CHAPTER V

CONDUCTING TISSUE

All cells of which the primary or secondary function is that of conduction are included under conducting tissue. It will be understood how important the conducting tissue is when the enormous quantity of water absorbed by a plant during a growing season is considered. It will then be realized that the conducting system must be highly developed in order to transport this water from one organ to another, and, in fact, to all the cells of the plant. Special attention must be given to the occurrence, the structure, the direction of conduction, and to the nature of the conducted material.

The cells or cell groups comprising the conducting tissue are vessels and tracheids, sieve tubes, medullary ray cells, latex tubes, and parenchyma.

VESSELS

=Vessels= and =tracheids= form the principal upward conducting tissue of plants. They receive the soil water expressed from the cortical parenchyma cells located in the region of the root, immediately back of the root hair zone. This soil water, with dissolved crude inorganic and organic food materials, after entering the vessels and tracheids passes up the stem. The cells needing water at the different heights absorb it from the vessels, the excess finally reaching the leaves. When the stem branches, the water passes into the vessels of the branches and finally to the leaves of the branch. In certain special cases the vessels conduct upward soluble food material. In spring sugary sap flows upward through the vessels of the sugar maple.

Vessels are tubes, often of great length, formed from a number of superimposed cells, in which the end walls have become absorbed. The vessels therefore offer little resistance to the transference of water from the roots to the leaves of a plant. The combined length of the vessels is about equal to the height of the plant in which they occur. The length of the individual vessels varies from a fraction of a meter up to several meters.

ANNULAR VESSELS

The =annular vessels= are thickened at intervals in the form of rings (Plate 40, Fig. 1), which extend outward from and around the inner wall of the vessel. In fact, it is the inner wall which is thickened in all the different types of vessels. The ring-like thickening usually separates from the wall when the drug is powdered. Such separated rings occur frequently in powdered digitalis, belladonna, and stramonium leaves. Annular vessels are not, however, of diagnostic importance, because more characteristic cells are found in the plants in which they occur. Not infrequently a vessel will have annular thickenings at one end and spiral thickenings at the other. Such vessels are found in the pumpkin stem (Plate 40, Fig. 1).

Vessels are distinguished from other cells by their arrangement, by their large size when seen in cross-section, and by the thickening of the wall when seen in longitudinal sections of the plant or in powders. The side walls of vessels are thickened in a number of striking yet uniform ways. The chief types of thickening of the wall, beginning with one that is the least thickened, are annular, spiral, sclariform, pitted, and pitted with bordered pores.

SPIRAL VESSELS

In the =spiral vessel= the thickening occurs in the form of a spiral, which is readily separated from the side walls. This is

## particularly the case in powdered drugs, where the spiral thickening

so frequently separates from the cell wall. There are three types of spiral vessels: those with one (Plate 41, Fig. 1), those with two, and those with three spirals. Single spirals occur in most leaves; double spirals occur in many plants (Plate 41, Fig. 2), but they are particularly striking in powdered squills. Triple spirals are characteristic of the eucalyptus leaf (Plate 41, Fig. 3); in fact, they form a diagnostic feature of the powder. Frequently a spirally thickened wall indicates a developmental stage of the vessel. Many such vessels are spirally thickened at first, but later, when mature, an increased amount of thickening occurs and the vessel becomes a reticulate or pitted vessel. Many mature vessels, however, are spirally thickened as indicated above. In herbaceous stems and in certain roots and leaves spiral vessels are associated with the sclariform reticulate and pitted type. In certain cases a single spiral band will branch as the vessel matures.

There is a great variation in the amount of spiral thickening occurring in a vessel. In leaves, particularly, the spiral appears loosely coiled; while in squills and other rhizomes and roots the spiral appears as a series of rings. When viewed by high power only half of each spiral band is visible. At either side of the cell the exact size and form of the thickening appear in two parallel rows of dark circles or projections from the walls. This thickening of the wall is rendered visible from the fact that the light is retarded as it passes through that portion of the spiral extending from the upper to the under side of the spiral; while the light readily traverses the upper and lower cross bands of the vessel.

