CHAPTER VII

SYNTHETIC TISSUE

Under synthetic tissue are grouped all tissues and cells which form substances or compounds other than protoplasm. Such compounds are stored either in special cavities or in the cells of the plant, as the glandular hairs; internal secreting cavities of barks, stems, leaves, fruits, seeds, and flowers; photosynthetic cells or cells with chlorophyll, and the parenchymatic cells which form starch, sugar, fats, alkaloids, etc.

PHOTOSYNTHETIC TISSUE

The most important non-glandular synthetic tissue is the photosynthetic tissue, which is composed of the chlorophyll-bearing cells of the plant. These are the so-called green cells of leaves, of stems of herbs, of young woody stems, and in the older woody stems of plants like wild cherry, birch, etc. The greater part of the tissue of leaves is composed of chlorophyll-bearing cells.

Leaves collectively constitute the greatest synthetic manufacturing plant in the world, because the green cells of the leaf produce most of the food of men and animals. The two compounds utilized in the manufacture of food are carbon dioxide (CO₂) and water (H₂O). These two compounds are combined by chlorophyll through the agency of light into starch. Chemically this reaction may be expressed as follows:

6CO₂ + 5H₂O = 2C₆H₁₀O₅ + 6O₂.

During the day a large quantity of starch is formed. At night through the action of a ferment the excess of starch remaining in the leaf is converted into sugar (C₆H₁₂O₆) - C₆H₁₀O₅ + H₂O = C₆H₁₂O₆. In this form it is distributed to the living cells of the plant. The presence or absence of starch in leaves is easily ascertained by placing the leaf in hot alcohol to remove the chlorophyll, and by adding Lugol’s solution. If starch is present, the contents of the cells will become bluish black; but if no starch is present, the cells remain colorless.

GLANDULAR TISSUE

The =glandular tissue= of the plant is divided into two groups, according to where it occurs. These groups are, first, =external= glandular tissue, and secondly, =internal= glandular tissue. The most important external glandular tissue is composed of the glandular hairs. These are divided into two groups: first, =unicellular=; and secondly, =multicellular= glandular hairs.

UNICELLULAR GLANDULAR HAIRS

The =unicellular glandular hairs= are either sessile or stalked.

=Sessile unicellular hairs= occur in digitalis leaves.

=Stalked unicellular hairs= of digitalis are shown on Plate 60, Fig. 2.

=Unicellular uniseriate stalked glandular hairs= occur on the stems of the common house geranium (Plate 61, Fig. 2), on the leaves of butternut, the leaves and stems of marrubium peregrinum (Plate 98, Fig. 5), and in arnica flowers. The stalk varies from two to ten cells; in eriodictyon the cells vary from four to eight cells.

Unicellular multiseriate stalked glandular hairs are not of common occurrence.

MULTICELLULAR GLANDULAR HAIRS

=Multicellular glandular hairs= are divided into two groups: first, sessile; and secondly, stalked hairs.

Multicellular sessile glandular hairs occur on the leaves of peppermint (Plate 60, Fig. 3), horehound (Plate 97, Fig. 7), and in hops (Plate 60, Fig. 4). In each of these hairs there are eight secretion cells.

=Stalked glandular hairs= are divided into two groups: first, uniseriate stalked; and secondly, multiseriate stalked glandular hairs.

=Multicellular uniseriate stalked glandular hairs= occur on the leaves of tobacco (Plate 61, Fig. 4), belladonna (Plate 61, Fig. 1), and digitalis (Plate 60, Fig. 2), and of the fruit of rhus glabra.

[Illustration: PLATE 60

GLANDULAR HAIRS

1. Kamala (_Mallotus philippinensis_, [Lam.] [Muell.] Arg.). 2. Digitalis leaf (_Digitalis purpurea_, L.). 3. Peppermint leaf (_Mentha piperita_, L.). 4. Lupulin. 5. Cannabis indica leaf (_Cannabis saliva_, L.).]

=Multicellular multiseriate stalked glandular hairs= occur on the stems and leaves of cannabis indica (Plate 60, Fig. 5).

In the glandular hair of kamala (Plate 60, Fig. 1) the number of secretion cells is variable and papillate in form, and the cuticle is separated from the secretion cells.

In the glandular hair of hops the outer wall or cuticle is torn away from the secretion cells, and the cavity thus formed serves as a storage cavity. This distended cuticle of the hops shows the outline of the cells from which it was separated.

In the glandular hairs of the mints the secreted products (volatile oils) are stored between the secretion cells and the outer detached cuticle. This cuticle is elastic, and it becomes greatly distended as the volatile oil increases in amount.

In many of the so-called glandular hairs, tobacco, belladonna geranium, etc., the synthetic products are retained in the glandular cells, there being no special cavity for their storage.

These hairs usually contain an abundance of chlorophyll.

