The following points highlight the ten modern trends in taxonomy of Indian flowering plants. The trends are: 1. Gross Morphological 2. Anatomical 3. Pollen Morphology or Palynological 4. Chemotaxonomy 5. Serotaxonomy 6. Palaeontological 7. Ontogenetical 8. Cytogenetics and Biosystematics 9. Embryology and Taxonomy 10. Numerical Taxonomy.
Taxonomy: Trend # 1. Gross Morphological:
Turril says, the morphology, either of plants or animals, now days includes a good deal more than the mere study of shape. The term ‘morphology’ is very wide and includes anatomy, and much of cytology, ontogeny, embryology, palaeontology and genetics. He says, “even within morphology in a very strict sense the taxonomist is forced to consider matters which involve physiology.”
Turril says, that someone or other morphological classification provides the best basis for a general classification of the widest possible use, because morphological characters are, in general easier to use than others. According to him the great value of a classification based essentially on gross morphology is that it enables correlations of the characters to be easily determined.
Taxonomy: Trend # 2. Anatomical:
The study of morphology with a compound microscope is known as anatomy (histology). According to some anatomists the anatomical features are more conservative than those of-gross morphology and are, therefore, of greater use in tracing phylogeny or organogenesis. Vesque has given much emphasis upon the value of anatomy in taxonomic and phylogenetic research.
Tippo has proposed the lines of evolution of stem structure, more particularly for the secondary xylem of angiosperms. The view has been accepted by most plant anatomists. Stem anatomy shows that the Magnoliales are a relatively primitive of the angiosperms.
Chalk advocates that the structure is probably more conservative than that of the flower and the minor floral differences by which many species are distinguished from one another are not reflected in the wood; thus the wood of different species of a genus are often indistinguishable.
According to Chalk there is one danger in using wood anatomy in classification is that wood usually means secondary wood, and the stressing of criteria derived, at least, mainly from the secondary wood of trees and large shrubs may give results statistically biased against herbaceous parts of woody groups.
The following anatomical characters are of taxonomic significance:
1. Epidermal appendages, trichomes and emergences of all kinds.
2. Structure of stomata.
3. The wood anatomy including the elements of the secondary xylem; this is very useful in modem taxonomic studies, particularly in establishing the phylogenetic relationships among taxa, and in many cases serves as an aid in the identification of orders.
4. The nodal anatomy of the axis, the leaf trace nature.
5. Floral anatomy.
Epidermal emergences of all kinds:
All unicellular and multicellular appendages are known as trichomes; more massive structures, e.g., warts, spinous bodies both epidermal and sub-epidermal are called emergences. The use of trichomes in taxonomic studies is of much value, and has been known for many years.
In identification of some families of angiosperms, the presence of a particular type of trichomes helps a lot, e.g., simple unicellular or multicellular uniseriate trichomes occur in many taxa; similarly the occurrence of glandular trichomes of various kinds forms an aid to taxonomy in some respects.
Structure of stomata:
Stomata which also belong to the epidermal tissue system have been the subject matter of studies, which is very useful in the fields of taxonomy. Stomata have been classified according to the position of subsidiary cells, guard cells in relation to the aperture.
They are of the following types:
1. Ranunculaceous or anomocytic. Type A:
(Anomocytic = irregular celled). This type is characterised by having a limited number of subsidiary cells which are quite alike the remaining epidermal cells; the accessory or subsidiary cells may be four or five in number. In most of the cases, these subsidiary cells are just like the other epidermal cells. This type of stomata occur in Ranunculaceae, Capparidaceae, Malvaceae and some other families.
2. Cruciferous or anisocytic. Type B:
(Anisocytic = unequal celled). This type of stomata occur in Cruciferae (Brassicaceae) and many genera of Solanaceae. In this type, each stoma remains surrounded by three subsidiary cells of which one is distinctly smaller than the other two.
3. Rubiaceous or paracytic. Type C:
(Paracytic =parallel celled). In this type, the stoma remains surrounded by two subsidiary or accessory cells which are parallel to the long axis of the pore and guard cells. This type of stomata occur in Rubiaceae and allied families.
4. Caryophyllaceous or diacytic. Type D:
(Diacytic = cross celled). In this type each stoma remains surrounded by a pair of subsidiary or accessory cells and whose common wall is at right angles to the guard cells. This type of stomata occurs in Caryophyllaceae and allied families.
