CHAPTER II THE EPIDERMIS AND PERIDERM (Part 2)

Crete (Plate 11, Fig. 3) hair is smooth-walled, and the branches are alternate. In mullen (Plate 11, Fig. i) the hairs have whorled branches, the walls are smooth, and the cell cavity usually contains air. The lavender hairs (Plate 11, Fig. 2) have mostly opposite MuLTicELLULAK MuLTisEBiATE Non-Bkanchbd Hajks 1. Cumin (Cuminum cyminum, L.)> 3. Marigold (Calendula oJUinalis, L.). PLATE 11 Multicellular Uniseriate Branched Hairs 1. Mullen leaf (Verbascum thapsus, L.). 2. Lavender flowers (Lavandula vera, D. C.)» 3. Dittany of Crete (Origanum diciamnus, L.). THE EPIDERMIS AND PERIDERM 77 branches, and the walls are rough. Thus the multicellular branched hairs may be divided into subgroups which have alternate, opposite, whorled, or in certain hairs irregularly ar- ranged branches. Each class may be again subdivided accord- ing to color, character of cell termination, etc., as cited at the beginning of the chapter. Occasionally multicellular hairs assume the form of a shield (Plate 12, Fig. i); in such cases the hair is termed peltate, as in the non-glandular multicellular hair of shepherdia canadensis. Hairs grow out from the surface of the epidermis in a per- pendicular, a parallel, or in an oblique direction. Hairs which grow parallel or oblique to the surface are usually curved, and the outer curved part of the wall is usually thicker than the inner curved wall. The mature hairs of some plants consist of dead cells. In other plants the cells forming the hair are living. When dried, those hairs, which were dead before drying, contain air; while those hairs which were living before drying, show great variation in color and in the nature of the cell contents. The contents are either organic or inorganic. The commonest organic con- stituent is dried protoplasm. In cannabis indica are de- posits of calcium carbonate. Multicellular multiseriate branched hairs are the ultimate division of the pappus of erigeron, aromatic goldenrod, arnica, grindelia, boneset, and life-everlasting. The hairs of erigeron (Plate 13, Figs, i and 2) are slender; the walls are porous. Each hair terminates in two cells, which are greatly extended and sharp-pointed; the branches from the basal part of the hairs (Plate 13, Fig. i) are of about the same length as the apical branches. The hairs of aromatic goldenrod (Plate 13, Figs. 3 and 4) are larger than those of erigeron; the diameter is greater and the walls are non-porous. The apex of the hair terminates in a group of about four cells of imequal length, which are sharp- pointed. The branches of the basal cells (Plate 13, Fig. 3) are similar to the branches of the apical cells. The hairs of arnica (Plate 14, Figs, i and 2) have thick, strongly porous walls; the branches terminate in sharp points. The apex of the hair terminates in a single cell. The basal n-Glandular Multicellular Haiks Skepherdia eanadeniii, [L.] Nutt. Multicellular Ml-lti seriate Bkanched Haibs t. Baaal hairs of crwcron {Ertgeron canadensis, L.)- 2. Apical hairs of cngeron lErigeron (anadensis, L.). 3. Basal hairs of aromatic golHenmd {Solidaffi odora. Ait.)- 4. Apical hairs of aromatic guldenrod {Solidago odora, Ait.)> 80 HISTOLOGY OF MEDICINAL PLANTS branches (Plate 14, Fig. 2) are much longer than special branches. The hair of grindelia (Plate 14, Figs. 3 and 4) has very thick walls with numerous elongated pores. The apex of the hair terminates in a cluster of cells with short, free, sharp-pointed ends. The basal branches (Plate 14, Fig. 4) are longer than the apical branches. Boneset hair (Plate 15, Figs, i and 2) has non-porous walls. The apex of the hair terminates in two blunt-pointed cells. The terminal wall is thicker than the side wall. Some of the branches lower down terminate in cells with very thick or solid points. The basal branches (Plate 15, Fig. i) are longer, but the cells are narrower and more strongly tapering than are the branches of the apical part of the hair. Life-everlasting (Plate 