated or shorter cells not united into a vessel. The former are the vessels, the latter the tracheides. The markings in the two forms correspond, and there are intermediate varieties. The markings are spiral, annular, reticu lated, pitted, and trabeculate (juniper and lycopod), with the varie ties of bordered pits and scalariform markings. Short tracheides form the velamen or outer modification of the epidermis of the aerial orchid roots, also the outer tissue of the stein of Sphagnum. In Nclumbrium speciosum the tracheides are 1 2 centimetres long. Many of the structures usually called vessels are tracheides. Large vessels frequently exhibit tyloses or cells filling up the cavity of the vessel. They have been observed in many monocotyledons and dicotyledons, both in stems and roots, and in herbaceous as well as in woody plants. 5. Sieve-tubes. These resemble vessels in being elongated cylin drical or prismatic cells joined in long rows, the individual cells always remaining distinctly marked. The transverse wall separating the two cavities becomes perforated at the unthickened parts, forming the sieve-plate perforated by the sieve-pores. The contents of the sieve- tubes are colourless and transparent, and the Avail is coated Avith a thin layer of protoplasm-like substance, not unfrequently Avith small starch granules. Sieve-tubes form a special part of the bast of plants, and are met with in pteridophytes, gymnosperms, and angiosporms, exhibiting occasionally in different groups slight structural differ ences. 6. Laticiferous Tubes are tubes containing the peculiar milky sap or latex occurring in special groups of plants. These run through the plant usually for very long distances, and when a portion is injured the milk-sap floAvs out at the opening. The Avails are ahvays soft, of pure cellulose, and readily giving the characteristic reaction Avith iodine and sulphuric acid. The tubes contain no protoplasm and nucleus ; but a quantity of a rarely Avatery, usually milky juice, occasionally, hoAvever, orange or yellow, and sometimes containing peculiar starch granules. The tubes are either simple or segmented. Segmented tubes occur in Cichoracece, Campanulacece, Lobeliaccce, Papayaccaz, many Papavcraccce, as Papaver, Argcmone, and Cheli- donium, but not in Glaucium or Sanguinfiria, many aroids, and Musaccoe. Simple tubes are met with in Euphorbiacece, Urticacece, Apocynaccce, and Asclepiadaceav. These latter do not exhibit the net-like anastomoses of the segmented forms, and usually have the branches terminating in blind extremities. 7. Intercellular spaces are the cavities betAveen the elements of full-groAvn tissues, the cells in the meristem stage being in unin terrupted continuity. Some of the intercellular spaces are produced Cavitiei by the splitting of the cell-wall betAveen three or more cells, others in are formed by the destruction of the Avails of a cell or group of cells tissues, during the formation, by desorganization, of some secretion. Lastly, large cavities appear in plants as the result of mechanical rupturing and tearing of the inner tissues during rapid groAvth of the part. De Bary distinguishes all these by separate names, viz., sehizo- genous Avhen formed by splitting of the common Avail betAveen cells, lysigenous AA hen formed by the destruction of certain cells and cell- groups, and rhexigenous when produced by mechanical disruption. From the nature of the contents, the intercellular spaces can bo divided into two groups, the one containing substances or mixtures similar to those contained in gland-cells, the other containing air, or rarely Avater. Of the intercellular glands, spaces, or canals the following varieties may be distinguished:- (a.) mucilage or gum canals, of which examples may be seen in MarattiaceoK, LycopocliacccR, Cycadacccc, Canna, Opuntia, andsome^ratocco? ; (b.) resin, ethereal oil, or gum-resin canals, either in long canals, as in Conifercc, Alismacece, aroids, Composite (Tubuliflora:), Umbclliferce, Araliacccc, &c., or short spaces as in Itutacsce, Hypcricum, Oxalis, Myrtaccce, Lysimachia, &c. Of the intercellular air or water spaces there are several modifications. First there are the minute spaces between the walls of parenchymatous cells, the interstitial air spaces ; and Avhen the spaces are larger and accompanied Avith irregular groAvth of the wall, lacunae are produced, as in the root of Sagittaria sagittifolia, or in the pith of Juncus or petioles of Musa, &c. Large schizogenous air-spaces with smooth Avails are met with in Isoctcs, Potamogeton, Hippuris, Trapa, Nymphccaccce, and many others. Lysigeuous spaces having the remains of the destroyed cells more or less marked on the walls are seen in Equisctum, Cyperaccce, Gramincce, Typha, Iris ; Avhile the large hollow stems of UmbelliferoK, Compositce, grasses, and the leaves of Allium, &c., are rhexigenous. Occasionally flat cell-surfaces or diaphragms interrupt the continuity of long air-spaces, and not unfrequently internal hairs or peculiar hair-like idioblasts are formed, projecting into the intercellular spaces as in Nupliar and Monstcra. It is only in the neighbourhood of Avater stomata that the spaces contain Avater for a short time. Systems of Tissues. Sachs describes three systems of tissues, complex aggregations consisting of different kinds of tissue, but all so combined as to form readily recogniz able parts of the root, stem, or leaf of a plant. Externally there is the epidermal or limitary system equivalent to De Bary s first division, excluding his parenchyma. This system is taken to include the epidermis of plants, with its cuticle, stomata, and hairs, and also to include the secondary modi fications produced by the development of cork and bark. In the interior of most parts of the higher plants, and follow ing in the direction of the long axis of growth, separate or united strings or bundles are seen running and usually branching or anastomosing. Generally these bundles are harder than the surrounding tissues and readily separable from them. Consisting as they do of many kinds of tissue of vessels, cells, and sclerenchyma, these structures are known as fibre-vascular bundles. Lastly, there exists a quantity of parenchyma or a mixture of parenchyma with other forms, packing up all the space between the fibro- vascular bundles on the one hand and the epidermal system on the other. This forms the ground tissue, and includes the parenchyma proper of De Bary. 1. The Epidermal or Limitary System. The epidermal system takes its name from the chief member of Epi- the group, namely, the epidermis or outer skin of the plant. It is dermal the superficial layer, and is variously developed in the higher and tissues. loA r er plants. In the loAA r er forms, algse, fungi, lichens, the external cells are usually smaller than those below, or the walls are thicker and coloured ; while in many mosses and liverworts a true separable epidermis is only slightly indicated. In others, as in Marchantia, capsules of most mosses, and in Sphagnum, a specially differentiated epidermis appears, resembling that in the higher plants. The nature of the epidermis varies in accordance Avith the conditions to which it is exposed, as to air and light, or in water, or in the soil, and in darkness. The nature of the limitary tissue also varies with the stage of grovth in such parts as are of perennial duration. Usually the epidermis is a single layer of cells producing stomata and hairs. In many plants the epidermis is strengthened by the formation of a corky outer layer, the cuticle, vhich develops Avax ; or in other cases a JICAV formation takes place below the epidermis, usually in the ground tissue, and by the formation of layers of cork a secondary epidermal or limitary tissue is produced. Other parts of the ground tissue assist in forming the outer covering of plants, and