CHAPTER II

THE EPIDERMIS AND PERIDERM

The epidermis and its modifications, the hypodermis and the periderm, form the dermal or protective outer layer or layers of the plant.

The epidermis of most leaves, stems of herbs, seeds, fruits, floral organs, and young woody stems consists of a single layer of cells which form an impervious outer covering, with the exception of the stoma.

LEAF EPIDERMIS

The cells of the =epidermis= vary in size, in thickness of the side and end walls, in form, in arrangement, in character of outgrowths, in the nature of the surface deposits, in the character of wall--whether smooth or rough--and in size.

In cross-sections of the leaf the character of both the side and end walls is easily studied.

In surface sections--the view most frequently seen in powders--the side walls are more conspicuous than the end wall (Plates 2 and 3). This is so because the light is considerably retarded in passing through the entire length of the side walls, while the light is retarded only slightly in passing through the end wall. The light in this case passes through the width (thickness) of the wall only. The outer walls of epidermal cells are characteristic only when they are striated, rough, pitted, colored, etc. In the majority of leaves the outer wall of the epidermal cells is not diagnostic in powders, or in surface sections.

The thickness of the end and side walls of epidermal cells differs greatly in different plants.

As a rule, leaves of aquatic and shade-loving plants, as well as the leaves of most herbs have thinner walled epidermal cells than have the leaves of plants growing in soil under normal conditions, or than have the leaves of shrubs and trees.

[Illustration: PLATE 2

LEAF EPIDERMIS

1. Uva-ursi (_Arctostaphylos uva-ursi_, [L.] Spring). 2. Boldus (_Peumus boldus_, Molina). 3. Catnip (_Nepeta cataria_, L.). 4. Digitalis (_Digitalis purpurea_, L.). 4-A. Origin of hair.]

[Illustration: PLATE 3

LEAF EPIDERMIS

1. Upper striated epidermis of chirata leaf (_Swertia chirata_, [Roxb.] Ham.). 2. Green hellebore leaf (_Veratrum viride_, Ait.). 3. Boldus leaf (_Peumus boldus_, Molina). 4. Under epidermis of India senna (_Cassia angustifolia_, Vahl.).]

The widest possible range of cell-wall thickness is therefore found in the medicinal leaves, because the medicinal leaves are collected from aquatic plants, herbs, shrubs, trees, etc.

The outer wall is always thicker than the side walls. Even the side walls vary in thickness in some leaves, the wall next to the epidermis being thicker than the lower or innermost portion of the wall. Frequently the outermost part of the side walls is unequally thickened. This is the case in the beaded side walls characteristic of the epidermis of the leaves of laurus, myrcia, boldus, and capsicum seed, etc. The thickness of the side walls of the epidermal cells of most leaves varies in the different leaves.

In most leaves there are five typical forms of arrangement of epidermal cells: First, those over the veins which are elongated in the direction of the length of the leaf; and, secondly, those on other parts of the leaf which are usually several-sided and not elongated in any one direction. If the epidermis of the leaf has stoma, then there is a third type of arrangement of the epidermal cells around the stoma; fourthly, the cells surrounding the base of hairs; and fifthly, outgrowths of the epidermis, non-glandular and glandular hairs, etc.

It should be borne in mind that in each species of plant the five types of arrangement are characteristic for the species.

The character of the outer wall of the epidermal cells differs greatly in different plants. In most cases the wall is smooth; senna is an example of such leaves. In certain other leaves the wall is rough, the roughness being in the form of striations. In some cases the striations occur in a regular manner; belladonna leaf is typical of such leaves. In other instances the wall is striated in an irregular manner as shown in chirata epidermis. Very often an epidermis is rough, but the roughness is not due to striations. In these cases the epidermis is unevenly thickened, the thin places appearing as slight depressions, the thick places as slight elevations. Boldus has a rough, but not a striated surface.

=Surface deposits= are not of common occurrence in medicinal plants; waxy deposits occur on the stem of sumac, on a species of raspberry, on the fruit of bayberry, etc. Resinous deposits occur on the leaves and stems of grindelia species, and on yerba santa.

In certain leaves there are two or three layers of cells beneath the epidermis that are similar in structure to the epidermal cells. These are called hypodermal cells, and they function in the same way as the epidermal cells.

Hypodermal cells are very likely to occur on the margin of the leaf. Uva-ursi leaf has a structure typical of leaves with hypodermal marginal cells. Uva-ursi, like other leaves with hypodermal cells has a greater number of hypodermal cells at the leaf margin than at any other part of the leaf surface.

