The importance of taxonomy

The exact naming (taxonomy) and an understanding of the species’ relationship to other species is an essential basis for pharmacognostical work. Only such endeavour allows the correct identification of a botanical drug, and consequently is the basis for further pharmacological, phytochemical, analytical or clinical studies.

Flower

The flower (Fig. 3.1) is the essential reproductive organ of a plant. It is frequently very showy in order to attract pollinators, but in other instances the flowers are minute and difficult to distinguish from the neighbouring organs or from other flowers.

Fig. 3.1 Schematic line drawing of a flower.

For an inexperienced observer, two characteristics of a flower are particularly noteworthy: the size and the colour. Although these are often good characteristics of a species, others aspects are more important from a botanical point of view.

Such characteristics include the form of the various parts of a flower, whether these parts are fused (joined) or separate (free), how many of each of these structures normally exist per flower, whether or not all flowers on a plant (or in a group of plants of the same species) are similar. Morphologically speaking, many parts of the flower are modified leaves, which during the development of higher plants have taken on specific functions for reproduction:

Another essential aspect of the flower’s morphology is the position of the gynaecium with respect to the position of the corolla on the pistil: i.e. epigynous (the corolla and other elements of the flower are attached to or near the summit of the ovary), or hypogynous (the corolla and other elements of the flower are attached at or below the bottom of the ovary).

Fruit and seed

The development of seeds occurred relatively late in the evolution of plants. The lower plants, such as algae, mosses and ferns, do not produce seeds. Gymnosperms such as the maiden hair tree (Ginkgo biloba) (see below) were the first group of organisms to produce seeds, from which the angiosperms or fruit-bearing plants evolved. The gymnosperms are characterized by seeds that are not covered by a secondary outer protective layer, but only by the testa – the seed’s outer layer.

In the angiosperms, the ovule and, later, the seed are covered with a specialized organ (the carpels) which in turn develops into the pericarp (Fig. 3.2). This, the outer layer of the fruit, can either be hard as in nuts, all soft as in berries (dates, tomatoes), or hard and soft as in a drupe (cherry, olives). Drugs from the fruit thus have to be derived from an angiosperm species.

Fig. 3.2 Simplified schematic representation of (a) a gymnosperm seed sitting on a scale (as in a fir tree) and (b) an angiosperm fruit, covered by both the testa and the pericarp.

The morphology of a fruit provides important information as to the identity of a plant species or medicinal drug. Another distinction of fruits is based on the number of carpels and gynaecia per fruit, which may be:

Leaves

The leaves arise out of the stem; their key function is the assimilation of glucose and its derivative, starch, from water and carbon dioxide (photosynthesis) using energy provided by sunlight.

The function of the leaves, as collectors of the sun’s energy and its assimilation, results in their typical general anatomy with a petiole (stem) and a lamina (blade). In many cases, the petiole is reduced and may be missing completely. Plants have adapted to a multitude of environments and this adaptation is reflected in anatomical and morphological features of the leaf. For example, adaptation to dry conditions gives rise to leaves that conserve moisture, which may be fleshy or possess a thick cuticle. These are termed xerophytic leaves, and include oleander (Nerium oleander L.).

The lower surface of the leaf is generally covered with stomata, pores which are surrounded by specialized cells and which are responsible for the gaseous exchange between the plant and its environment (uptake of CO₂ and emission of water vapour and O₂).

The nodes (or ‘knots’) are the parts of the stem where the leaves and lateral buds join; the intermediate area is called the internodium. A key characteristic of a species is the way in which the leaves are arranged on the stem. For example, they may be (Fig. 3.3):

Fig. 3.3 Types of arrangements of leaves.

