CHAPTER III MECHANICAL TISSUES The mechanical tissues of the plant form the framework around which the plant body is built up. These tissues are constructed and placed in such a manner in the different organs of the plant as to meet the mechanical needs of the organ. Many underground stems and roots which are subjected to radial pressure have the hypodermal and endodermal cells arranged in the form of a non-compressible cylinder. Such an arrangement is seen in sarsaparilla root (Plate 38, Fig. 4). The mechanical tissue of the stem is arranged in the form of solid or hollow columns in order to sustain the enormous weight of the branches. In roots the mechanical tissue is combined in rope-like strands, thereby effectively resisting pulling stresses. The epidermis of leaves subjected to the tearing force of the wind has epidermal cells with greatly thickened walls, particularly at the margin of the leaf. The epidermal cells of most seeds have very thick and lignified cell walls, which effectively resist crushing forces. The cells forming mechanical tissues are: bast fibres, wood fibres, collenchyma cells, stone cells, testa epidermal cells, and hypodermal and endodermal cells of certain plants. The walls of the cells forming mechanical tissues are thick and lignified, with the exception of the collenchyma cells and a few of the fibres. Lignified cells are as resistive to pulling and other stresses as similar sized fragments of steel. The hardness of their wall and their resistance to crushing explain the fact that they usually retain their form in powdered drugs and foods.<Callout type="important" title="Identifying Crystal-Bearing Fibres">Crystal-bearing fibres occur in the barks of frangula (Plate 19, Fig. i); cascara sagrada (Plate 19, Fig. 2); witch-hazel (Plate 19, Fig. 4); in cocillana (Plate 20, Fig. i); in white oak (Plate 20, Fig. 2); in quebracho (Plate 20, Fig. 3); and in Spanish licorice root (Plate 19, Fig. 3).</Callout> Crystal-Bearing Fibres of Barks 1. Frangula {Rhamnus Jrangula, L.). 2. Cascara sagrada {Rhamnus purshiana, D.C.). 3. Spanish licorice {Clycyrrkias glabra, L.)- 4. Witch-hazel bark {Hamamelis virginiana, L.).<Callout type="risk" title="Crystal Cells and Fibres Separation">The fibres which are crystal-bearing may be striated or porous, etc.; but owing to the fact that the grouping of the fibres and crystals is so characteristic, little or no attention is paid to the structure of the individual fibres. Crystal cells are exterior to the fibres, and in separating the fibres during the milling process the crystal cells are broken down and removed from the fibres.</Callout> Crystal-Bearing Fibres of Leaves 1. Cocillana (Cedrela odorata, L.). 2. White oak (Quercus alba, L.). 3. Quebracho (Aspidosperma quebracho-bianco, Schlechtendal). 4. Eucalyptus leaf (Eucalyptus globulus, Labill). 5. Senna leaf (Cassia angustifolia, Vahl.).<Callout type="tip" title="Leaf Fibre Identification">Having observed that a leaf has crystal-bearing fibres, in order to identify the powder it is necessary to locate one of the other diagnostic elements of the leaf — as the papillae of coca (Plate 21, Fig. i), or the hair of senna (Plate 21, Fig. 3), or the vessels in eucalyptus (Plate 21, Fig. 2).</Callout> Branched Bast Fibres occur in only a few of the medicinal plants, notable examples being tonga root and sassafras root. Occasionally one is found in mezereum bark.<Callout type="beginner" title="Branched Fibre Identification">The bast fibre of tonga root (Plate 22, Fig. 2) often has seven branches, but four- and five-branched forms are more common. The walls are non-porous, non-striated, and nearly white.</Callout> Porous and Striated Bast Fibres occur in blackberry bark of root, wild-cherry bark, and in cinchona bark.<Callout type="gear" title="Bark Sample Preparation">The fibres of blackberry root bark (Plate 23, Fig. i) have distinctly porous and striated walls; the cavity, which is usually greater than the diameter of the wall, contains starch.</Callout> Porous and Non-Striated Bast Fibres occur in marshmallow root and echinacea root.<Callout type="important" title="Fibre Length and Structure">The fibres of marshmallow (Plate 24, Fig. 3) usually occur in fragments. The walls have simple pores, and the diameter of the cell cavity is very wide; the pores on the upper or lower wall are circular or oval in outline (end view).</Callout> Non-Porous and Striated Bast Fibres occur in elm bark, stillingia root, and cundurango bark.<Callout type="risk" title="Fibre Breakage">The bast fibres of elm bark (Plate 25, Fig. i) occur in broken, curved, or twisted fragments. The central cavity is very small, and the walls are longitudinally striated.