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)

 Home Vegetation Topography






"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.


'Species Plantarum'

'Genera Plantarum'



1. Introduction                                                                                

          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.        

 2. Nomenclature                                                                                                                       Top   

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.


Category                                                                   Ending of names Examples
Division -phyta Spermatophyta
Class -mae Angiosperms
Subclass -neae Dicotyledoneae
Superorder -idae Rosidae
Order -ales Fabales
Family -aceae Fabaceae
Subfamily -oideae Papilionoideae
Tribe -eae Vicieae
Subtribe -ineae Vicineae

        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.

3. A Brief History of Classification                                                    Top

        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.

Class 1. Acotyledones -Cotyledons absent  
  2. Monoctyledones -With stamens hypogynous  
  3. Monoctyledones -With stamens perigynous  
  4. Monoctyledones -With stamens epigynous  
  5. Dicotyledones -Apetalae with stamens epigynous  
  6. Dicotyledones -Apetalae with stamens perigynous  
  7. Dicotyledones -Apetalae with stamens hypogynous  
  8. Dicotyledones -Monopetalae with corolla hypogynous  
  9. Dicotyledones -Monopetalae with corolla perigynous  
  10. Dicotyledones -Monopetalae with corolla epigynous and anthers united  
  11. Dicotyledones -Monopetalae with corolla epigynous and anthers distinct  
  12. Dicotyledones -Polypetalae with stamens epigynous  
  13. Dicotyledones -Polypetalae with stamens perigynous  
  14. Dicotyledones -Polypetalae with stamens hypogynous  
  15. Diclines -Irregulares.  

                A.P. de Candolee (1778-1841) modified Jussieu's classification by giving maximum stress to the importance of morphological characters. the chief merit of his classifications was the creation of subdivisions like Thalamiflorae, Calyciflorae and Corolliflorae of Diplochlamydeous (Dichlamydeous) group. Like his predecessors, the main demerit of his classification was the grouping of Gymnosperms with Dicotyledons without realizing its primitive status. Another demerit of his classification was the placing of Cryptogams along with Monocotyledons, which were distinctly separated by Jussieu. Yet, his classification had helped many of his followers to make a new concept in dividing the plant kingdom. the outline of his classification is as follows.


A. Vasculares (Vascular plants with cotyledons)

      Class a. Dicotyledones or Exogenae

           1. Diplochlamydae   (Both calyx and corolla present)

                 Series 1. Thalamiflorae


                                 Natural order                        1-8

                                 Natural order                        9-21

                                 Natural order                       22-44

                                 Natural order                       45-46

                  Series 2. Calciflorae

                                 Orders  (Families)                47-84

                  Series 3. Corolliflorae

                                 Orders (Families)                 85-108  

           II. Monochlaydae

                                  Orders (Families)               109-128

                                             (Order 128 is Coniferae)

       Class b.  Monocotyledons or Endogenae

            1. Phanerogamae

                                    Orders (Families)             129-150

                                    Orders (Families)             151-155 (Ferns etc.)

B. Cellulares (Plants without vascular bundles and cotyledons)

        Class a. Foliaceae

                                     Orders (Families)            156-157 (Mosses and Liverworts)

        Class b. Aphyllae

                                      Orders (Families)           158-161 (Algae, Fungi and Lichens)

           George Bentham (1800-1884) and Joseph Hooker (1817-1911) in their 'Genera Plantarum' contributed an outstanding system of classification of their own, in which they described 7,569 genera of flowering plants grouped into 202 orders (families in the current series) which in turn were grouped into cohorts (now recognized as orders). It is mainly based on the system of classification proposed by de Jussieu and modified by de Candolle. In this classification dicotyledons, gymnosperms and monocotyledons are placed on equal status. Dicotyledons are divided into three sub-classes, namely polypetalae, gamopetalae and monochlamydeae. The placing of polypetalae at the beginning is justified because free petals are considered to be a primitive character. In addition to the two divisions of polypetalae by de Candolle, they created one more divisions (series in Bentham and Hooker's system), disciflorae and placed in between thalamiflorae and calyciflorae. These three series show gradual evolutionary advance from marked hypogyny to epigyny through the transitional perigynous condition. The series are then divided into cohorts (orders) and orders (families).                                                                                             Top 

            It was during this time the great evolutionist Charles Darwin (1809-1882) published his 'Origin of Species' (1859), which helped the systematists to get a better idea of phylogenetic relationships in grouping plants on an evolutionary basis.

             Among the post-Darwinian systems, one of the renowned classifications is that of Adolf Engler (1844-1930) and Karl Prantl(1849-1893). Their system, an improved one of Bentham and Hooker's system, appeared in 'Die Naturalichen Pflanzenfamilien' (1887-1915), is a comprehensive world treatment of the plant kingdom. They divided phanerogams into gymnosperms, monocotyledons and dicotyledons. In this system, they corrected Bentham and Hooker's position of Gymnosperms and placed the latter before Angiosperms, but it is with the position of monocotyledons before dictos that most adverse criticism is leveled against this classification. According to them flowers without perianth are primitive and those with one whorl of perianth or two whorled (sepals and petals) perianth are considered advanced.  The chief merit of this classification is the merger of polypetalae and monochlamydeae into archichlamydeae. Another merit is the arrangement of families on the basis of evolutionary sequence, to a certain extent, and closely related families Liliaceae, Iridaceae, Amaryllidaceae, etc. are placed close to each other. Though there are drawbacks in this system, especially the position of monocotyledons before dicotyledons, Engler and Prantl's system is widely accepted and still followed in arranging the families of flowering plants in most part of the world.

