In addition to measuring the size of cells and tissues with the eyepiece or stage micrometers, a number of other quantitative values are utilized for the microscopic evaluation of botanicals (predominantly leaves). These include palisade ratios, vein islet and stomatal numbers, and sto-matal index:
Palisade ratios. The palisade ratio is the average number of palisade cells that occur beneath an epidermal cell. The palisade ratio of many plants remains constant regardless of geographical location of the plant. This can give general subjective information about the characteristics of a plant and can be used to distinguish some closely related species from each other (e.g., Agathosma spp.). The palisade ratio, however, is not applicable to monocot leaves due to a lack of consistent differentiation within the mesophyll (Mukherjee 2002).
Vein islet (veinlet) and vein termination. The term "vein islet" is used to indicate the smallest unit of photo-synthetic tissue encircled by the ultimate divisions of the conducting strands of leaves. The vein islet number is the number of vein islets per square millimeter and is calculated from four contiguous square millimeters in the central part of the lamina, midpoint between the midrib and margin. Various botanicals exhibit consistent values that can allow for the differentiation of closely related species (e.g., Erythroxylum spp.) and can be used when other measures, such as palisade ratio, cannot distinguish between closely allied species. The vein or veinlet termination is the ultimate free termination of a vein or branch of a vein (Trease and Evans 1966).
Stomatal number and stomatal index. The quantification of stomata is a specific assessment tool for leaves. The stomatal number is the average number of stomata per square millimeter of leaf epidermis and the total number of stomata on the leaf. The stomatal index is the percentage proportion of stomata on one surface of the leaf plus the epidermal cells and stomata on the opposite side. Although the stomatal number varies greatly with the age of the leaf, the stomatal index remains highly consistent.
More detailed information on how these measurements are developed is readily available in the primary pharma-cognosy literature.
Table 4.2 Microscopic Characters Used in Computer-Aided Identification of Botanical Ingredients
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