The cutinized walls of epidermal cells are stained red with saffranin.

TESTA EPIDERMIS

=Testa epidermal cells= form the epidermal layers of such seeds as lobelia, henbane, capsicum, paprika, larkspur, belladonna, scopola, etc.

In surface view the end walls are thick and wavy in outline; frequently the line of union--middle lamella--of two cells is indicated by a dark or light line, while in others the wall between two cells appears as a single wall. The walls are porous or non-porous, and the color of the wall varies from yellow to brown, to colorless. These cells always occur in masses, composed partially of entire and partially of broken fragments.

In lobelia seed (Plate 4, Fig. 2) the line of union of adjacent cell walls appears as a dark line. The walls are wavy in outline, of a yellowish-red color and not porous.

In henbane seed (Plate 4, Fig. 3) the line of union between the cells is scarcely visible; the walls are decidedly wavy, more so than in lobelia, and no pits are visible.

In capsicum seed (Plate 4, Fig. 1) the cells are very wavy and decidedly porous, the line of union between the cell walls being marked with irregular spaces and lines.

In belladonna seed (Plate 5, Fig. 1) the walls between two adjacent cells are non-striated and non-porous, and extremely irregular in outline.

[Illustration: PLATE 4

TESTA EPIDERMAL CELLS

1. Capsicum seed (_Capsicum frutescens_, L.). 2. Lobelia seed (_Lobelia inflata_, L.). 3. Henbane seed (_Hyoscyamus niger_, L.).]

[Illustration: PLATE 5

TESTA CELLS

1. Belladonna seed (_Atropa belladonna_, L.). 2. Star-aniseed (_Illicium verum_, Hooker). 3. Stramonium seed (_Datura stramonium_, L.).]

In star-anise seed (Plate 5, Fig. 2) the walls are irregularly thickened and wavy in outline.

In stramonium seed (Plate 5, Fig. 3) the walls are very thick, wavy in outline, and striated.

PLANT HAIRS (TRICHOMES)

In histological work plant hairs are of great importance, as they offer a ready means of distinguishing and differentiating between plants, or parts of plants, when they occur in a broken or finely powdered condition. There is no other element in powdered drugs which is of so great a diagnostic value as the plant hair. The same plant will always have the same type of hair, the only noticeable variation being in the size. In microscopical drug analysis the presence of hairs is always noted, and in many cases the purity of the powder can be ascertained from the hairs. Botanists seem to have given little attention to the study of plant hairs. This accounts for the fact that information concerning them is very meagre in botanical literature, and, as far as the author can learn, no one has attempted to classify them. In systematic work, plant hairs could be used to great advantage in separating genera and even species. Hairs are, of course, a factor now in systematic work. The lack of hairs is indicated by the term glabrous. Their presence is indicated by such terms as hispid, villous, etc. In certain cases the term indicates position of the hair as ciliate when the hair is marginal. When hairs influence the color of the leaf, such terms as cinerous and canescent are used. In all the cases cited no mention is made of the real nature of the hair.

In systematic work, as in pharmacognosy, we must work with dried material, and it is only those hairs which retain their form under such conditions which are of classification value.

Hairs are the most common outgrowths of the epidermal cells. They are classified as glandular or non-glandular, according to their structure and function. The glandular hairs will be considered under synthetic tissue.

Each group is again subdivided into a number of secondary groups, depending upon the number of cells present, their form, their arrangement, their size, their color, the character of their walls, whether rough or smooth, whether branched or non-branched, whether curved, twisted, straight, or twisted and straight, whether pointed, blunt, or forked.

FORMS OF HAIRS

PAPILLÆ

=Papillæ= are epidermal cells which are extended outward in the form of small tubular outgrowths.

Papillæ occur on the following parts of the plant: flower-petals, stigmas, styles, leaves, stems, seeds, and fruits. Papillæ occur on only a few of the medicinal leaves.

The under surface of both Truxillo (Plate 6, Fig. 3) and Huanuca coca have very small papillæ. The outermost wall of these papillæ are much thicker than the side walls. The papillæ of klip buchu (Plate 6, Fig. 4), an adulterant of true buchu, has large thick-walled papillæ.