5. Gramineous type:
The gramineous stoma possesses guard cells of which the middle portions are much narrower than the ends so that the cells appear in surface view like dump-bells. They are commonly found in Gramineae (Poaceae) and Cyperaceae of monocotyledons (see fig. 8.6).
Wood anatomy:
The plant anatomists have established a number of lines of phylogenetic specialisation in the stelar structure on the evolutionary basis and have applied the same in establishing data regarding the primitiveness of taxa and relationships between them and ultimate determination of taxon position.
These lines of anatomical specialisation are as follows:
1. The protostele is more primitive than siphonostele or dictyostele.
2. In angiosperms the woody axis of trees and shrubs is more primitive than the stem of herbs.
3. Vessel elements with scalariform perforation plates are more primitive than vessels with single opening in the perforation plate (i.e., simple perforation).
4. The type with numerous bars and narrow openings is more primitive than the type with few bars and wide openings.
5. Vessel elements, long, small in diameter, angular in cross section are more primitive than those with short ones having broad circular cross sections; also the vascular elements with long sloping end walls are more primitive than those having non-sloping transverse walls.
6. Scalariform type of pitting on the lateral walls of the vessel is more primitive; opposite and alternate pitting are advanced.
7. Solitary or single pores occurring in vessel arrangement throughout the wood (i.e., solitary pores) is more primitive than aggregate groupings of various types, such as pore multiples, pore fascicles, pore chains, etc.
8. Diffuse porous conditions of wood is more primitive than ring porous condition.
9. Evolutionary line of origin runs as Tracheids-to fibre-tracheids to libriform wood fibres-to scalariform tracheids-to circular bordered pitted tracheids.
10. Diffuse wood parenchyma is more primitive than various aggregate arrangement. Apo-tracheal wood parenchyma is more primitive than paratracheal wood parenchyama.
Heterogeneous rays, i.e., rays with both the vertically as well as radially elongated cells are less specialised and primitive than homogeneous rays in which cells are only radially elongated.
Schemes based on floral morphology and together with anatomical characters must go together in determination, of taxonomic position, generic identification, etc.
For example, Amentiferae according to Engler is most primitive in the Dicotyledons, but the wood anatomy shows that this taxon is very advanced in comparison to Magnoliales-Ranales.
Amentiferae:
Simple perforation plates, i.e.; an advanced character; vessels are ring porous, short, round in cross section, large in diameter with alternate or opposite pitting, i.e., an advanced character; libriform wood fibres prevail, an advanced character.
Phylogenetic specialisation of tracheary and fibre cells:
The specialisation of tracheary elements (i.e., tracheids and vessels) was concomitant of the separation of functions of conduction and imparting mechanical rigidity in the vascular plants which took place during the evolution of terrestrial plants (Bailey, 1953). In the less specialised state, conduction of water and mechanical support are both undertaken by tracheids; with the increasing specialisation woods evolved with better conducting elements, the vessels or tracheae which are more efficient in conduction than in support.
On the other hand fibres took their origin as mainly strengthening elements. Thus, from primitive tracheids two lines of specialisation evolved in two directions, one towards the vessel, another towards the fibres.
Vessels arose separately in several groups (taxa) among Tracheophyta, i.e., vascular plants. The common accepted idea is that the vessels evolved independently in angiosperms. Evidence supports that in the Dicotyledons vessels originated and undergone specialisation first in the secondary xylem, then in metaxylem and were lost in the early primary xylem, i.e., protoxylem.
The evolutionary sequence of the vessel members of the secondary xylem of the Dicotyledons began with long scalariform pitted tracheids (primitive character) as found in the very primitive dicotyledons, such as Winteraceae, Trochodendraceae, Tetracentraceae, Chloranthaceae, etc. These tracheids were succeeded by vessel members of long and narrow form with tapering ends.
The cells became short progressively became wider and less inclined and finally transverse. In very primitive state the perforation plate was scalariform with numerous bars resembling a wall with scalariformly arranged pits without pit membranes; further specialisation went on, as a result number of bars decreased and finally total elimination of bars took place resulting in the simple perforation (advanced).
The pitting on vessel walls also underwent change in course of evolution. In inter-vessel pitting bordered pit pairs in opposite arrangement were changed into smaller bordered pit pairs in opposite arrangement then to alternate arrangement. The evolutionary sequence was from tracheids to fibre tracheids to libriform fibre.