It should be remembered that, when the upper part of the spiral vessel is in focus, the bands appear to bend in a direction away from the eye; while when the under side of the bands are in focus, the bands appear to bend toward the eye. These facts will show that it is necessary to focus on both the upper and lower walls in studying spiral vessels. In double spiral vessels the spirals are frequently coiled in opposite directions; therefore the bands appear to cross one another. In eucalyptus leaf the three bands are coiled in the same direction. In all cases the thickening occurs on all sides of the wall. Its appearance will, therefore, be the same no matter at what angle the vessel is viewed.

SCLARIFORM VESSELS

=Sclariform vessels= have interrupted bands of thickening on the inner walls. Two or more such bands occur between the two side walls. The series of bands are separated by uniformly thickened portions of the wall extending parallel to the length of the vessel. Sclariform vessels are usually quite broad, so that it is necessary to change the focus several times in order to bring the different series of bands in focus. The series of bands are usually of unequal width and length.

[Illustration: PLATE 40

ANNULAR AND SPIRAL VESSELS

1. Pumpkin stem (_Cucurbita pepo_, L.). 2. Two characteristic views of spiral vessels. 3. (_A_) Upper part of spiral vessel in focus. (_B_) Under part of spiral vessel in focus. 4. Spiral vessel of the disk petal matricaria (_Matricaria chamomilla_, L.).]

[Illustration: PLATE 41

SPIRAL VESSELS

1. Single spiral vessel of pumpkin stem (_Cucurbita pepo_, L.). 2. Double spiral vessel of squill bulb (_Urginea maritima_, [L.] Baker). 3. Triple spiral vessel of eucalyptus leaf (_Eucalyptus globulus_, Labill).]

Sclariform vessels occur in male fern (Plate 42, Fig. 2), calamus, tonga root (Plate 42, Fig. 3), and sarsaparilla (Plate 42, Fig. 1). In each they are characteristic. Sclariform vessels, with these few exceptions, do not occur in drug plants. In fact, drugs derived from dicotyledones rarely have sclariform vessels. They occur chiefly in the ferns and drugs derived from monocotyledenous plants. Their presence or absence should, therefore, be noted when studying powdered drugs.

RETICULATE VESSELS

=Reticulate vessels= are of common occurrence in medicinal plants. In fact, they occur more frequently than any other type of vessel. The basic structure of reticulate vessels (Plate 43, Fig. 1) occurring in different plants is similar, but they vary in a recognizable way in different plants (Plate 43, Fig. 2). The walls of reticulate vessels are thickened to a greater extent than are the walls of spirally thickened vessels.

PITTED VESSELS

=Pitted vessels= are met with most frequently in woods and wood-stemmed herbs. There are two distinct types of pitted vessels--_i.e._, simple pitted vessels and pitted vessels with bordered pores.

The pitted vessel represents the highest type of cell-wall thickening. The entire wall of the vessel is thickened, with the exception of the places where the pits occur. The number and size of the pits vary greatly in different drugs. In quassia (Plate 44, Fig. 1) the pits are numerous and very small, and the openings are nearly circular in outline. In white sandalwood (Plate 44, Fig. 3). the pits are few in number, but when they do occur they are much larger than are the pits of quassia.

PITTED VESSELS WITH BORDERED PORES

=Pitted vessels with bordered pores= are of common occurrence in the woody stems and stems of many herbaceous plants (Plate 45, Figs. 3 and 4). In such vessels the wall is unthickened for a short distance around the pits. This unthickened portion may be either circular or angled in outline, a given form being constant to the plant in which it occurs. The pits vary from oval to circular. Pitted vessels with bordered pores occur in belladonna and aconite stems.

[Illustration: PLATE 42

SCLARIFORM VESSELS

1. Sarsaparilla root (_Smilax officinalis_, Kunth). 2. Male fern (_Dryopteris marginalis_, [L.] A. Gray). 3. Tonga root.]