15, Figs. 3 and 4) has uniformly thickened but non-porous walls. The hair terminates in two blunt-pointed, greatly elongated cells. The basal branches (Plate 15, Fig. 4) are narrower, slightly tapering, and the base of the branches frequently curve down- ward. The cell cavities of these hairs are filled with air. The walls of hairs are composed of cutin, of lignin, and of cellulose. PERIDERM The periderm is the outer protective covering of the stems and roots of mature shrubs and trees. The periderm replaces the epidermis. The periderm may be composed of cork cells, stone cell-cork, or a mixture of cork, parenchyma, Sbres, stone cells, etc. CORE PERIDERM The typical periderm is made up of cork cells. Cork cells vary in appearance, according to the part of the cell viewed. On surface view (Plate 16, Fig. A) the cork cells are angled in outline and are made up of from four to seven side walls; five- and six-sided cells are more common than the four- and seven-sided cells. Surface sections of cork cells show their Multicellular Mm.TiSEtiiATB Branched Hairs I. Apical hairs arnica (Arnica montana, L.). 3. Basal hairs arnica {Arnica montana, L.)' 3. Apical hairs grindelia (Crindelia sguarrosa, [Pursh] Dunal). 4. Basal hairs griodelia {Crinddia iquarrosa, [PurshI Dunal). MULTICBLLULAK MuLTISEUATB BRANCHED HaIRS I. Apical hairs boneaet (Eupatorium perfolialum, L.). 3. Basal hairs boneset {Eupatorium perfolialum, L.). 3. Apical hairs life-everlasting (Gnaphalium obtusifolium, L.). 4. Basal hairs life-everlasting (ffnaphaiiurn obtusifolitim, L,). THE EPIDERMIS AND PERIDERM 83 length and width. These side walls usually appear nearly white, while the end wall, particularly of the outermost cork cells, usually appears brown or reddish-brown, or in some cases nearly black. Cork cells on cross-section are rectangular in form, and they are arranged in superimposed rows, the number of rows being gradually increased as the plant grows older. Such an increase in the number of rows of cork cells is shown in the cross-section of cascara sagrada (Plate i6, Fig. C). Cork cells fit together so closely that there is no intercellular spaces between the cells. ^ In this case two rows of cork cells occupy no greater space than the solitary row of cork cells immediately over and external to them. As a rule, the outer- most layers of cork cells have a narrower radial diameter than the cork cells of the underlying layers. This is due to the fact that these outer cells are stretched as the stem increases in diameter. This view shows the height of cork cells, but not always the length, which will, of course, vary according to the part of the cell cut across. In a section a few millimeters in diameter, however, all the variations in size may be observed. The color of the walls is nearly white. The cavity may contain tannin or other substances. When tannin is present, the cavity is of a brownish or brownish-red color, or it may be nearly black. Most barks appear devoid of any colored or colorless cell contents. The radial section (Plate i6, Fig. B) of cork cells shows the height of the cells and the width of the cells at the point cut across. Some cells will be cut across their longest diameter, while others will be cut across their shortest diameter. Cork cells are, therefore, smaller in radial section than they are in cross-section. The color of the walls is white, and the color and nature of the cell contents vary for the same reasons that they vary in cross-sections. The number of layers of cork cells occurring in cross- and radial-sections varies according to the age of the plant, to the type of plant, and to the conditions under which the plant is growing. The number of layers of cork cells is not of diagnostic im- portance, nor is the surface view of cork cells diagnostic except in certain isolated cases. PERiDEUt OF Cascara Sachada i.liliamttns punhiana, O.C) A. 1, Outline of cork cells; 2, Line uf contact of adjoining cork celU. B. Radial longitudinal section of