Plant Diversity of Saudi Arabia
An introduction to the early history of
(Courtesy - The identification of Vascular Plant-Families in Saudi Arabia- Alfarhan & J. Thomas, 1994)
"In spite of all patterns of evolution in flowering plants that emerged from various studies made of them through the years, many unsolved riddles still exist" C.L. Porter.
Plant Taxonomy, one of the oldest of all biological disciplines, is often shunned by students of biology and the beginners in particular due to the tiresome terminology used in identifying the plant specimens, which forces them to study intensively hundreds of Latin terms without knowing what exactly they refer to. Taxonomy is a common term now used in all branches of science, deals with the laws and principles of classification. The term taxonomy, coined by French botanists De-Candolle in 1813, derives from two Greek words 'taxis' meaning arrangement and 'nomia' meaning distribution. The prime aim of taxonomy is to identify all kinds of plants one comes across and to arrange them into a scheme of classification that will show their true relationships. This includes three different aspects, namely identification, naming and classification. Though these terms are independent in their respective category, they are, in practice all mutually connected. Identification means to perceive a plant material as already known or different form another known entity. Naming involves the process of giving names to the plants according to an International Code of Nomenclature in order to have a means of communication, and classification is the grouping of plants on the basis of similar characters.
The usage of the "Scientific Names"
Previously there were no scientific names for the plants. All languages have common or vernacular names for almost all the important plants of the countries concerned, but the vernacular names of a given plant naturally differ in various languages. This had been a problem for the botanists and for the laymen in general in the absence of a common basis for communication. an attempt to reduce this perplexity started during the Theophrastus period and later on by many scientists, but their systems of naming plants could not handle the difficulties associated with naming a large number of plants by the use of names with only the generic terms. Bauhin and Tournefort postulated a new system in which they started applying adjectives in distinguishing different species. Thus, for example, the name of a species belong to the genus Caryophyllum that grows in rocks with grass-like leaves and umbellate corymbs was Caryophyllum, sextalis, folies, gramineus, umbellatus, corymbis. Their system also did not work because the names became very long and could not be used with ease.
In 1753, Linnaeus devised a new system, the "Binomial Nomenclature", which became universally accepted by botanists and is in vogue up to the present day. However, in the beginning, due to inadequate flow of information among botanists of the world, the same species was, in many cases, described under different names by different authors from different parts of the world. The result was the creation of about 830,000 names recorded by Index Kewensis for the period 1753-1965, where as the actual number of species referred to by these names was only about one third of this figure. This revealed the need for the formulation of an internationally accepted rules and regulations in order to stabilize the plant names. Thus an International Botanical Congress was set up during the middle of the 19th century for the establishment of a 'Code' for botanical nomenclature. Since then the code has been periodically reviewed at a number of international Botanical Congresses and subjected to modifications. The latest version of the International Code of Botanical Nomenclature which governs the botanical nomenclature all over the world was adopted by the XII International Botanical Congress in Leningrad and published in 1978. According to the Code, every plant is given a scientific name which has two Latin parts, i.e., 1. The generic name or genus name starting with a capital letter and, 2. The specific name or species' name starting with a small letter. Thus the scientific name of the common date palm tree is Phoenix dactylifera. The specific name dactylifera cannot be given to any other species belonging to the genus Phoenix but can be given to any other species belonging to another genus of the same family or another family. When a species is divided into two or more subspecies these will receive a third, subspecies, name as for example Ipomoea sinensis ssp. blepharosepala.
The International Code of Botanical Nomenclature gives broad outline for the use of the Code including typification, author citation, effective and valid publication etc. The first part or the Code deals with the ranks of taxa in plant classification. Every individual plant is treated as belonging to a number of categories with species as the basic unit. The Code also has certain directives to arrange these categories in their descending order and stipulates that the name of every taxonomic group should end in a specific manner. The main categories and the stipulated ending of their names are as follows.