</Callout> Non-Porous and Non-Striated Bast Fibres occur in mezereum bark, in Ceylon cinnamon, in sassafras root bark, and in soap bark.<Callout type="important" title="Fibre Length Variation">In Saigon cinnamon (Plate 26, Fig. i) the bast fibres measure up to .900 mm. in length, so that in powdering the bark the fibre is rarely broken.</Callout> The pores, which are absent in many drugs, are, when present, either simple, as in echinacea root (Plate 24, Fig. 4), or they are branched, as in yellow cinchona (Plate 23, Fig. 3). In each of the above fibres the length and width of the fibre are shown.<Callout type="tip" title="Fibre Identification in Powdered Drugs">In powdered drugs bast fibres occur singly or in groups. The individual fibres may be broken, as in mezereum and elm bark, or they may be entire, as in Ceylon cinnamon and in sassafras bark (Plate 26, Figs. 2 and 3).</Callout> Wood Fibres always occur in cross-sections associated with vessels and wood parenchyma, from which they are distinguished by their thicker walls, smaller diameter, and by the nature of the pores, which are usually oblique, and fewer in number than the pores in the walls of wood parenchyma, and different in form from the pores of vessels. The wood fibre on cross-section (Plate 105, Fig. 4) shows an angled outline, except in the case of the fibres bordering the pith-parenchyma, etc., in which case they are rounded on their outer surface, but angled at the points in contact with other fibres.<Callout type="gear" title="Wood Fibre Identification">The wood fibres of santalum album are whitish-brown; of quassia, whitish-yellow; of logwood and santalum rubum, red.</Callout> Collenchyma cells form the principal medicinal tissue of stems of herbs, petioles of leaves, etc. In certain herbs the collenchyma forms several of the outer layers of the cortex of the stem. In motherwort, horehound, and in catnip the col- lenchyma cells occur chiefly at the angles of the stem.<Callout type="important" title="Collenchyma Cell Identification">In motherwort (Plate 29, Fig. B) there are twelve bundles, one large bundle at each of the four angles, and two small bundles, one on either side of the large bundle.</Callout> The coUenchyma cells retain their living contents at maturity. Many coUenchyma cells, particularly of the outer layers of the stem, have no cell contents. In some cases, however, starch occurs, as in the bast fibres of rubus.<Callout type="risk" title="Cell Content Loss">The color of the bast fibres varies, being colorless, as in Ceylon cumamon; or yellowish-white, as in echmacea; or bright yellow, as in bayberry bark.</Callout>
Key Takeaways
- Crystal-bearing fibres are important for accurate drug identification.
- Bast fibres can be identified by their structure and presence in certain plant parts.
- Wood fibres have distinct characteristics that differentiate them from bast fibres.
Practical Tips
- Learn to identify crystal-bearing fibres, as they are crucial for distinguishing between different medicinal plants.
- Understand the varying structures of bast fibres to accurately analyze powdered drugs.
- Recognize the importance of wood fibre identification in plant-based drug analysis.
Warnings & Risks
- Be cautious when separating crystal cells from fibres during milling, as this can affect their identification.
- Avoid misidentifying wood fibres with other types of fibres due to their similar appearance.
- Ensure proper handling and preparation of samples to prevent loss of cell contents.
Modern Application
While the techniques for identifying plant fibres have advanced, understanding these historical methods still provides valuable insights into plant anatomy. Modern tools can enhance accuracy but cannot replace the foundational knowledge gained from studying these traditional practices.
Frequently Asked Questions
Q: What are crystal-bearing fibres and how do they differ from other types of bast fibres?
Crystal-bearing fibres contain crystals within their structure, which distinguishes them from non-crystal bearing fibres. These fibres are found in the barks of specific plants like frangula and cascara sagrada.
Q: How can one identify crystal cells during the separation process?
During milling, crystal cells are often broken down and removed from the fibres. Therefore, it is important to note that crystal-bearing fibres may appear free of crystals in powdered form after processing.
Q: What are some practical applications of identifying wood fibres in plant-based drugs?
Identifying wood fibres helps in distinguishing between different medicinal plants and ensures accurate drug analysis. This is particularly useful for ensuring the correct species and quality of the herbal material being used.