cascara. sagrada. i, Cork cells; z.Phel- logen; 3, Forming parenchyma cells: 4, Cortical parenchym; l.Corki-eils; . ing parenchyma cells; 4, Cortical parenchyma cclU, , Pliullogen; 3, Form* 3 THE EPIDERIOS AND PEIODERM 85 The presence or absence of cork or epidermal tissue in pow- ders must always be noted. The presence of cork enables one to distinguish Spanish from Russian licorice. In like manner, the presence of epidermis enables one to distinguish the pharma- copodal from the unofficial peeled calamus. The absence of epi^ennis in Jamaica ginger is one of the means by which this variety is distinguished from the ot]^er varieties of ginger, etc. : In canella alba the periderm is replaced by stone cell-cork. That iSy the cells forming the periderm are of a typical cork shi^y but the walls are lignified, unequally thickened, and the inner or thicker walls are strongly porous, and the walls are of a yeUowish color. Stone cell-cork forms the periderm of clove bark also, but the cells are narrower and longer, and the inner wall is not so thick or porous as is the case in canella alba bark. STONE CELL PERIDERM In canella alba (Plate 17, Fig. B) cork periderm is frequently replaced by stone cells, particularly in the older barks. These stone cells form the periderm because they replace the cork periderm, which fissures and scales off as the root increases in diameter. The side and end walls of cork cells are of nearly uniform diameter. Exceptions occur, but they are not common. In buchu stem (Plate loi, Fig. 3), the cork cells have thick outer walls, but thin sides and inner walls. The cell cavity contains reddish-brown deposits of tannin. PARENCHYMA AND STONE CELL PERIDERM As the trees and shrubs increase in diameter, cracks or fis- sures occur in the periderm, or corky layer. In such cases the phellogen cells divide and redivide in such manner as to cut off a portion of the parenchyma cells, stone cells, and fibres of the cortex which is inside of and below the fissure. All the parenchyma cells, etc., exterior to the newly formed cork cells soon lose their living-cell contents, since their food-supply is cut off by the impervious walls of the cork cells. In time they are forced outward by the developing cork cells until they 1 \ Z PLATE 17 r"^^^^^^^^^^^H hr '^^ k 3 W^ # % 5( KS- J^ k A IPilP WBi 1^ ^^^^^w jg^^^^ SjQiJl-j ^^^ €i^S^^^^^^^9h ^^H '^Q H^^sP ^h|^ ^=1 ^r^^i~^^M^ ^^^ j^^ ^^^^^^ \ ^^^ 1^^ ^^^^^^3 plielg i^^ H^^^ — 2^§ _■ i^-^ B ■ ^^^^^^^ A. CrOBs-eection of Mandrake Rhi ^nic {Podophyllum pfllalum. L.). ^| W I. Epidc rmis. ^1 I 2. Phclbgcn. ^H 1 3- Corti a! parcnchy la. ^1 1 B, Stone cell periderm o white cm Coi^/Ia i/iQ. Murr.). ^1 88 fflSTOLOGY OF MEDICINAL PLANTS partially or completely fill the break in the periderm. In white oak bark (Plate 1 8), as in other barks, a large part of the peri- derm is composed of dead and discolored cortical cells. ORIGIN OF CORK CELLS The cork cells are formed by the meristimatic phellogen cells, which originate from cortical parench3mia. These cells divide into two cells, the outer changing into a cork cell, while the inner cell remains meristimatic. In other instances the outer cell remains meristimatic, while the inner cell changes into a cortical parenchyma ceU. The development of a cortical parenchyma cell from a divided phellogen cell is shown in Plate loi, Fig. 6. Both the primary and secondary cork cells originate from the phellogen or cork cambrium layer. Cork cells do not contain living-cell contents; in fact, in the majority of medicinal barks the cork cells contain only air. The walls of typical cork cells are composed, at least in part, of suberin, a substance which is impervious to water and gases. In certain cases layers of cellulose, lignin, and suberin have been identified. Suberin, however, is present in all cork cells, and in some cases all of the walls of cork cells are composed of suberin. Suberized cork cells are colored yellow with strong sodium hydroxide solutions and by chlorzinciodide.