Barks are all obtained from dicotyledonous plants. In studying barks there should be ascertained the thickness, arrangement, form, structure, color, and cell contents of the cells occurring in the outer, middle, and inner barks.

The outer bark includes the cork cells and the phellogen layer. The middle bark includes all the cells occurring between the phellogen layer and the beginning of the medullary rays. The inner bark includes the medullary ray cells and all cells associated with them. The plan of structure of all barks is similar, but in each species of plant the structure of the bark is uniform and characteristic for the species.

A great number of drugs consist of the bark of woody plants; for this reason the bark is considered in a separate chapter from the stem.

White Pine Bark

The cross-section of white pine bark (Plate 103) has the following structure:

Outer Bark. The periderm consists of several layers of reddish-brown cork cells (1) which are narrow, elongated, and with thin walls.

Middle Bark. The cells forming the middle bark are parenchyma and secretion cells.

The parenchyma cells vary greatly in size, form, and thickness of the walls. The cells beneath the cork cells and around the secretion cells are tangentially elongated and oval in shape, while the other parenchyma cells are more irregular in shape.

The secretion cells are arranged around the schizogenous secretion cavities. The cells are tangentially elongated, and the walls, which are slightly papillate, are white.

Inner Bark. The cells forming the inner bark are medullary rays, parenchyma, sieve cells, and storage cavities.

The medullary rays form wavy lines. The medullary ray cells are radially elongated, rectangular in shape, and they contain granular cell contents. The sieve cells are either square or rectangular in shape. The walls are thin and white. The storage cavities are either filled with starch or with prisms and tannin.

Plate 103. Cross-Section of Unrossed White Pine Bark (Pinus strobus, L.).

1. Cork cells of the epidermis. 2. Parenchyma cells filled with chlorophyl. 3. Intercellular space. 4. Secretion cavity with resin. 5. Secretion cells. 6, One or more circles of parenchyma filled with chlorophyl. 7. Parenchyma. 8. Medullary rays. 9. Sieve cells. 10. Storage cavities.

Powdered White Pine Bark

White pine bark (Plate 104) when powdered shows the following characteristic elements:

The microscopic structure of a powdered white pine is as follows: The epidermis (1) consists of reddish-brown masses, irregular in outline. The outer parenchyma cells are of a bright-green color, owing to the presence of chlorophyll. (The above elements are not usually found in the rossed bark.) The parenchyma (3) with starch usually occurs in longitudinal sections accompanied with sieve cells. Often the tissue separates transversely, showing the medullary rays (4) with their granular cell contents (9) and the inner parenchyma cells filled with starch and the surrounding sieve cells.

The crystals are nearly perfect cubes and occur singly (5) or in groups (6). On the longitudinal section of the bark the crystals occur in parenchyma cells surrounded by a reddish cell content and form parallel rows which are very characteristic. The resin occurs either as white, angled fragments (7) in a water mount, or as globular mass (8) or as reddish-brown pieces (10). The starch is very abundant and is distributed through the field. The diagnostic grain is lens-shaped, with a cleft hilum, which is nearly straight, or slightly curved, and runs parallel to the long diameter of the grain. The addition of ferric chlorid T. S. will show the presence of tannin by forming a dark coloration. The identification of the starch is facilitated by the addition of a weak Lugol's solution, which. imparts a blue coloration to the starch grain.

The form, amount, and distribution of the cells composing the bark differ greatly in different plants.

In cramp bark the cork and phellogen cells are very large, while in cascara sagrada the phellogen and the cork cells are very small.

Plate 104. Powdered White Pine Bark (Pinus strobus, L.).

1. Epidermis. 2. Parenchyma cells. 3. Parenchyma with starch. 4. Medullary rays. 5. Solitary crystals. 6. Solitary crystals and tannin. 7, 8 and 10. Resin masses. 9. Starch.

In canella alba bark the periderm is composed of stone-cell cork or stone cells arranged in superimposed rows, which form the outer layers of the bark.

In white oak and most barks from woody trees the periderm consists of lifeless parenchyma, medullary rays, sieve cells, bast 'fibres, and in some cases stone cells and of phellogen cells.

In young wild cherry, cascara sagrada, and frangula are several layers of tangentially elongated collenchyma cells with chlorophyll. In the older barks of the above and in many other barks no collenchyma cells occur.

In cramp bark and in tulip tree bark the outer layers of the cortical parenchyma cells are beaded. In most barks there is no beaded walled parenchyma. The outer layers of most cortical parenchyma cells are tangentially elongated while the inner parenchyma cells are mostly circular in outline.

In white oak, cascara sagrada and prickly ash are groups of stone cells; in the cinnamon barks are bands of stone cells; in cinchona bark are isolated stone cells. In cramp bark, mezerum, elm, and white pine bark no stone cells occur.

In frangula, cascara sagrada, cocillina, cinnamon, cinchona, sassafras, and wild cherry barks the bast fibres occur in groups. In frangula, cascara sagrada, and cocillina the bast fibres are surrounded by crystal cells with crystals.

In sassafras bark mucilage cells occur. In canella alba, white pine, and sassafras barks secretion cells occur; but in most barks no secretion cells occur.

In sassafras bark the medullary ray cells are nearly as broad as long; in cramp bark they are elongated and oval in shape. In cascara sagrada, as in most barks, the cells are longer than broad and rectangular in shape.

In cascara sagrada the sieve cells are very large; in granatum bark the sieve cells are very small.

In cassia cinnamon and in canella alba bark the walls of the sieve cells have collapsed, with the result that the sieve cells have become partly obliterated.

In witch-hazel, mountain maple, willow, and black walnut are found prisms; in cramp bark, black haw, wahoo, pomegranate, and cotton root bark are found rosette crystals; in the cinnamon barks are found raphides; in cinchona bark, micro-crystals.

In cocillina, frangula, cascara sagrada, white oak, poplar and Jamaica dogwood barks are found crystal-bearing fibres (Plates 19 and 20).

When studying barks we must consider the kind, structure, and amount of the periderm; the nature of the phellogen; the nature and amount of the cortical parenchyma; the occurrence, distribution, and amount of stone cells, when present; the occurrence and structure of the bast fibres; the presence or absence of secretion cells; the width, distribution, and structure of the medullary rays.