The velvety appearance of most flower-petals (Plate 6, Figs. 2 and 5) is due to the presence of papillæ. The papillæ of flower-petals are very variable. In calendula flowers (Plate 6, Fig. 1) they are small, yellowish in color, and the outer wall is marked with parallel striations which appear as small teeth in cross-section. The ray petal papillæ of anthemis consist of rather large, broad, blunt papillæ with slightly striated walls. The papillæ of the ray petals of the white daisy consist of papillæ which have medium sized, cone-shaped papillæ with finely striated walls. The papillæ of the flower stigma vary greatly in different flowers. In some cases two or more types of papillæ occur, but even in these cases the papillæ are characteristic of the species.

The papillæ differ greatly in the case of the flowers of the compositæ, where two types of flowers are normally present--namely, the ray flowers and the disk flowers.

In all cases observed the papillæ of the stigma of the ray flowers are always smaller than the papillæ of the stigma of the disk flowers. It would appear from extended observation that the papillæ of the ray flower stigma are being gradually aborted. The papillæ of the style are always different from the papillæ of the stigma. The style papillæ are always smaller, and they are of a different form.

[Illustration: PLATE 6

PAPILLÆ

1. Calendula flowers (_Calendula officinalis_, L.). 2. White daisy ray flower (_Chrysanthemum leucanthemum_, L.). 3. Coca leaf (_Erythroxylon coca_, Lamarck). 4. Klip buchu. 5. Anthemis ray petal (_Anthemis nobilis_, L.).]

UNICELLULAR NON-GLANDULAR HAIRS

=True plant hairs= are tubular outgrowths of the epidermal cell, the length of these outgrowths being several times the width of the hair.

The unicellular hairs are common to many plants. The two groups of non-glandular unicellular hairs are, first, the solitary; and secondly, the clustered hairs.

=Solitary unicellular hairs= occur on the leaves of chestnut, yerba santa, lobelia, cannabis indica, the fruit of anise, and the stem of allspice, senna, and cowage.

Chestnut hairs (Plate 7, Fig. 1) have smooth yellowish-colored walls, and the cell cavity contains reddish-brown tannin. These hairs occur solitary or clustered; the clustered hairs normally occur on the leaf, but in powdering the drug, individual hairs of the cluster become separated or solitary.

Yerba santa hairs (Plate 7, Fig. 4) are twisted, the lumen or cell cavity is very small, and the walls, which are very thick, are grayish-white.

Lobelia hairs (Plate 7, Fig. 5) are very large. The walls are grayish-white, and the outer surface extends in the form of small elevations which make the hair very rough. The hair tapers gradually to a solid point.

Cannabis indica hairs (Plate 7, Fig. 6) are curved. The apex tapers to a point and the base is broad, and it frequently contains deposits of calcium carbonate. The walls are grayish-white in appearance, and rough. The roughness increases toward the apex.

The hairs of anise (Plate 7, Fig. 7) are mostly curved; the walls are thick, yellowish-white, and the outer surface is rough; this is due to the numerous slight centrifugal projections of the outer wall.

Allspice stem hairs (Plate 7, Fig. 2) have smooth walls. The cell cavity is reddish-brown. The hair is curved.

The hair of senna (Plate 7, Fig. 10) is light greenish-yellow with rough papillose walls. The hair is usually curved and tapering, and it does not have any characteristic cell contents.

[Illustration: PLATE 7

UNICELLULAR SOLITARY HAIRS

1. Chestnut leaf (_Castanea dentata_, [Marsh] Borkh). 2. Allspice stems (_Pimento, officinalis_, Lindl.). 3. Cowage. 4. Yerba santa (_Eriodictyon californicum_, [H. and A.] Greene). 5. Lobelia (_Lobelia inflata_, L.). 6. Cannabis indica (_Cannabis saliva_, L.). 7. Anise fruit (_Pimpinella anisum_, L.). 8. Hesperis matronalis (_Hesperis matronalis_, L.). 9. Galphimia glauca (_Galphimia glauca_, Cav.). 10. Senna (_Cassia angustifolia_, Vahl.).]

[Illustration: PLATE 8

CLUSTERED UNICELLULAR HAIRS

1. and 2. European oak (_Quercus infectoria_, Olivier). 3. Kamala (_Mallotus philippinensis_, [Lam.] [Muell.] Arg.). 4. Witch-hazel leaf (_Hamamelis virginiana_, L.). 5. Althea leaf (_Althæa officinalis_, L.).]

Cowage hairs (Plate 7, Fig. 3) are lance-shaped, and they terminate in a sharp point. The outer wall contains numerous recurved teeth-like projections. The cell cavity is filled with a reddish-brown contents which are somewhat fissured.

=Clustered unicellular hairs= occur on the leaves of chestnut, witch-hazel, althea, European oak, etc. In European oak (Plate 8, Figs. 1 and 2) clusters of two and three hairs occur. The walls are yellowish-white, smooth, and the tip of the hair is solid.

