Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/25

15 VEGETABLE.] HISTOLOGY 15 fying the different kinds of tissue we shall follow De Bary ( Vergleichende Anatomic der Vegetationsorgane der Phanero- gamen und Fame), and in the systems we shall adopt the threefold divisions of Sachs (Lehrbuch der Botanik), now generally used. I. KINDS OF TISSUE. (A.) Meristem Tissue. 1. Primary. 2. Secondary. Permanent Tissue. (B.) 1. b. Cork. 2. 3. 4. 5. 6. ,-lmary eristem erome. Cell Tissue, a. Epidermis. c. Parenchyma proper. Sclerenchyma. Gland Cells, Tracheal Tissue. Sieve Tubes. Laticiferous Tubes. 7. Intercellular Spaces. II. SYSTEMS OF TISSUES. (A.) Epidermal or Limitary System. (B.) Fibro-vascidar System. (C.) Ground System. (A.) Meristem Tissue. Primary meristem can be observed in the embryo in its young stages, and at the apex of the stem and root. In the embryo at an early stage, as described by Hanstein (Botanische Abhandlungen, i.), the meristem be comes separable into three zones, differing in the appearance and arrangement of the cells and in the mode of dividing. These zones were called by Hanstein (1) dermatogen, or primary epidermis ; (2) the plerome or central series of more elongated cells with marked longitudinal division of the cells; and (3) a series between the plerome and derma togen, dividing more or less irregularly or transversely, the periblem. These three zones remain distinctly marked at the apex of the stem, and in Hippuris the three can be easily seen, while as the stem elongates new cells continually form, the initial cells or cell, as there may be one or more for each zone. Usually the dermatogen layer is the most constant in angiospermous plants, the separation into periblem and plerome being sometimes a little obscure. In the root a fourth zone of meristem has to be distinguished, called by Janczewski the calyptrogen layer, from which the calyptra, pileorhiza, or root-cap is formed. Various modi fications of the arrangement of the different layers in the meristem of roots have been described. Very rarely, as in Hydrocharis and in Pistia, four distinct layers are formed the plerome, periblem, dermatogen, and calyptrogen. In Cuciirbita, Pisum, and a few others there is a common mass of meristem at the apex, from which the others are all differentiated. In Zea Mays and most monocotyledons two distinct zones are seen, the plerome and the calyptrogen, while between them a short distance from the apex the initial layer forms, which separates into the periblem arid dermatogen. In Fayopyrum and most dicotyledons the plerome and periblem are sharply separated, but the peri blem above the apex of the plerome passes into a common layer with the initial cells of the dermatogen and calyptra, the dermocalyptrogen. In gymnosperms the root possesses a sharply-defined plerome with a periblem mantle, in Thuja formed by from 12 to 14 regular concentric layers; there is no trace either of a calyptrogen or dermatogen layer, the outer cells of the periblem serving as a calyptra. In the stems of gymnosperms the condition of the layers is some what intermediate between those formed in the angiosperms and lycopods. In Araucaria and Dammara the dermatogen, periblem, and plerome are separate and distinct, but in Abietinew and in Cycas they run into a common initial group, and it is only at some distance from the apex that in the Abietinece the separation becomes very marked, and in Cycas only slightly marked. In lycopods the end of the stem shows a series of cells, the initial group from which the periblem and dermatogen (or the external layer representing it) arise. Further down the initial cells of the plerome are developed from the side of the periblem. In the root of lycopods the arrangement of the layers is exactly the same as in Hydrocharis and Pistia. In the Ligulatai and the remaining Pteridophyta there is a single cell at the apex of root and stem which divides into two. The one daughter-cell forms the new apical cell, the other is the segment cell. The segment cell divides still further, and forms a meristem from which at a later stage zones corresponding more or less accurately to dermatogen, periblem, and plerome are produced. In the roots a segment is cut off in front of the apical cell, which is the first cell of the calyptra, and from which, by repeated divisions, that structure arises. Secondary meristem is intimately connected with the Second secondary circumferential growth of stems and roots in ar y gymnosperms and dicotyledons. One of the zones of menste secondary meristem arising from permanent cells is the cork-cambium or phellogen layer, which is described under the epidermal system of tissues. The other example of secondary meristem is the cambium layer separating the wood and bast in the stems and roots of gymnosperms and dicotyledons. When the fibro-vascular bundles first appear, either in Procarr the periblem or plerome, the cells become distinguishable by bium. their form and arrangement, and as the cells are still in the condition of meristem, the term procambium has been given to the whole. The cells of the procambium are gradually converted into permanent tissue, generally chang ing their appearance completely, although in some cases the change is but slight, the cells being cambiform and hardly differentiated into the two parts of the fibro-vascular bundle, the wood and bast, to be described under the fibro-vascular tissues. In some plants all the procambium is converted into permanent tissue, while in others a small zone between the wood and bast remains in the condition of meristem. If the bundles are separate, secondary meristem forms in the ground tissue between the bundles, bridging over the space between the bundles, but uniting so as to form the Cambii cambium-ring, which consists of fascicular cambium in rni g- the bundle, derived from the procambium, and interfasci- cular cambium, a secondary meristem formed in the ground tissue. It is by the growth of this cambium ring that the secondary circumferential growth, so marked in our ordinary forest trees, takes place. (B.) Permanent Tissue. It will be sufficient to give only Per- a general sketch of the seven kinds of tissue described by mailcir De Bary, and to refer for full details to his Vergleichende tissue - Anatomie above mentioned. 1. Cell-tissue is permanent tissue, the cells of which are little if Cell- at all altered iu form and appearance from their meristem stage. In tissue, some cases the cells are short, in others elongated. The wall may bo thin, and enclose the protoplasm and other contents, the chlorophyll, starch, sugar, iriulin, &c. In others the wall is thick and changed in composition. As varieties of cell-tissue De Bary includes (1) epidermis and its appendages, equivalent to the epidermal system of Sachs, and to be considered below; (2) cork, parenchymatous cella chemically altered, and forming usually a part of the secondary epidermal system ; and (3) parenchyma proper, all the cell-tissue inside the epidermis and cork cells, a division almost but not quite equivalent to the ground tissue of Sachs. 2. Sclerenchyma. De Bary includes under the name of scleren- Scleren chyma all the Lard thickened cells of plants, whether long or short, chynia. which have become greatly thickened, and whoso cavity is nearly if not quite obliterated, the cell-contents also, as a consequence, having entirely disappeared, or left only slight traces. In this state these cells act in conveying water through their walls, und also serve to givo rigidity to the plant, forming the mechanical system of Schwendener. Two forms are distinguished: (1) the short scleren- chymatous cells, and (2) long sclerenchymatous fibres. Of the former, examples are met with in the flesh of the pear, in the root-tubers of Dahlia, in the rhizome of Dentaria, the pith of Hoya carnosa, and
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