General principles of botany: morphology and systematics

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Chapter 3 General principles of botany

morphology and systematics

The chapters in this section provide a short introduction to the bioscientific basis for all aspects of the use of plants in pharmacy required for understanding herbal medicines and pure natural products.

The following case study shows that knowledge about medicinal plants is not only relevant, because pharmacy uses many pure natural products derived from plants, but also that pharmacists can and should advise patients about common medicinal plants.

A (Hypothetical) Case Study Based on G Hatfield’s Research About the Usage of Medicinal Plants in Norfolk

While you are working as a locum pharmacist, a patient informs you that his general practitioner is worried about unexplained low levels of potassium (hypokalaemia). Among other things, the patient is complaining of chronic constipation and requests several pharmaceuticals. He also reports that he uses a ‘herbal tea’, which he prepares from the plant he calls ‘pick-a-cheese’ and grows in his back garden. This tea helps him to overcome the problem of constipation.

How do you react? Is the patient using a little known, but unproblematic, herbal product? Further inquiry about the case gives you the following information:

image ‘Pick-a-cheese’ may be a widely distributed garden plant and weed known also as ‘common mallow’ which has the botanical name of Malva sylvestris or it may be some other botanical species known under the same common name.

image Upon your request the patient brings you a branch of the plant and with the help of the scientific (botanical) literature you make a positive identification of this plant as Malva sylvestris. The identification is based on the features of the plant (leaves, fruit, flowers) that you are able to observe.

image In checking the active constituents (especially polysaccharides) you come to the conclusion that the plant is unlikely to contribute to the symptoms as they were reported by the patient. The plant is widely used as a local food item (also, for example, in Mediterranean France) and as a household remedy. Toxic natural products seem to be absent. Therefore, the search for a cause of the hypokalaemia continues…

[For further information on Norfolk country remedies readers are referred to Hatfield G 1994 Country remedies. The Boydell Press, Woodbridge.]

Plants and drugs

Pharmacognosy is the study of medical products derived from our living environment; especially those derived from plants and fungi. From the botanical point of view, the first concern is how to define a pharmaceutical (or medical) plant-derived drug.

In the context of pharmacy a botanical drug is a product that is either:

• derived from a plant and transformed into a drug by drying certain plant parts, or sometimes the whole plant, or

• obtained from a plant, but no longer retains the structure of the plant or its organs and contains a complex mixture of biogenic compounds (e.g. fatty and essential oils, gums, resins, balms).

The term ‘drug’ is linguistically related to ‘dry’ and is presumably derived from the Middle Low German droge (‘dry’).

Isolated pure natural products such as the numerous pharmaceuticals used in pharmacy are thus not ‘botanical drugs’, but rather chemically defined drugs derived from nature. Botanical drugs are generally derived from specific plant organs of a plant species. The following plant organs are the most important, with the Latin name that is used, for example in international trade, in parentheses:

• Aerial parts or herb (herba).

• Leaf (folia).

• Flower (flos).

• Fruit (fructus).

• Bark (cortex).

• Root (radix).

• Rhizome (rhizoma).

• Bulb (bulbus).

The large majority of botanical drugs in current use are derived from leaves or aerial parts.

Botanically speaking, a plant-derived drug should be defined not only in terms of the species from which it is obtained but also the plant part that is used to produce the dried product. Thus, a drug is considered to be adulterated if the wrong plant parts are included (e.g. aerial parts instead of leaves).

In the following sections of this chapter a brief overview of botanical taxonomy is given; then the higher plants are discussed on the basis of their main organs, function, morphology and anatomy. Since most of the pharmaceutical products derived from plants are from the higher plants (or Magnoliopsida), little reference is made here to other plants such as lichens, mosses, algae, or to mushrooms or micro-organisms.

Microscopic characteristics play an important role in identifying a botanical drug. Although microscopy is now only rarely used in everyday pharmaceutical practice, there are a large number of features that allow the identification of botanical material. Since classical textbooks provide an extensive description of such features, microscopic identification is only occasionally discussed in this introductory work.

These days, drug identification is achieved using a combination of methods, including thin-layer chromatography, high-performance liquid chromatography and microscopic methods. In large (phyto-)pharmaceutical companies, near-infrared spectroscopy has become an essential tool.

Taxonomy

The species is the principal unit within the study of systematics. Biological diversity is subdivided into > 500,000 discontinuous units (the botanical species) and > 2 million zoological species. The species is thus the basic unit for studying relationships among living organisms. Systematicists study the relationships between species.

Taxonomy is the science of naming organisms and their correct integration into the existing system of nomenclature. Each of these names is called a taxon (pl. taxa), which thus stands for any named taxonomic unit. In order to make this diversity easier to understand, it is structured into a series of highly hierarchical categories, which ideally should represent the natural relationship between all the taxa.

Example of Botanical Classification

The opium poppy, Papaver somniferum L.

Binomial: this is the genus and species names, plus the authority. Thus, in this example, Papaver somniferum is the binomial (the basic unit of taxonomy and systematics). It is followed by a short acronym (in this case ‘L.’), which indicates the botanist who provided the first scientific description of the species and who assigned the botanical name [in this example, ‘L.’ stands for Carl von Linnaeus (or Linné), a Swedish botanist (1707–1778) who developed the binomial nomenclature].*

Species: somniferum, here meaning ‘sleep-producing’.

Genus: Papaver (a group of species, in this case poppies, which are closely related).

Family: Papaveraceae (a group of genera sharing certain traits, named after one of the genera).

Order: Papaverales.

Class: Magnoliatae.

Subphylum: Magnoliphytina (seed-bearing plants with covered seeds).

Division (= Phylum): Spermatophyta (seed-bearing plants).

Kingdom: Plantae (the plants), one of three kingdoms, the others being the animals and fungi.

* In some cases, there is first a name in parentheses, followed by a second name not in parentheses. For example, in the case of the common aloe, Aloe vera (L.) Burm. f., the name in parentheses indicates the author (Linnaeus) who first described the species but assigned it to a different genus. The second name in this case, Burm. f., stands for the 18th century botanist Nicolaas Laurens Burman; f. stands for filius (son), since he is the son of another well-known botanist who provided numerous first descriptions of botanical species.

A species is generally characterized as having morphologically similar members and being able to inbreed. Since Carl Linnaeus, the names of species are given in binomial form: the first part of the name indicates the wider taxonomic group, the genus; the second part of the name is the species. In order to better understand biological diversity, the species are arranged into clusters of varying degree of similarity, forming a hierarchy.

The basic classification of the plant kingdom into divisions circumscribes the main groups of plants, including the following:

• Algae, including the green algae (Chlorophyta) and the red algae (Rhodophyta)

• Mosses (Bryophyta)

• Ferns (Pteridophyta)

• Seed-bearing plants (Spermatophyta).

As mentioned above, only a few algae, mosses and ferns have yielded pharmaceutically important and will therefore only be discussed very cursorily.

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.

Morphology and anatomy of higher plants (spermatophyta)

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