Wood parenchyma:
In the secondary wood (xylem), there are two types of parenchyma, i.e.:
(a) Axial parenchyma,
(b) Ray parenchyma, i.e., radial parenchyma, these two types of parenchyma are almost similar in their wall structure and contents. They store starch, oils and many other ergastic substances; tannins and crystals are common, the types of crystals and their arrangements serve much in the identification of wood. The distribution of axial parenchyma in the secondary wood exhibits many intergrading patterns.
The touch relation of the parenchyma with regard to the vessels as observed in transverse sections under the microscope shows two principal patterns, e.g., (a) apotracheal and (b) paratracheal; the former is primitive condition, the latter is advanced.
Apotracheal parenchyma is further classified into (a) diffuse, in which single parenchyma cell or parenchyma strand scattered among fibres and (b) banded apotracheal in which parenchyma cells in a line form band (not in touch) over and under the vessels.
The paratracheal parenchyma presents two types also, e.g.:
(a) Vasicentric (very advanced), it forms a complete sheath in touch and round the vessel and
(b) Confluent in which coalesced parenchyma form irregular or tangential or diagonal bands.
Structure of the rays:
In contrast to the predominantly uniseriate rays of Pinus (Gymnosperm-conifer) those of the Dicotyledons may be one too many cells wide, i.e., the rays (secondary rays) may be uniseriate and multiseriate. The structure of rays also plays an important role in helping to identify the primitiveness of the wood anatomy.
Rays may range in height from one to many cells. The appearance of rays in radial and tangential sections can be used as basis for classification.
Arrangement of vessels:
The arrangement of vessels in the secondary xylem of Dicotyledons is a characteristic feature and is used in the identification of species. When the vessels are more or less equal in diameter and are uniformly distributed throughout the wood or when there is gradual change in size and distribution throughout the growth ring the wood is known as “diffuse porous”, as found in Salix, Populus, Acacia, etc.
When the wood consists of vessels of different diameters and in which those produced at the beginning of the season are distinctly larger than the late wood, the wood is called “ring porous”, as found in Fraxinus, Quercus, etc. From the phylogenetic point of view diffuse porous wood is primitive than ring porous wood.
Wall thickenings of the vessels:
In many vascular plants (Tracheophyta) the secondary wall thickenings of the protoxylem are annular or spiral. From an ontogenetic viewpoint the annular thickenings develop prior to the spiral. In later formed tracheary elements the spiral bands became joined in certain areas giving rise to scalariform thickenings.
In tracheary elements developed at later ontogenetic stage, the thickening of wall becomes reticulate; when the openings in the secondary wall of such a network are elongated in a direction perpendicular to the longitudinal axis of the element, the thickening is known as scalariform-reticulate.
Ontogenetically the most advanced type of thickening is pitted which occurs in the late primary and secondary xylem (wood). The pitting according to their arrangements are known as (a) scalariform, (b) opposite and (c) alternate.
When the pits are transversely elongated and are consequently arranged in ladder-like manner, it is termed scalariform; when circular pits are arranged in horizontal or transverse rows they are known as opposite; when in diagonal series, known as alternate. The different types of pitting are of taxonomic importance.
Vessels and the structure of perforation plates:
Vessels have limited length and in those vessel members which end in a tube bearing perforation at the terminal end, sometimes perforations also are present on the side wall. The end or side wall bearing perforation is known as perforation plate. The perforation plate consisting of a single large pore is known as simple perforation.
In case of end wall containing numerous perforations they may be scalariform perforation plate in which perforations are arranged in parallel series in ladder-like fashion, reticulate perforation plate is distinguished by the arrangement of perforations in reticulate or netlike fashion; when end wall has many circular perforations arranged in scattered fashion, it is known as foraminate type characteristic of many gymnosperms and a few angiosperms.
The following structural features of angiosperm tracheary elements are used as the basis for the study of evolution:
1. The length of the element, e.g., tracheids are long cells, the average length in Trochodendron is 4.35 mm, in the monocots the same is 5.07 mm.
2. Diameter of tracheary elements (vessels).
3. The perforation plates; the scalariform oblique perforation plates with numerous perforations are most primitive; simple horizontal one is the most advanced.
4. Shape of the cross-section of the vessel; angular primitive; circular recent and advanced.
Origin and specialisation of vessels:
In the Dicotyledons the evolution of vessels took place first in the woody plants, apparently they arose independently in taxa with vessels which occur in most existing dicotyledons.