In kamala (Plate 8, Fig. 3) clusters of seven or more hairs occur; the walls are yellowish, and the cell cavity is reddish-brown. In witch-hazel leaf (Plate 8, Fig. 4) clusters of a variable number of hairs occur. The hairs, which are of various lengths, have yellowish-white, thick, smooth walls, and reddish cell contents.

In althea leaf (Plate 8, Fig. 5) the hairs are nearly straight and the walls are smooth. The basal portions of the hair are strongly pitted.

=Branched solitary unicellular hairs= occur on the leaves of hesperis matronalis (Plate 7, Fig. 8), and on galphimia glauca (Plate 7, Fig. 9).

The hair of hesperis matronalis has smooth walls, and the two branches grow out nearly parallel to the leaf surface.

The hair of galphimia glauca has rough walls, and the two branches grow upward in a bifurcating manner.

MULTICELLULAR HAIRS

=Multicellular= hairs are divided into the uniseriate and the multiseriate hairs. Both of these groups are divided into the branched and the non-branched hairs, as follows:

1. =Uniseriate=. (_A_) =Non-branched.= (_B_) =Branched.=

2. =Multiseriate.= (_A_) =Non-branched.= (_B_) =Branched.=

=Multicellular uniseriate non-branched hairs= occur on the leaves of digitalis, Western and Eastern skullcap, peppermint, thyme, yarrow, arnica flowers, and sumac fruit.

[Illustration: PLATE 9

MULTICELLULAR UNISERIATE NON-BRANCHED HAIRS

1. Digitalis leaf (_Digitalis purpurea_, L.). 2. Arnica flower (_Arnica montana_, L.). 3. Western skullcap plant (_Scutellaria canescens_, Nutt.). 4. Eastern skullcap plant (_Scutellaria lateriflora_, L.). 5. Peppermint leaf (_Mentha piperita_, L.). 6. Thyme leaf (_Thymus vulgaris_, L.). 7. Yarrow flowers (_Achillea millefolium_, L.). 8. Wormwood leaf (_Artemisia absinthium_, L.). 9. Sumac fruit (_Rhus glabra_, L.).]

Digitalis hairs (Plate 9, Fig. 1) are made up of a varying number of uniseriate-arranged cells of unequal length, frequently placed at right angles to the cells above and below; the walls are of a whitish color, and are rough or smooth.

Eastern skullcap (Plate 9, Fig. 4) has hairs with not more than four cells; these hairs are curved, and the walls are whitish, sometimes smooth, but usually rough. In Western skullcap (Plate 9, Fig. 3) the hairs have sometimes as many as seven cells. The walls are white and rough, and the individual cells of the hair are much larger than are the cells of the hairs of true skullcap.

Peppermint (Plate 9, Fig. 5) has from one to eight cells. The hair is curved, and the walls are very rough.

Thyme (Plate 9, Fig. 6) has short, thick, rough-walled trichomes, the terminal cell usually being bent at nearly right angles to the other cells.

Yarrow hairs (Plate 9, Fig. 7) have a variable number of cells. In all the hairs the basal cells are short and broad, while the terminal cell is greatly elongated.

Arnica hairs (one form, Plate 9, Fig. 2) have frequently as many as four cells, the terminal cell being longer than the basal cells. The walls are white and smooth.

Sumac-fruit hairs (Plate 9, Fig. 9) have spindle-shaped, reddish-colored hairs.

=Multicellular multiseriate non-branched hairs= occur on cumin fruit and on the tubular part of the corolla of calendula.

The hairs on cumin fruit vary considerably in size. All the hairs are spreading at the base and blunt or rounded at the apex. The cells forming the hair are narrow and the walls are thick. Three differently sized hairs are shown in Plate 10, Fig. 1.

The hairs of the base of the ligulate petals of calendula (Plate 10, Fig. 2) are biseriate. The hairs are very long and the walls are very thin.

=Multicellular uniseriate branched hairs= occur on the leaves of dittany of Crete, mullen, and on the calyx of lavender flowers.

The dittany of Crete (Plate 11, Fig. 3) hair is smooth-walled, and the branches are alternate.

In mullen (Plate 11, Fig. 1) the hairs have whorled branches, the walls are smooth, and the cell cavity usually contains air.

