Lecture 01

Taxonomy of Flowering Plants

Plant taxonomy is the science that finds, describes, classifies, identifies, and names plants. It thus is one of the main branches of taxonomy.

Plant taxonomy is closely allied to plant systematics, and there is no sharp boundary between the two. In practice, "plant systematics" is involved with relationships between plants and their evolution, especially at the higher levels, whereas "plant taxonomy" deals with the actual handling of plant specimens.

Goals of plant taxonomy

The goals are the identification, nomenclature and classification of plants.

A. Plant identification is the determination of the identity of an unknown plant by comparison with previously collected specimens or with the aid of books or identification manuals.

B. Plant nomenclature is the determination of the correct name of a known plant according to a known system (International Code of Botanical Nomenclature; ICBN).

Vernacular name versus Scientific name

A vernacular name of an organism (a common name) is a name in general use within a community; it is often contrasted with a scientific name. On the other hand the scientific name has Greek or Latin origin. Linnaeus 1753 gave the Binomial Nomenclature (each plant should have two epithets; the first is the generic name written in capital letter and the second is the specific epithet).

C. Plant classification is the placing of known plants into groups or categories to show some relationship. Scientific classification follows a system of rules that standardizes the results, and groups successive categories into a hierarchy. For example, the family to which the lilies belong is classified as follows:

Kingdom: Plantae, Division: Magnoliophyta, Class: Liliopsida, Order: Liliales, Family: Liliaceae, Genera : ... ...

The classification of plants results in an organized system for the naming and cataloging of future specimens, and ideally reflects scientific ideas about plant inter-relationships.

Systems of Classification

A. Artificial Classifications classifies organisms by means of one or few characters.

B. Natural Classification reflects the situation as it is believed to exist in nature and utilizes all information available at the time such as habit of the plant.

C. Phylogenetic Classification classifies organisms according to their evolutionary sequence.

Determination the degree of primitiveness or evolutionary advancement of a plant group

Bessey's system was based on a series of "dicta" or statements of the guiding principles he used in determining the degree of primitiveness or evolutionary advancement of a plant group. Following are Bessey's dicta:

A. General dicta

1. Evolution is not always upward, but often it involves degradation and degeneration.

2. In general, homogeneous structures (with many and similar parts) are lower, and heterogeneous structures (with fewer and dissimilar parts) are higher.

3. Evolution does not necessarily involve all organs of the plant equally in any particular period, and one organ may be advancing while another is retrograding.

4. Upward development is sometimes through an increase in complexity, and sometimes by a simplification of an organ or a set of organs.

5. Evolution has generally been consistent, and when a particular progression or retrogression has set in, it is persisted in to the end of the phylum.

6. In any phylum the holophytic (chlorophyll-green) plants precede the colorless (hysterophytic) plants, and the latter are derived from the former.

7. Plant relationships are up and down the genetic lines, and must constitute the framework of phylogenetic taxonomy.

B. Dicta related to the general structure of the flowering plants

8. The stem structure with collateral vascular bundles arranged in a cylinder is more primitive than that with scattered bundles and the latter are to be regarded as derived from the former.

9. Woody stems (as of trees) are more primitive than herbaceous stems, and herbs are held to have been derived from trees.

10. The simple, unbranched stem is an earlier type, from which branching stems have been derived.

11. Historically the arrangement of leaves in pairs on the stem is held to have preceded the spiral arrangement in which the leaves are solitary at the nodes.

12- Historically simple leaves preceded branched ("compound") leaves.

13. Historically leaves were first persistent ("evergreen") and later deciduous.

14. The reticulated venation of leaves is the normal structure, and the parallel venation of some leaves is a special modification derived from it.

C. Dicta related to the structure of the flower

15. The polymerous flower structure precedes, and the oligomerous structure follows from it, and this is accompanied by a progressive sterilization of sporophylls.  

16. Petaly is the normal perianth structure, and apetaly is the result of perianth reduction (aphanisis).

17. The apochlamydeous perianth is earlier and the gamochlamydeous perianth is derived from it by symphysis of the members of perianth whorls.                                               

18. Actinomorphy is an earlier .structure than zygomorphy and the latter results from a change from similar to a dissimilar growth of the members of the perianth whorls.

19. Hypogyny is the more primitive structure, and from it epigyny was derived later.

20. Apocarpy is the primitive structure, and from it syncarpy was derived later.

21. Polycarpy is the earlier condition, and oligocarpy was derived from it later.

22. The endospermous seed is primitive and lower, while the seed without endosperm is derived and higher.

23. Consequently, the seed with a small embryo (in endosperm) is more primitive than the seed with a large embryo (in scanty or no endosperm).

24. In earlier (primitive) flowers there are many stamens (polystemonous) while in later flowers there are fewer stamens (oligostemonous).

25. The stamens of primitive flowers are separate (apostemonous), while those of derived flowers are often united (synstemonous).

26. The condition of powdery pollen is more primitive than that with coherent or massed pollen.

27. Flowers with both stamens and carpels (monoclinous) precede those in which these occur on separate flowers (diclinous).

28. In diclinous plants the monoecious condition is the earlier, and the dioecious later.

Trends in plant taxonomy

1. Orthodox Criteria

a-The macro-morphological characters

include the characters of the root system and other underground parts (rhizome, tuber, bulb). The macro-morphological characters of the aerial parts include the characters of stem, leaf, flower, inflorescence, fruit... etc.

b- The micro-morphological characters

The anatomical characters of the vegetative structures have been of

importance in separating higher categories, such as gymnosperms from

angiosperms and monocot from dicot.

2. Palynological criteria

Palynology is the study of pollen grains and spores including pollen morphology such as:

1. Composition of pollen grains

2. Stratification of the pollen wall (sporoderm)

3. Exine sculpture

4. Number, type and position of apertures in sporoderm

5. Shape of pollen.

3. Chemotaxonomy Criteria

Plant Chemotaxonomy is the application of chemical data to systematic problems.

4. Embryology Criteria

The embryological characters of taxonomic value in delimiting plant groups

include the anther; development and organization of the pollen grains; development and structure of the ovule; megasporogenesis and development of the embryosac; fertilization; endosperm; embryo and seed coat.

5. Cytology

The utilization of the characters and phenomena of cytology for the explanation of taxonomic problems is referred as cytotaxonomy.

6. Molecular Systematics

The molecular analyses of the three angiosperm genomes: chloroplast DMA

(cp DMA), mitochondrial DMA (mt DNA) and nuclear DMA (n DNA) are applicable to the study of phylogenetic and taxonomic relationships.

7- Physiological Criteria

Plants show differences in their metabolic process and chemical reaction. On this basis, similarities and differences can be established amongst the plants.