[Illustration: PLATE 43

RETICULATE VESSELS

1. Hydrastis rhizome (_Hydrastis canadensis_, L.). 2. Musk root (_Ferula sumbul_, [Kauffm.] Hook., f.).]

[Illustration: PLATE 44

PITTED VESSELS

1. Quassia, low magnification (_Picræna excelsa_, [Swartz] Lindl.). 2. Quassia, high magnification. 3. White sandalwood (_Santalum album_, L.).]

[Illustration: PLATE 45

VESSELS

1. Reticulate vessel of calumba root (_Jateorhiza palmata_, [Lam.] Miers). 2. Reticulate tracheid of hydrastis rhizome (_Hydrastis canadensis_, L.). 3. Pitted vessel with bordered pores of belladonna stem. 4. Pitted vessel with bordered pores of aconite stem (_Aconitum napellus_, L.).]

Vessels and tracheids lose their living-cell contents when fully developed. In the vessels the cell contents disappear at the period of dissolution of the cell wall.

The walls of vessels and tracheids are composed of lignin, a substance which prevents the collapsing of the walls when the surrounding cells press upon them, and which also prevents the tearing apart of the wall when the vessel is filled with ascending liquids under great pressure. Lignin thus enables the vessel to resist successively compression and tearing forces.

Tracheids are formed from superimposed cells with oblique perforated end walls. The side walls of tracheids are thickened in a manner similar to those of vessels. The tracheids in golden seal are of a bright-yellow color, and groups of these short tracheids scattered throughout the field form the most characteristic part of the powdered drug. In ipecac root the tracheids are of a porcelain-white, translucent appearance, and they are much longer than are the tracheids of golden seal.

The cellulose walls of parenchyma cells are stained blue with hæmatoxylin and by chlorzinciodide. Cellulose is completely soluble in a fresh copper ammonia solution.

SIEVE TUBES

=Sieve tubes= are downward-conducting cells. They conduct downward proteid food material. This fact is easily demonstrated by adding iodine to a section containing sieve tubes, in which case the sieve tubes are turned yellow.

Developing sieve tubes have all the parts common to a living cell; but when fully mature, however, the nucleus becomes disorganized, but a layer of protoplasm continues to line the cell wall.

Sieve tubes (Plate 46, Fig. 1) are composed of a great number of superimposed cells with perforated end walls and with non-porous cellulose side walls. The end walls of two adjoining cells are greatly thickened and the pores pass through both walls. This thickened part of the porous end walls of two sieve cells is called the sieve plate, and it may be placed in an oblique or a horizontal position.

[Illustration: PLATE 46

1. Longitudinal section of sieve tube (_Cucurbita pepo_, L.). 2. Cross-section of sieve tube just above an end wall--sieve plate.]

In a longitudinal section the sieve tubes are seen to be slightly bulging at the sieve plate, and through the pores extend protoplasmic strands. The strands are united on the upper and lower side of the sieve plate to form the protoplasmic strands of the living sieve tubes and the callus, layers of dried plants. This callus is frequently yellowish in color, and in all cases is separated from the cell wall. In certain plants the sieve plate occurs on the side walls of the sieve tubes in contact with other sieve tubes.

SIEVE PLATE

=Sieve plates= on cross-section (Plate 46, Fig. 2) are polygonal in outline, and the pores are either round or angled. Large sieve tubes and sieve plates occur in pumpkin stem; but, almost without exception, in drug plants the sieve tubes are small and the sieve plate is inconspicuous. When the drug is powdered, the sieve tubes break up into undiagnostic fragments. When studying sections of the plants, the extent, size, and arrangement of the sieve tubes must always be noted.

MEDULLARY BUNDLES, RAYS, AND CELLS

Function

The medullary ray cells are the lateral conducting cells of the plant. They conduct outwardly the water and inorganic salts brought up from the roots by the vessels and tracheids; and they conduct inwardly toward the centre of the stem the food material manufactured in the leaves and brought down by the sieve cells. The medullary rays thus distribute the inorganic and organic food to the living cells of the plant, and they conduct the reserve food material to the storage cells, and, lastly, they function in certain plants as storage cells.

