Macroscopic identity of medicinal plant materials is based on shape, size, colour, surface characteristics, texture, fracture characteristics and appearance of the cut surface. However, since these characteristics are judged subjectively and substitutes or adulterants may closely resemble the genuine material. It is often necessary to substantiate the findings by microscopy and/or physicochemical analysis.
Microscopic inspection of medicinal plant materials is indispensable for the identification of broken or powdered materials, in detection of adulterants, in identifying plant tissue characteristic features and as an initial screening test for impurities; the specimen may have to be treated with chemical regents. An examination by microscopy alone cannot always provide complete identification, though, when used in association with other evaluation methods, it can frequently supply invaluable supporting evidence.
Every plant has a characteristic tissues structure which can be seen by study of tissue arrangement, cell wall and configuration when properly mounted and viewed in a microscope.
Types of Microscopic Evaluation
Microscopic evaluation of herbal drugs is divided into two
- Qualitative Microscopy
- Quantitative Microscopy
1. Qualitative Microscopy
This method is used to identify herbal drug by their known histological characters through Transverse section (T.S.) or Longitudinal Section (L.S.). Some of the characteristic evaluated are below:
- Stomata: There are several types of stomata, distinguished by the forms and arrangement of the Surrounding cells, e.g. Anomocytic (irregulated celled eg Digitalis), Anisoytic (unequal – celled eg Datura), Diacytic (cross – celled eg Mentha, Paracytic ( parallel celled eg Senna).
- Trichomes (hairs): they are tubular elongated outgrowth of the epidermal cells. Different types of trichomes are unicellular trichomes eg Canabis, ulticelluler trichomes eg digitalis.
- Calcium oxalate crystals: it is an inorganic crystalline compound present in medicinal plant. They have specific shapes that can be utilized for the identification of herbal drug. Due to this reason, they are used as a diagnostic character in plants. Some of the shapes of calcium oxalate crystals seen are cubic as seen in senna, rhombic (diamond), rosettes as seen in cloves.
2. Quantitative Microscopy
It is used to determine the proportion of substance present by means of microscope. It involves different parameters like:
- Palisade Ratio: It is defined as average number of palisade cells beneath each epidermal cell. e.g. Atropa belladonna (6-10), Digitalis Lanata (2.5-6.5)
- Stomatal No: It is defined as average number of stomata per square millimeter area of epidermis.
- Stomatal Index: It is the percentage which the number of stomata forms to the total number of epidermal cells. It is calculated by, ( ) E S S S I + × = 100. Where, S.I. = Stomatal Index; S= Number of stomata per unit area; E= Number of Epidermal cells in the same unit area.
- Vein Islet Number: It is defined as average number of Vein Islet per square millimeter of the leaf surface midway between midrib and the margin.
- Vein Termination Number: It is defined as average number of Vein terminations per square millimeter of the leaf surface midway between midrib and the margin.