As a result of the facts collected, it has been assumed that the vessels arose first in the secondary xylem, and later in meta-xylem of the primary xylem, the specialisation and consequent evolutionary steps gradually advanced from secondary to primary xylem.
It could also be assumed that herbaceous plants have evolved from woody plants as a result of reduction of cambial activity only after obvious development of vessels had taken place in the woody ancestral plants. In some specialised Dicotyledons, e.g., in certain members of Cactaceae the secondary xylem lacks vessels which are replaced by so-called vascular tracheids, here the lack of vessels is the result of secondary reduction (Bailey, 1957).
From the evolutionary point of view, vessels in the Monocotyledons first appeared in roots and later in stems and leaves. The specialization of the vessels followed in the same pattern Phylogenetically the vessels first appeared and became specialised in late formed metaxylem and advanced gradually in early formed metaxylem and finally into protoxylem (Cheadle, 1944).
In the existing Monocotyledons of present day the last formed metaxylem roots contain vessels having scalariform perforation plates. A few aquatic Monocotyledons lack vessels in their xylem of roots, stems and leaves; their character is probably secondary one, i.e., reduction stage and advanced.
Floral anatomy:
Floral anatomy for delimiting taxa has been utilized by B. Tiagi and G. Dixit (1965) in Asclepiadaceae; Y.D. Tiagi (1964), B. Tiagi (1965, 1966) in Opuntia, Cuscuta and Evolvulus V. Puri (1970) has reviewed the opinions concerning the nature of ovule in angiosperms and suggested that the angiospermic ovule has possibly arisen independently of the gymnosperms and the two, due to lack of any direct link, should not be homologised.
Y.D. Tiagi (1961, 1963, and 1968) on the basis of evidence derived from floral morphology, anatomy, embryology and cytology in Cactaceae considers the family as closely related to Calycanthaceae. The floral anatomy of two species of Cuscuta has been examined by B. Tyagi (1966) and its retention in the family Convolvulaceae has been suggested by C.M. Govil (1968). V Singh (1977) has studied the primary pattern of floral development in all the taxa of the Alismatales as trimerous.
Establishment of Hydrangeaceae and Escalloniaceae of sub-families Hydrangeoideae and Escallonioideae of Saxifragaceae, as separate families has been supported by N.P. Saxena (1966) on anatomical and embryological grounds. Eames (1953) has suggested the removal of Paeonia from Ranunculaceae and elevation of a new family Paconiaceae on the basis of floral anatomy.
Anatomy of Leaf:
B.C. Kundu and B.Gupta (1963, 1964 a, b) have suggested the use of quantitative microscopy of leaves in distinguishing taxa which is found useful in Solanaceae.
Anatomy of Axis:
This has widely been used for comparing taxa, in Corchoms (R.M. Datta and K.Roy, 1963), in Ipomoea (C.M. Govil, 1971), Phaseolus (P.C. Datta and A.Saha, 1970).
Epidermal and Cuticular Anatomy:
This has also been used for of medicinal plants (P. Singh and B.C. Kundu, 1962, Y.N. Pandey, 1970; K.J. Ahmad, 1964 a, b).
Seed Coat Anatomy:
Anatomy of seed coats of pulses has been utilised for delimitation of taxa by P.C. Datta and R.K. Maiti (1968). Chuang and Lawrence (1972) studied the anatomy of seed coat in the genus Cordylanthus of Scrophulariaceae.
Taxonomy: Trend # 3. Pollen Morphology or Palynological:
The science of palynology or study of pollen morphology deals with the detailed study of pollen grains, i.e., the microspores of Spermatophyta, particularly of Angiosperms. It mainly deals with the structure, walls, etc., of pollen grains. Palynological studies help in the confirmation of relationship and affinities between the related taxa.
The pollen grains of related families and genera are usually of more or less of same type. The sculpture and pattern of the outer wall exine, number of apertures on the wall, size and the shape of the pollen grains, etc., help in taxonomy.
Mature pollen grains usually have two walls, the outer exine, and the inner intine. The intine is thinner and more tender than exine. The exine at first appears within the special wall as a thin membrane. In further development the exine thickens considerably and two layers can be distinguished on it, the outermost of which is the sexine and the inner is nexine.
The sexine is thin and has high refractive index, and therefore is not easily visible; the surface of sexine is at first smooth, but later on after the formation of nexine many types of projections may develop on it In the aperture regions of characteristic shapes from which pollen tube comes out, the exine may be completely wanting or it may be represented by nexine (Erdtman, 1952).