Occurrence

The form, size, wall structure, and the distribution of the medullary ray bundles, rays, and cells are best ascertained by studying: first, the cross-section of the plant; secondly, the radial section; and, thirdly, the tangential section.

Students should be careful to distinguish between the medullary ray bundle, the medullary ray, and the medullary ray cell. In some plants the bundles are only one cell wide, but in other plants the medullary ray bundle is more than one cell wide, frequently several cells wide.

THE MEDULLARY RAY BUNDLE

The =medullary ray bundle= is made up of a great many medullary ray cells. These bundles (Plate 106, Fig. 5) are of variable length, height, and width. The bundles are isolated, and they occur among and separate the other cells of the plants in which they occur. Tangential sections show the medullary ray bundle in cross-section. Such sections are lens-shaped, and they show both the width and the height of the medullary ray bundle. The length of the medullary ray bundle is shown in cross-sections.

THE MEDULLARY RAY

The =medullary ray= (Plate 47) is a term used to indicate that part of a medullary ray bundle which is seen in cross-sections and in radial sections. In cross-sections the length of the ray will be as great as the length of the bundle, and the width of the ray will be as great as the width of the medullary ray bundle at the point cut across. In longitudinal sections the medullary ray will differ in height according to the thickness of the bundle at the point cut.

When the medullary rays extend from the centre of the stem to the middle bark, they are termed primary medullary rays; when they extend from the cambium circle to the middle bark, they are termed secondary medullary rays. As the plant grows, the diameter of the organ becomes greater and the number of medullary rays are increased. In each of these cases the medullary rays may be one or more than one cell wide, according to whether the medullary ray bundle is one or more than one cell wide. Even in the same plant the width of the medullary rays will vary if the bundle is more than one cell wide, according to width of the medullary ray bundle at the point cut across.

[Illustration: PLATE 47

RADIAL LONGITUDINAL SECTION OF WHITE SANDALWOOD (_Santalum album_, L.)

1. Medullary ray. 2. Wood fibres and wood parenchyma.]

On cross-section the medullary rays are seen to vary greatly. In many plants they are more or less straight radial lines, as in quassia (Plate 105, Fig. 2); while in other plants they form wavy lines where they bend or curve around the conducting cells, as in piper methysticum, kava-kava (Plate 48, Fig. A).

In the study of powdered drugs the radial view of the medullary rays is most frequently seen.

In a perfect radial section (Plate 107, Fig. 2) the medullary rays are seen as tiers of cells in contact throughout their long diameter, and they run at right angles to the long diameter of the other cells. This view of the rays shows the length and height of the medullary ray. In logwood the rays are often forty cells high. In powdered barks, woods (Plate 47), and woody roots the radial view of the medullary rays is frequently diagnostic.

In guaiacum officianale wood the medullary rays are one cell wide on cross-section, and up to six cells high on the tangential section. In santalum album the rays are from one to three cells wide on cross-section, and up to six cells high on tangential section. In the greater number of plants the rays are more than one cell wide.

THE MEDULLARY RAY CELL

The =medullary ray cell= (Plate 48, Fig. 1) is one of the individual cells making up the medullary ray bundle and the medullary ray.

The cross-sections of the cells which are seen in tangential sections show the cells to be mostly circular in outline when they occur in the central portion of medullary ray bundles of more than two cells in width; but they are more irregular in outline when the medullary ray bundle is only one cell wide. Even the cells of the three or more cell-wide bundles have irregular, outlined cells at the ends of the bundle and on the sides in contact with the other tissues.

The length and height of the medullary ray cell are shown in radial sections; while the width and length of the medullary ray cells are shown in cross-sections.

Structure of Cells

The structure of the individual cells forming the medullary rays differs greatly in different plants, but is more or less constant in structure in a given species.

The medullary rays of the wood usually have strongly pitted side and end walls, while the medullary rays of most barks are not at all, or only slightly, pitted. In most plants the cells are of nearly uniform size. Frequently, however, the cells vary in size in a given ray, as shown in the cross-section of kava-kava.