Another important characteristic is the form of the leaves. Typically, the main distinction is between simple or compound. Simple leaves have blades that are not divided into distinct morphologically separate leaflets, but form a single blade, which may be deeply lobed. In compound leaves, there are two or more leaflets, which often have their own small petioles (called petiolules). The form and size of leaves are essential characteristics (Fig. 3.4a). For example, leaves may be described as oval, oblong, rounded, linear, lanceolate, ovate, obovate, spatulate or cordate. The margin of the leaf is another characteristic feature. It can be entire (smooth), serrate (saw-toothed), dentate (toothed), sinuate (wavy) or ciliate (hairy) (Fig. 3.4b). Also, the base and the apex often have a very characteristic form.

Fig. 3.4 (a) Characteristic shapes of leaves. (b) Characteristic margins of leaves.

Microscopic characteristics of leaves include the form and number of stomata, the inner structure of the leaves, specialized secretory tissues including trichomes (glandular hairs), covering trichomes or bristles, and the presence of calcium oxalate structures which give a characteristic refractive pattern under polarized light.

The powdered leaves of several members of the nightshade family (Solanaceae), which yield some botanical drugs that are important for the industrial extraction of the alkaloid atropine, cannot be distinguished using normal chemical methods since they all contain similar alkaloids. On the other hand, they can easily be distinguished microscopically by the presence of different forms of crystals formed by the different species and deposited in the cells (see Fig. 3.5).

Fig. 3.5 Calcium oxalate crystals, main forms: (a) rosette (e.g. Datura stramonium, Solanaceae); (b) sand (e.g. Atropa belladonna, Solanaceae); (c) monoclinic prism (e.g. Hyosyamus niger, Solanaceae); (d) needles (e.g. Iris germanica, Iridaceae); (e) raphides (e.g. Urginea maritima, Hyacinthaceae).

Shoots (= stem, leaves and reproductive organs)

An essential differentiation needs to be made between herbaceous (‘herbs’) and woody plants (trees and shrubs). In both cases, the function of the stem is to provide the physical strength required for positioning the leaves/flowers and fruit in the most adaptive way. The stem is a cylindrical organ which, together with the root, forms the main axis of a plant. Herbaceous species are generally short-lived and often grow rapidly and the distinction between the outside and the inner stem can only be made by detailed examination. Woody species, on the other hand, show a clear distinction between the bark and the (inner) wood.

In the stem, the transport of water and inorganic nutrients (upward transport) is achieved in the xylem, which only occurs in the inner parts of the stem and forms an essential part of the wood. The phloem, on the other hand, is the plant part responsible for the transport of assimilates (sugars and polysaccharides), which generally occurs from the leaves downwards. Between the wood and the bark is the cambium, the tissue that gives rise to new cells which then differentiate and form the outer (bark) and inner (wood) parts of a secondary stem. The fine structure of a bark or wood is an important diagnostic criterion for identifying a drug. The bark as an outer protective layer frequently accumulates biologically active substances; for example, several of the pharmaceutically important barks accumulate tannins.

Root

Three functions of a typical root are of particular importance to a plant:

The root is generally composed of an outer layer (the bark of the root including the hypodermis) and an inner cylinder, containing the xylem and the phloem. The two organs of the root are separated by the endodermis, an inner protective layer. Water and inorganic nutrients are transported upwards in the xylem; assimilates are transported in the phloem.

Very young plants have a primary root, which during development soon becomes thicker and adds layers of secondary tissue. It is the secondary roots – often roots or rootstocks with a special storage function – that are used in pharmacy.

Rootstock and specialized underground organs

Some underground organs can be distinguished on botanical grounds from the root. Although they may have some functions similar to roots, botanically they are derived from other parts of the plants; they are therefore a separate group of plant organs and yield another group of botanical drugs. They include rhizomes and tubers (generally, both are morphologically a stem) and underground bulbs (morphologically derived from parts of the leaves).

Diverse and unspecified botanical drugs

Some drugs are derived from the whole plant or from specialized organs (e.g. the bulbs in the case of garlic, Allium sativum L.). The exudates of Aloe vera (L.) Burman f. (syn. Aloe barbadensis) leaves are used as a strong purgative.