CHAPTER V CONDUCTING TISSXJE (Part 2)

in a given ray, as shown in the cross-section of kava-kava. Arrangement of the Cells in a Ray The imion 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 obUque wall. In most plants the medullary ray celb have blunt or square or oblique end walls, so that the line of imion 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 medul- lary rJLys of most roots and stems of herbs are composed of cellu- lose; while the walls of medullary ray cells occurring in woods are frequently lignified. There is a great variation in the character 'of the cell con- tents of medullary rays. In white pine bark (Plate 48, Fig. Bi) are deposits of tannin; in quassia wood, starch; in canella alba, rosette crystals of calcimn oxalate, etc. LATEX TUBES Living latex tubes, like sieve tubes, have a layer of proto- plasm lining the walls, and, in addition, have nimierous nuclei. In drug plants the nuclei are not distinguishable, but the proto- plasm is always clearly discernible. 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 I. Cross-section of kava-kava root {Piper methyslicum. Font., (.)• I. Unequal diameter medullary ray cells. 3. Wood parenchyma. 4. Wood fibres. I. Cross-section of white pine bark (Piniu strobus, L.). 1. Wavy medullary rays with tannin. 3. Parenchyma cells. 3, Seve cells. 144 HISTOLOGY OF MEDICINAL PLANTS 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 diflfer 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 or- dinary 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 nmnberless branches (Plate 50, Fig. i) 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: (i) cortical parenchyma, (2) pith parenchyma, (3) wood par- enchyma, (4) leaf parenchyma, (5) aquatic plant parenchyma, (6) endosperm parenchyma, (7) phloem parenchyma. Parenchyma cells, cortical, pith, aquatic plant, leaf, flower. A. Craes-Kction ot black Indian hemp (Apocynvm cannabmum, L.}. I. Longitudinal section of a latex tube. 3. Cross'^ection o( latex tube. 3. Parenchyma. B. Crass-section of a part of blacic Indian hemp root. 4. Cross-section tA a large latex tube. 5. Parenchyma. Weber). 3. Cross-section of eanguinaria n 3. Cross-section of dandelion roo 41 canadenHs, L.}. CONDUCTING TISSUE 147 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 walk 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 greaUy in size, thick- ness 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 arrange- ment 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 ap- proach the square form. All typical cortical parenchyma cells have uniformly thick- ened 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., accord- ing 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. PLATE 51 Parenchyma Cells I. Longitudinal section of the cortical parenchyma of celandine root (Chflidonium majus^ L.) 2. Cross-section of the cortical parenchyma of sirsiiiKirilla root (Smilax officinalis, Kunth). A. Loc^tudioal section of the inth parenchyma of grindeUa stem (Grin- deUa i^uarma, (Purah) Dunal). I. Cell cavity. 3. CitMB-Bcction of the porouB end wall. 3. Surface view of the porous side wall. B. CTDM-aection of the pith parenchyma of grinddia stem. I. Cell cavity. 3. Porous walls. 3. Pitted end walls 150 HISTOLOGY OF MEDICINAL PLANTS LEAF PARENCHYMA The parenchyma cells (Plate 109, Fig. i) 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 sur- face. This arrangement makes it possible for the parench>Tna cells of the leaf to absorb more readily the enormous amoimt 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 occuring in the plant. Their walls are alwaj-s lignifiied 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 un- doubtedly 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 verv small, however, and the walls are very thin and non-pitted.