The nexine is relatively thick and cutmized. The specific cutin is called sporopollenin (Frey-Wyssling, 1959).
This substance is more stable than cutin. The intine is not of constant thickness, it is thicker at apertures. Numerous characteristic projections and sculpturings are formed on sexine, in addition to various types of sculpturings of the sexine there are other morphological characters which are used in the classification of the pollen and also imparts great value in angiosperm taxonomy.
Pollen grains are apertures, i.e., provided with pores or apertures. The position, shape, structure and number of apertures are of taxonomic significance. As a result of research on palynology based on the variability of the grains, a detailed nomenclature of the structural characteristics of pollen has been developed by which exact morphological descriptions of various pollen grains can be done accurately.
The categories of the structural morphology of pollen that help in plant taxonomy are as follows:
(i) Size of pollen grain to be measured in µ = (µ = .001 mm);
(ii) Structure of pollen grain, i.e., oval, elliptical, triangular, etc.;
(iii) Whether grains are free or in pollinia, or in tetrads;
(iv) The nature of sculpturing of the pollen grains, the nature of exine, e.g., smooth, spiny, warty, pitted, etc.;
(v) Sulcus-an elongated furrow perpendicular to the long axis at the pole of the grain;
(vi) Colpa-this is an elongated furrow at right angle to the equatorial plane, the ends of the furrow are directed towards the poles of the grain;
(vii) Ruga—an elongated furrow, the direction of which differs from both of the above types;
(viii) Pore- a pore is a circular aperture, when the number of pores is small, they are restricted in the equatorial region; when large they occur on all over the surface.
The comparative study of the structure of pollen grains of living and fossil plants plays as important role in the determination of classification of the plants. According to Wodehouse the pollen grain characters of gymnosperms are primitive to those of angiosperms.
Wodehouse says, that the one furrowed grain is a primitive character in Saururaceae, Piperaceae, Chloranthaceae and Magnoliaceae all of which families have been considered by various authors as primitive.
According to P.K.K. Nair, pollen morphology of primitive angiosperms has indicated that there are two primary dicot stocks namely the “Magnolian” stock (Magnoliaceae and its allies), and the “Ranalian” stock (Ranunculaceae and its allies), each stock being characterized by monocolpate and tricolpate pollen respectively.
Cronquist, A. (1968) has proposed that the two subclasses Ranunculideae and Magnoliideae, should be considered to represent the two palynological stocks, and should be raised to higher taxa levels.
Taxonomy: Trend # 4. Chemotaxonomy:
The science of chemical taxonomy (chemotaxonomy) is based on the classification of plants on the basis of their chemical constituents which are deeply concerned with the molecular characteristics. The method of chemical taxonomy is simple in principle and is based on the investigation of the distribution of chemical compounds, or groups of biosynthetically related compounds in series of related plants.
Different plants sometimes contain substances which although belong to different chemical compounds appear to be biosynthetically analogous.
Such plants may contain similar enzyme system, and the compounds produced by such enzymes infer that relationships exist between related plants. However, the chemotaxonomic studies include the investigations of the patterns of the compounds existing in plants, and in all the individual parts of the plants, such as bark, wood, leaves, roots, etc.
The chemical constituents usually differ much in different organs. Such investigations are needed for obtaining real evidence for alliance and non- alliance of plants.
According to Turril, the physiological aspects, to some extent, lead the taxonomist towards the determination of phylogeny. It has been biochemical rather have been used in taxonomy. The presence or absence of essential oils, resins, latex, glucosides, alkaloids, etc., has been used in classification.
According to Molisch, a definite chemical substance may appear in a single species, in several species of the same genus, in a single genus, in several genera of one family, in an entire family, in two to many related families, or in large divisions of the plant kingdom.
Mc Nair showed the distribution of certain chemical substances in the families of angiosperms and its relation to climate. Climatic conditions have a major influence on the distribution of plants containing certain substances, e.g., fats, volatile oils, alkaloids, etc.
He concluded, that for tropics, and perhaps for all climates, the chemical products are highly organized. According to Reichert, it is possible to identify many plants by their starch grains.
Alston and Turner (1963) in their work “Biochemical Systematics”; Hegnauer (1965) in his “Chemotaxonomic der Pflanzen”; Leone (1964) in his “Taxonomic Biochemistry and Serology”; Swain (1963) in his “Chemical Plant Taxonomy” and Harborne (1967) have given a review of phytotaxonomical chemistry and emphasized on the role of various chemicals in plant taxonomy.