Arrangement of the Cells in a Ray

The union of any two cells in a ray is also of importance. In quassia the medullary ray cells have oblique end walls, so that on cross-section the line of union between two cells is an oblique wall. In most plants the medullary ray cells have blunt or square or oblique end walls, so that the line of union is a straight line.

In most plants the cells are much longer than broad, but the cells of sassafras bark are nearly as broad as long.

The walls of the cortical medullary ray cells and the medullary rays of most roots and stems of herbs are composed of cellulose; while the walls of medullary ray cells occurring in woods are frequently lignified.

There is a great variation in the character of the cell contents of medullary rays. In white pine bark (Plate 48, Fig. B1) are deposits of tannin; in quassia wood, starch; in canella alba, rosette crystals of calcium oxalate, etc.

LATEX TUBES

Living =latex tubes=, like sieve tubes, have a layer of protoplasm lining the walls, and, in addition, have numerous nuclei. In drug plants the nuclei are not distinguishable, but the protoplasm is always clearly discernible.

[Illustration: PLATE 48

_A._ Cross-section of kava-kava root (_Piper methysticum_, Forst., f.). 1. Unequal diameter medullary ray cells. 2. Vessels. 3. Wood parenchyma. 4. Wood fibres.

_B._ Cross-section of white pine bark (_Pinus strobus_, L.). 1. Wavy medullary rays with tannin. 2. Parenchyma cells. 3. Sieve cells.]

Latex tubes function both as storage and as conducting cells. They, like the sieve tubes, contain proteid substances chiefly, yet frequently starch is found. The cells bordering the latex tubes absorb from them, as needed, the soluble food material. While our knowledge concerning the function of latex in some plants is meagre, still in other plants it is practically certain that the latex is composed of nutritive substances which are utilized by the plant as food. In certain other plants the latex appears to be used as a means of resisting insect attacks and as a protection against injury.

There are two types of latex tubes common to plants, namely, latex cells and latex vessels. Latex tubes developing from a single cell do not differ materially from a latex tube originating from the fusion of several cells. In each case the latex tube branches to such an extent that it bears no resemblance to ordinary cells. It would seem that the ultimate branches are formed and develop in much the same manner as root hairs--that is, by a growing tip of the branch. A mature plant may therefore have latex tubes with almost numberless branches (Plate 50, Fig. 1) and be of very great length.

The branches of latex tubes develop in such an irregular manner that it is possible to obtain a cross and a longitudinal section of the latex tubes by making a cross-section of stem. Such a section is shown in the drawing of the cross-section of the rhizome of black Indian hemp (Plate 49, Fig. B).

The color of the latex in medicinal plants varies from a gray white in papaw (carica papaya), aromatic sumac, black Indian hemp, and bitter root, to white in the opium poppy, light orange in celandine, and deep orange in bloodroot (Plate 50, Fig. 2). In each of these cases it is the latex which yields the important medicinal products.

PARENCHYMA

The larger amount of plant tissue is composed of =parenchyma= cells. These cells vary from square to oblong, or they may be irregular and branched. The end walls are square or blunt, and the wall is composed of cellulose, with the exception of the wood parenchyma, which has lignified walls.

There are seven characteristic types of parenchyma cells: (1) cortical parenchyma, (2) pith parenchyma, (3) wood parenchyma, (4) leaf parenchyma, (5) aquatic plant parenchyma, (6) endosperm parenchyma, (7) phloem parenchyma.

[Illustration: PLATE 49

_A._ Cross-section of black Indian hemp (_Apocynum cannabinum_, L.). 1. Longitudinal section of a latex tube. 2. Cross-section of latex tube. 3. Parenchyma.

_B._ Cross-section of a part of black Indian hemp root. 4. Cross-section of a large latex tube. 5. Parenchyma.]