The division wall of multicellular glandular hairs may be vertical, as in the two-celled hair of digitalis (Plate 60, Fig. 2); as in horehound (Plate 97, Fig. 6), and as in peppermint (Plate 60, Fig. 3); in this case there are eight cells, and they form a more or less flat plate of cells.

In other hairs the division wall is horizontal; this produces a chain of superimposed secreting cells, as in some of the glandular hairs of belladonna leaf (Plate 61, Fig. 1), etc.

In other hairs the division walls are both vertical and horizontal, as in tobacco (Plate 61, Fig. 4), henbane (Plate 61, Fig. 3), belladonna (Plate 61, Fig. 1).

Other characters to be kept in mind in studying glandular hairs are the following: Color of cell contents; size of the cells, whether uniform or variable; character of wall, whether smooth or rough.

SECRETION CAVITIES

=Secretion cavities= are divided into three groups, according to the nature of the origin of the cavity: first, schizogenous cavities, which originate by a separation of the walls of the secretion cells; secondly, lysigenous cavities, which arise by the dissolution of the walls of centrally located secretion cells; and thirdly, schizo-lysigenous cavities, which originate schizogenously, but later become lysigenous owing to the dissolution of the outer layers of the secretion cells.

[Illustration: PLATE 61

STALKED GLANDULAR HAIRS

1. Belladonna leaf (_Atropa belladonna_, L.). 2. Geranium stem (_Geranium maculatum_, L.). 3. Henbane leaf (_Hyoscyamus niger_, L.). 4. Tobacco leaf (_Nicotiana tabacum_, L.).]

SCHIZOGENOUS CAVITIES

=Schizogenous cavities= occur in white pine bark (Plate 62, Fig. B). The cells lining the cavity are mostly tangentially elongated, and the wall extends into the cavity in the form of a papillate projection. Immediately back from these cells are two or three layers of cells which resemble cortical parenchyma cells, except that they are smaller and their walls are thinner.

In white pine bark there is a single layer of thin-walled cells lining the cavity. Immediately surrounding the secretion cells is a single layer of thick-walled fibrous cells.

In klip buchu (Plate 63, Fig. B), as in white pine leaf (Plate 64, Fig. B), there is a single layer of thin-walled secretion cells which are surrounded on three sides with parenchyma cells and on the outer side by epidermal cells.

LYSIGENOUS CAVITIES

=Lysigenous cavities= occur on the rind of citrus fruits--bitter and sweet orange, lemon, grapefruit, lime, etc., and in the leaves of garden rue, etc.

In bitter orange peel, (Plate 64, Fig. A) the cavity is very large, and the cells bordering the cavity are broken and partially dissolved. The entire cells back of these are white, thin-walled, tangentially elongated cells. There is a great variation in the size of these cavities, the smaller cavities being the recently formed cavities.

SCHIZO-LYSIGENOUS CAVITIES

=Schizo-lysigenous= cavities are formed in white pine bark and many other plants owing to the increase in diameter of the stem. In such cases the walls of the secreting cells break down. The resulting cavity resembles lysigenous cavities.

=Unicellular secretion cavities= occur in ginger, aloe, calamus, and in canella alba barb.

[Illustration: PLATE 62

_A._ Cross-section of calamus rhizome (_Acorus calamus_, L.). 1, Intercellular space; 2, Parenchyma cells; 3, Secretion cavity. _B._ Cross-section of white pine bark (_Pinus strobus_, L.). 1, Parenchyma; 2, Secretion cavity; 3, Secretion cells.]

[Illustration: PLATE 63

_A._ Cross-section of a portion of canella alba bark (_Canella alba_, Murr.). 1. Excretion cavity. _B._ Cross-section of a portion of klip buchu leaf. 1. Epidermal cells. 2. Secretion cavity. 3. Secretion cells.]

[Illustration: PLATE 64

_A._ Cross-section of bitter orange peel (_Citrus aurantium_, _amara_, L.). 1, Internal secretion cavity formed by the dissolution of the walls of the central secreting cells; 2, Secretion cells. _B._ Cross-section of white pine leaf (_Pinus strobus_, L.). 1, Epidermal and hypodermal cells; 2, Parenchyma cells with protruding inner walls; 3, Endodermis; 4, Secretion cavity; 5, Secretion cells.]

In calamus (Plate 62, Fig. A) the cavity is larger than the surrounding cells; it is rounded in outline, and it contains oleoresin. These cavities are in contact with the ordinary parenchyma cells, from which they are easily distinguished by their larger size and rounded form.

The =unicellular oil cavity= of canella alba (Plate 63, Fig. A) is rounded or oval in cross-section and is many times larger than the surrounding cells. The wall, which is very thick, is of a yellowish color.

Secretion cavities vary greatly in form, according to the part of the plant in which they are found. In flower petals and leaves they are spherical; in barks they are usually elliptical; in umbelliferous fruits they are elongated and tube-like.

Mucilage cavities are not of common occurrence in medicinal plants. They occur, however, in the stem and root bark of sassafras, the stem bark of slippery elm, the root of althea, etc.

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