Molery (1967) has stressed on the importance of β-cyanins and β-xanthins in plant taxonomy. β-cyanins are commonly met with in the families of Order Centrospermae. The other chemicals are also found specifically in particular orders or families of flowering plants.
For example, isoquinoline (alkaloids) is found in the families of Ranales; retanone in the families of Leguminales; biflavonyls in Casuarina equisetifolia (Casuarinaceae). The presence of such chemicals in different groups of plants is of great taxonomic significance.
The choice of host plants by parasites is a part determined by the chemical constitution of the host. Thus, many animal parasites choose plants with similar chemical characters-mustard oil, glucosides, IICN glucosides, alkaloids, organic acids (especially oxalic), salts, etc. In correlation insect larvae which, when free to choose, limit themselves to one plant family or to such families as have similar chemical substances.
Applications and methods:
There are a few angiosperamic taxa which are characterized by specific compounds of general occurrence. For example, leaving aside the family Caryophyllaceae, the rest of the families, such as Chenopodiaceae, Amaranthaceae, Aizoaceac, etc., of the taxon Caryophyllales (Centrospermae) contain Betacyanin, a colured substance, but differs from anthocyanin.
It appears that except Caryophyllaceae, the remaining families are closely related, and therefore, the Caryophyllaceae may be isolated which may form a separate monotypic taxon.
Betacyanin also occurs in Cactaceae, and therefore, the members of Caryophyllales are phylogenetically related. There are certain other chemical connections between Cactaceae and members of Caryophyllales, e.g., common presence of isoquinole alkaloid in Salsola of Chenopodiaceae and Cactaceae.
Another example may be cited, i.e., in the family Cruciferae unsaturated acid, erucic acid is prominent; in Tropaeolum erucic acid is also present, it indicates the relationship between Geraniales and Rhoeadales.
In Umbelliferac and Araliaceae a structural isomer of oleic acid, i.e., petroselinic acid occurs, these two families are related and belong to the same order. The other examples are from Magnoliales-Ranales taxa, where it shows that in Magnoliaceae Lauraceae, Ranunculaceae, Annonaceae, the alkaloid isoquinoline is present; this supports that these families are closely related.
On the other hand, Asclepiadaceae and Gentianaceae are allied due to the common occurrence of pyridine. The Liliaceae and Amaryllidaceae are closely associated, and this is supported by the presence of isoquinoline in both.
Taxonomy: Trend # 5. Serotaxonomy:
Serotaxonomic test and consequent phylogenetic relationships between the taxa of angiosperms were established in Germany by Professor K. C. Mez (1866-1944) at the University of Koenigsberg in 1926, and this was modified in 1936.
He established that relationship between larger group of angiospermic plants could be determined by serological test; and the closely related taxa and plants could be arranged accordingly. Serotaxonomy consists of the study and analysis of protein reaction of plants of different families with the blood serum of either rabbit or guineapig.
The serum test is done as follows:
For example, two plants A and B of two different families, of controversial relationship are taken for study. The protein extract of plant A, i.e., extracted from tender shoot bud, is injected into the body of a rabbit, then after sometime, blood serum is collected from the injected rabbit in a sterilized test tube; protein extract of plant B is also collected from the tender shoot bud and is added to the rabbit serum having A protein; if there is precipitation then A and B plants are closely related and may be placed taxonomically close to each other if on the other hand, no precipitation taken place, the plants are not closely related.
The precipitation result is known as’ positive reaction, and non-precipitation result is known as negative reaction. For example, protein extract of Pass (flora spp., (plant A) injected into the body of a rabbit, the serum then collected in a sterilized test tube and to it the protein extract of Cucurbita spp., (plant B) is added; the result is non-precipitation, i.e., negative reaction, and therefore, it is considered that the families Passifloraceae and Cucurbitaceae are not related at all. If degree of precipitation is much, the families are closely related, if less the families are distantly related.
Taxonomy: Trend # 6. Palaeontological:
Darrah has proposed the following palaeontological evidences in support of the phylogenetic evolution of the plants:
1. The invasion of land by an undifferentiated thalloid plant took place not later than Silurian.
2. The simple upright undifferentiated and protostelic axis formed in Silurian.
3. Enlargement of the plant body with specialization towards a division of labour-sterile and fertile, took place apparently in late Silurian.