[Illustration: PLATE 50

LATEX VESSELS

1. Radial-longitudinal section of dandelion root (_Taraxacum officinale_, Weber). 2. Cross-section of sanguinaria root (_Sanguinaria canadensis_, L.). 3. Cross-section of dandelion root.]

Parenchyma cells, cortical, pith, aquatic plant, leaf, flower, and endosperm, conduct in all directions--upward, downward, and laterally. The direction of conduction depends upon the needs of the different cells forming the plant. The fluids pass from the cell with an abundance of cell sap to the cell with less cell sap. In this wall all cells are provided with food.

Parenchyma cells conduct water absorbed by the roots and soluble carbohydrate material chiefly.

The walls of all the different types of parenchyma cells are composed of cellulose with the exception of the wood parenchyma cells, the walls of which are lignified. The end walls of non-branched parenchyma cells and the cell terminations of branched cells are very blunt.

CORTICAL PARENCHYMA

=Cortical parenchyma= (Plate 51) differs greatly in size, thickness of the walls, and arrangement. A study of the longitudinal sections of different parts of medicinal plants reveals the fact that the cortical parenchyma cells form superimposed layers in which the end walls are either parallel, in which case the arrangement resembles that of several rows of boxes standing on end, or the end walls of the cells alternate with each other, in which case the arrangement is similar to that of the arrangement of the bricks in a building.

In certain plants the cortical parenchyma cells are long and narrow and rectangular in shape, while in other plants the cells, although still rectangular in outline, are very broad and approach the square form.

All typical cortical parenchyma cells have uniformly thickened non-pitted walls. In most barks the parenchyma cells beneath the bark are elongated tangentially, but are very narrow radially. The cells are always arranged around intercellular spaces, which vary from triangular, quadrangular, etc., according to the number of cells bordering the intercellular space.

PITH PARENCHYMA

=Pith parenchyma= (Plate 52) differs from cortical parenchyma cells chiefly in the character of the walls, which are usually thicker and always pitted.

[Illustration: PLATE 51

PARENCHYMA CELLS

1. Longitudinal section of the cortical parenchyma of celandine root (_Chelidonium majus_, L.) 2. Cross-section of the cortical parenchyma of sarsaparilla root (_Smilax officinalis_, Kunth).]

[Illustration: PLATE 52

_A._ Longitudinal section of the pith parenchyma of grindelia stem (_Grindelia squarrosa_, [Pursh] Dunal). 1. Cell cavity. 2. Cross-section of the porous end wall. 3. Surface view of the porous side wall. _B._ Cross-section of the pith parenchyma of grindelia stem. 1. Cell cavity. 2. Porous walls. 3. Pitted end walls.]

LEAF PARENCHYMA

The =parenchyma cells= (Plate 109, Fig. 1) of leaves, of flower petals, and the parenchyma cells of some aquatic plants are branched; that is, each cell has more than two cell terminations. These cell terminations are frequently quite attenuated and usually very blunt. Such a cell structure provides for a greater amount of intercellular space and a maximum exposure of surface. This arrangement makes it possible for the parenchyma cells of the leaf to absorb more readily the enormous amount of carbon dioxide needed in the photosynthetic process.

AQUATIC PLANT PARENCHYMA

The =parenchyma of aquatic plants= (Plate 59) has large intercellular spaces formed by the chains of cells.

WOOD PARENCHYMA

=Wood parenchyma= (Plate 105, Fig. 3) cells are the narrowest parenchyma cells occurring in the plant. Their walls are always lignified and strongly pitted, and in some cases the end walls common to two cells are obliquely placed.

PHLOEM PARENCHYMA

=Phloem parenchyma= (Plate 100, Fig. 8) cells are usually associated with sieve cells. They are very long, narrow, and have thin, non-pitted walls. The thinness of the walls undoubtedly enables the cells to conduct diffusible food substance more quickly than the cortical parenchyma cells.

PALISADE PARENCHYMA

=Palisade parenchyma= of leaves is of the typical parenchyma shape and the end walls are placed nearly on a plane, even when more than one layer is present. The cells are very small, however, and the walls are very thin and non-pitted.

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