The names formed before the formation of the Code, which do not conform to these endings should be changed accordingly. Thus, for example, the family names of Compositae (Asteraceae), Cruciferae (Brassicaceae), Gramineae (Poaceae), Guttiferae (Clussiaceae), Labiatae (Lamiaceae), Leguminosae (Fabaceae), Palmae (Arecaceae) Umbelliferae (Apiaceae) have been changed to comply with the rules of the Code. But in view of the previous extensive use of the old names, these may still be used as authorized by the Code.
Classification is the process of establishing and defining systematic groups on the basis of similar characters. As mentioned by Porter (1959), it has always been one of man's characteristics that he likes to have names for things and to arrange things in a somewhat orderly manner. It was the early philosophers and medical practitioners of Greek and Roman civilizations who started an attempt to classify the plants for the first time. Since then a number of classifications were recorded for the Angiosperms. None of these classifications are completely satisfactory and beyond criticism. All systems of classification including those of Benthem and Hooker, Engler and Prantle, Rendle or that of Hutchinson have their own merits and demerits and each one, to a certain extent, is an improvement of the older one.
All the earlier classifications were purely artificial, based on certain readily observed characters of plants, irrespective of their genetic or evolutionary significances, such as herbs, sub-shrubs, shrubs and trees or annuals, biennials and perennials. The natural or phylogenetic systems on the other hand, is framed on an evolutionary basis. Now in modern systems, the science of taxonomy is not simply based on floral or vegetative characters but also on data-like cytological, anatomical, embryological, palynological as well as various chemical analysis.
During the transition period, from artificial to phylogenetic through natural system to develop a classification that reflects the true phylogenetic relationship of the whole plant kingdom, a number of classifications have been postulated by various scientists. Since it is a laborious attempt to mention all the classifications, and also beyond the scope of this book, we can only introduce some of them in this brief discussion.
Taxonomy started with Greeks and Roman during the period of Theophrastus, Dioscorides and Pliny, the pioneers of their age. Theophrastus described about five hundred species of plants in his 'History of Plants', the oldest botanical work in existence. In those days plants were classified on the basis of habit. He divided plants into herbs, shrubs or trees and recognized annuals, biennials and perennials. After the decline of Greek and Roman civilizations, no attempt was made in the field of botany for a long time except for some works by herbalists. Among them, the most noteworthy work was that of Kaspar Bauhin in 1620, who made a remarkable attempt to utilize the binomial nomenclature. Though he did not use the names exclusively, in those days they were indeed significant.
This was followed by the transition period, a stage towards the modern period, starting from Andrea Caesalpino (1519-1603) to Carolus Linnaeus (1707-1778). The old concept of classifying the plants on the basis of herbs, shrubs or trees had been changed by Jaochim Jung (1587-1657). He clearly defined nodes, internodes, different types of leaves, perianth for the calyx, stamens, styles and the capitulum of Compositae (Asteraceae), etc. John Ray (1627-1705), also of this period added some important characters in classifying plants. He retained the categories herbs, shrubs or trees and divided each group into dicotyledons and monocotyledons. It was from his classification that his followers got the idea of the number of cotyledons and that it could be used in segregating the families.
'Species Plantarum' published in 1753 by Carolus Linnaeus described about 7300 plant species and used the system of binomial nomenclature. He divided the plants into 24 classes on the basis of sexual characters, i.e. the stamens and carpels, their cohesion, adhesion, etc. He also gave much importance to hypogyny, perigyny and epigyny. The defect of this classification mainly lies in giving no importance to the number of cotyledons and to the polypetalous and gamopetalous conditions. Another demerit of his classification is the failure in recognizing the importance of naked-seeded plants (Gymnosperms) and included among Angiosperms. Inspite of all these defects, the name of Linnaeus remains in the history of classification as one of the pioneers in this field.
The period of modern classification started towards the end of the eighteenth century. A.L. de Jussieu (1748-1836) grouped all plants into 15 classes. His system recognized the importance of cotyledons and their number. the subdivisions into classes were used on the position of stamens and ovary. Like Linnaeus, he too failed to segregate to segregate Gymnosperms from dicotyledons.