4. Origin of the photosynthetic leaf took place in Devonian.
5. Specialization in the service of support, with the resultant secondary body took place in Devonian.
6. Development of the strobilus also took place in Devonian.
7. The development of heterospory took place in upper Devonian if not earlier.
8. Retention of the gametophyte within megaspore, within the megasporangium, where fertilization takes place with the ultimate attainment of the seed took place probably in upper Devonian.
9. Evolution of the pollen grain took place in Carboniferous if not earlier.
10. Evolution of the bisporangiate flower took place in Triassic period.
11. Attainment of the condition of angiospermy took place in Jurassic and Triassic periods.
12. Evolution of the herbaceous habit of the angiosperms took place in Cretaceous (chiefly Cenozoic).
Taxonomy: Trend # 7. Ontogenetical:
According to Gunderson the evolution of the flowering plants is based on floral structure and presumed organogenesis. He points out that the flowering tendencies are widely accepted as progressive in organogenesis and agree with the findings in development.
Petals — from separate to united.
Petals — from actinomorphy to zygomorphy.
Sepals — from separate to united.
Ovary — from superior to inferior.
Carpels — from separate to united.
Placentation — from parietal to axile.
Taxonomy: Trend # 8. Cytogenetics and Biosystematics:
In modern days cytology has played an important role in taxonomy to achieve a classification which represents mutual relationships and is useful in indexing plants. Modern taxonomy takes up cytological evidence based on chromosome numbers, chromosome morphology, chromosome behaviour in meiosis and in aberrant forms of reproduction.
The branch of taxonomy principally based on cytology is known as ‘cytotaxonomy’, it is a part of experimental taxonomy. It includes cytological aspects, study of cytogenetics and phenomeria together with consideration of classical aspects of taxonomy.
Application of data from the fields of cytology and cytogenetics have become very much helpful in taxonomic studies, particularly in determining categories of genus, species and sub-species in controversial cases.
In this approach emphasis has been laid on cytogenetics and other aspects of cytotaxonomy supplemented by classical aspects of morphology, and therefore, the cytotaxonomy is the integration of cytology and taxonomy in the attempt of better understanding to solve problems of plant affinities.
The first step in cytotaxonomy research is the thorough sampling of the taxon (it may or may not be species) and its populations and the consequent cytological studies of chromosomes, i.e., the number of chromosomes, chromosome morphology, chromosome behaviour, etc., of many populations within geographic races, species, genera and so on.
Difference in chromosome number, chromosome morphology and chromosome behaviour at meiosis usually indicate generic or specific differences of taxonomic significance.
The second step consists in the capacity of different populations to hybridize and a study of vigour and fertility of hybrids. This enables to know the presence or absence of breeding barriers between groups and is of taxonomic value as indicating the natural limits of the taxa of various categories.
The third and last step includes the study of homologies of the chromosomes in the hybrids as determined in the meiosis, this is significant indicator in the degree of genetic relationship. Informations obtained from the above mentioned three steps are compared with the facts obtained from comparative morphology and geographical distribution.
The resultant classification of the taxa, i.e., the category of genus and taxa of lower rank, to which it is applied has an increased objective over one obtained from the consideration of morphology.
For example, Gundersen’s (1950) system of classification is based on basic chromosome numbers together with anatomical and morphological characters. He has shown in primitive taxon, such as Magnolifiorae the basic number of chromosomes are much less than advanced taxon Rubifiorae and others.
The relationship between genera and the relative primitiveness of the constituent genera in the family can be determined on cytological studies, e.g., cytological studies on Dipterocarpaceae (Roy and Jha, 1965) established such relationship, the taxonomic position and the probable evolution of the taxon.
Roy and Jha, 1965, estimated the chromosome numbers in different genera of the different tribes of Dipterocarpaceae.
The number of chromosomes estimated as such 2n = 22, n = 11; 2n = 14, n = 7; 2n = 12, n = 6 according to different genera, e.g., the tribe Vaticae and Dipterocarpae have some basic chromosome numbers as 2n = 22, n = 11; Shorea has 2n = 14, n = 7, so Dipterocarpae and Vaticae are more primitive and more closely related than those of others in the taxon.
Synthetic Biosystematic Approach:
Taxonomy is an unending synthesis whose basis is becoming broadened decade after decade.
It is therefore highly essential to bring about a coordinated synthesis between the “taxonomist” and “biosystematist”-the former posing the various ambiguities wherever located in the understanding of taxa while revising the families and preparing basic frame structure, and the latter working out such problems with the sophisticated technique utilising the relevant data and material supplied by the taxonomists.