             While delineating the history of classification, the names that should not be forgotten at this time are that of Charles Edwin Bessey (1845-1915) and Hans Halier (1868-1932). Bessey laid down certain principles called dicta or statements, regarding the primitiveness or evolutionary advancement of a plant group. He published his theories in his paper 'The phylogenetic Taxonomy of Flowering Plants' which evolved what is now known as the Besseyan system. His system of classification, applied for the group dicotyledones, has been the basis for arranging the families in all recent classifications.

               Layman benson (1957) in his  book 'Plant Classification" has agreed with most of the classificatory details given by Bentham and Hooker. Though his classification is a modified form of Bentham & Hooker's and Stebin's systems, the method adopted in arranging the orders has made his system more natural and phylogenetic. The orders, according to him, "are the real basis for the system of classification, may be made largely natural by thrusting aside preconceived notions of evolutionary or phylogenetic lines of development". John Hutchinson (1959) proposed a new system in his second edition of 'The Families of Flowering Plants' published in two volumes, which is somewhat like that of Bessey's system but differs from the latter in considering two main lines of flowering plant evolution in dicots. He divided the dicots  into Lignosae and Herbaceae on the basis of woody and herbaceous characters respectively.

              Among the contemporary taxonomists Armen Takhtajan is one of the prominent figures in this field who proposed another evolutionary system in his 'Flowering Plants: Origin and Dispersal' (1969). Unlike many other systems Takhtajan's classification is based on all available evidences which reflect a true phylogenetic relationship. According to him, the Angiosperms are monophyletic and monocotyledons originated from primitive dicotyledones. Arthur Cronquist (1988) in his 'Evolution and Classification of Flowering Plants', accepted most of the groupings proposed by Takhtajan and his predecessors except for certain changes. The two classifications differ in minute details but share in most of the major concepts. 

              Though we have  a number of classifications, ranging from artificial to natural to phylogenetic, sometimes we may find it difficult to locate the right family under the right order. One of the greatest difficulties in classifying these higher taxa of vascular plants is the creation of a reasonable alignment of the orders of Angiosperms. Segregation of Pteridophytes from Gymnosperms or the Gymnosperms from Angiosperms is relatively simple because of their distinct characters. Angiosperms, on the other hand, are the dominant plants of the present geological period, consists of a variety of plants and among them, the plants of many successful lines of evolutionary development are changing rapidly. Most groups are represented by a number of forms, and intermediate types between major groups are abundant. As C.L. Porter says "in spite of all patterns of evolution in the flowering plants that emerged from various studies made of them through the years, many unsolved riddles still exist".

4. Phylogeny                                                         Top 

                 It is now understood that all genera of a particular family or species or a genus do not evolve at the same speed. The rate of evolution depends upon the evolutionary potential and genetic plasticity operating upon them. Even though evolution involves all parts of a plant at a time. Some parts may proceed progressively whereas others may be retrogressive. However, the changes become permanent through several succeeding generations.

Some of the  primitive and advanced characters that determine the evolutionary position of a species are given in the following listing.

  Primitive Advanced
1. Perennials -Annuals
2. Erect Plants -Weak stemmed plants
3. Woody Trees -Shrubs and herbs
4. Green plants -Non Chlorophyllous plants
5. Terrestrial plants with normal habit -Aquatic plants, epiphytes, parasites and saprophytes
6. Dicotyledons -Monocotyledons
7. Tap root system -Adventitious roots
8. Few vascular bundles (Collateral) in a ring -Numerous vascular bundles in a scattered manner
9. Trachieds -Tracheae or vessels
10. Scalaeriform vessels -Pitted vessels
11. Alternate leaves -Opposite or whorled leaves
12. Simple leaves -Compound leaves
13. Reticulate venation -Parallel venation
14. Many flowered inflorescence -Solitary flower
15. Flowers with numerous flora parts with spiral arrangement -Flowers with fewer number of parts with whorled arrangement
16. Actinomorphic flowers -Zygomorphic flowers
17. Hypogyny -Perigyny and epigyny
18. Bisexual -Unisexual
19. Monoecious -Dioecious
20. Flowers possessing petals -Flowers without petals
21. Polypetalous -Gamopetalous
22. Numerous stamens -Few stamens
23. Free stamens -Stamens with adhesion or cohesion
24. Numerous carpels -Limited number of carpels
25. Apocarpy -Syncarpy
26. Simple and aggregate fruits -Multiple fruits
27. Seeds with two coats -Seeds with one coat
28. Endospermic seeds -Non-endospermic seeds
29. Embryo straight -Embryo curved
30. Epigeal germination -Hypogeal germination.

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