[Illustration: PLATE 10

MULTICELLULAR MULTISERIATE NON-BRANCHED HAIRS

1. Cumin (_Cuminum cyminum_, L.). 2. Marigold (_Calendula officinalis_, L.).]

[Illustration: PLATE 11

MULTICELLULAR UNISERIATE BRANCHED HAIRS

1. Mullen leaf (_Verbascum thapsus_, L.). 2. Lavender flowers (_Lavandula vera_, D. C.). 3. Dittany of Crete (_Origanum dictamnus_, L.).]

The lavender hairs (Plate 11, Fig. 2) have mostly opposite 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 arranged branches. Each class may be again subdivided according 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. 1); 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 perpendicular, 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 constituent is dried protoplasm. In cannabis indica are deposits 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. 1 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. 1) 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 unequal 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. 1 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 branches (Plate 14, Fig. 2) are much longer than special branches.

[Illustration: PLATE 12

NON-GLANDULAR MULTICELLULAR HAIRS _Shepherdia canadensis_, [L.] Nutt.]

[Illustration: PLATE 13

MULTICELLULAR MULTISERIATE BRANCHED HAIRS

1. Basal hairs of erigeron (_Erigeron canadensis_, L.). 2. Apical hairs of erigeron (_Erigeron canadensis_, L.). 3. Basal hairs of aromatic goldenrod (_Solidago odora_, Ait.). 4. Apical hairs of aromatic goldenrod (_Solidago odora_, Ait.).]

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. 1 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. 1) 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 downward.

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, fibres, stone cells, etc.

CORK PERIDERM

The typical periderm is made up of =cork cells=. Cork cells vary in appearance, according to the part of the cell viewed.

[Illustration: PLATE 14

MULTICELLULAR MULTISERIATE BRANCHED HAIRS

1. Apical hairs arnica (_Arnica montana_, L.). 2. Basal hairs arnica (_Arnica montana_, L.). 3. Apical hairs grindelia (_Grindelia squarrosa_, [Pursh] Dunal). 4. Basal hairs grindelia (_Grindelia squarrosa_, [Pursh] Dunal).]

[Illustration: PLATE 15

MULTICELLULAR MULTISERIATE BRANCHED HAIRS

1. Apical hairs boneset (_Eupatorium perfoliatum_, L.). 2. Basal hairs boneset (_Eupatorium perfoliatum_, L.). 3. Apical hairs life-everlasting (_Gnaphalium obtusifolium_, L.). 4. Basal hairs life-everlasting (_Gnaphalium obtusifolium_, L.).]

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 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 16, 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 outermost 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 16, 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 importance, nor is the surface view of cork cells diagnostic except in certain isolated cases.

[Illustration: PLATE 16

PERIDERM OF CASCARA SAGRADA (_Rhamnus purshiana_, D.C.)

_A._ 1, Outline of cork cells; 2, Line of contact of adjoining cork cells.

_B._ Radial longitudinal section of cascara sagrada. 1, Cork cells; 2, Phellogen; 3, Forming parenchyma cells; 4, Cortical parenchyma cells.

_C._ Cross-section of cascara sagrada. 1, Cork cells; 2, Phellogen; 3, Forming parenchyma cells; 4, Cortical parenchyma cells.]

The presence or absence of cork or epidermal tissue in powders 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 pharmacopœial from the unofficial peeled calamus. The absence of epidermis in Jamaica ginger is one of the means by which this variety is distinguished from the other varieties of ginger, etc.

In canella alba the periderm is replaced by stone cell-cork. That is, the cells forming the periderm are of a typical cork shape, but the walls are lignified, unequally thickened, and the inner or thicker walls are strongly porous, and the walls are of a yellowish 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 101, 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 fissures 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 partially or completely fill the break in the periderm. In white oak bark (Plate 18), as in other barks, a large part of the periderm is composed of dead and discolored cortical cells.

[Illustration: PLATE 17

_A._ Cross-section of Mandrake Rhizome (_Podophyllum peltatum_, L.). 1. Epidermis. 2. Phellogen. 3. Cortical parenchyma. _B._ Stone cell periderm of white cinnamon (_Canella alba_, Murr.).]

[Illustration: PLATE 18

PERIDERM OF WHITE OAK (_Quercus alba_, L.)

1. Outer layer of cork cells. 2. Cortical parenchyma cells. 3. Stone cells. 4. Phellogen. 5. Cortical parenchyma cells.]

ORIGIN OF CORK CELLS

The cork cells are formed by the meristimatic phellogen cells, which originate from cortical parenchyma. 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 cell. The development of a cortical parenchyma cell from a divided phellogen cell is shown in Plate 101, 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.

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