Such co-ordination between the Government Scientific Institutes and the University Departments would go a long way in solving several taxonomic problems of the Indian Flora (Rolla Seshagiri Rao and R. Sundra Raghavan, 1970).
Cytogenetics and Biosystematics in Synthetic Taxonomy:
During modern times, there has been spectacular extension of synthetic taxonomy. Taxonomists, unlike in the past, are not only busy collecting data but basic problems are discussed at the outset as to why should data be collected; how should it be collected and what information to get from them.
In all scientific experiments, howsoever, comprehensive and sophisticated it may be, we need some check to maintain balance between technique and concepts derived therefrom.
In this context cytogenetics, genecology, population genetics, plant breeding and-cytotaxonomy have a special role to play. Phylogenetic relationship at the specific and generic level and origin of species in many cases has been convincingly worked out by cytogeneticists. This approach has brought out new realizations and opened up new lines to study taxonomy (R.P. Roy and R.P. Sinha, 1970).
The cytogenetical data also indicate the origin and relationship of different taxa. Such studies include cross-ability between taxa and cytological analysis of artifical hybrids along with their parental forms. Similar studies can be carried on natural hybrids.
Variation in chromosome number is of special interest to the evolutionist as the new group may be biologically isolated by incompatibility or hybrid sterility and may have been formed as a result of sudden doubling of chromosome number.
Types formed by duplication of the same genome are difficult to distinguish from their progenitors as the differences are often in degree of expression of same characters.
As a general rule, alloploids are distinct in their morphological characteristics. The detection of more than one chromosome number within any one species may therefore demand its detailed study as the species may be a collective one and needs a taxonomic revision (S.L. Tandon and G R Rao 1970).
Taxonomy: Trend # 9. Embryology and Taxonomy:
The comparative or phylogenetic embryology deals with the embryological data, which are used as a tool for ascertaining inter-relationships and taxonomic positions. In a symposium on comparative Embryology of Angiosperms held at Delhi in 1967 the systematic positions of various families were discussed in the light of embryological data.
This symposium enabled to bring together in a comprehensive manner the scattered embryological literature on various families.
P. Maheshwari, B.M. Johri and S.N. Dixit (1957) pointed out that Loranthoideae and Viscoideae should be raised to the status of families. R.N. Kapil (1970) has pointed out several similarities between Crossosomataceae and Dilleniaceae like numerous centrifugal stamens, 2-3 middle layers, secretory tapetum, bitegmic crassinucellate ovules, micropyle formed by both the integuments. Polygonum type of embryo sac, nuclear endosperm showing aggregation of nude, at chalazal end and reniform arillate seeds. These show that the two families are related.
After a comparative study on the embryology of Magnoliaceae and Schizandra R.N. Kapil and S. Jalan (1964) supported the view that Schizandra should be raised to a family rank. According to K. Subramanyam (1970) Crassulaceae shows close affinity with Saxifragaceae. For example Crassula aquatica has a mode of habit similar to Podostemaceae and it has the most reduced endosperm in Crassulaceae.
This taxon according to Subramanyam (1962), forms a connecting link between Crassulaceae and Podostemaceae thus supporting Maheshwari (1945) that Podostemaceae are much reduced apetalous derivatives of the Crassulaceae.
V K Sharma (1968) has compared the embryological feature of Coriariaceae and allied families and suggested that it may represent an isolated stock distantly related to the Sapindales.
Dnyansagar (1970) stated that the sub-family Mimosoideae, Caesalpinoideae and Papilionatae should be raised to family levels.
Taxonomy: Trend # 10. Numerical Taxonomy:
Numerical plant taxonomy may be defined as the science in which for the purpose of classifying the plants, mathematical methods are used. In other words, the application of simple mathematical principles or techniques or methods in taxonomical studies of plants may be defined as numerical taxonomy.
Heywood (1967) defines numerical taxonomy as “the numerical evaluation of the similarity between groups of organisms and the ordering of these groups into higher-ranking taxa on the basis of these similarities.”
According to Sneath and Sokal (1973) “numerical taxonomy aims to develop methods that are objective, explicit and repeatable, both in evaluation of taxonomic relationships and in the erection of taxa.” Thus the establishment of this branch of taxonomy is mainly based on the fact that the taxonomists are becoming now a days more susceptible to more clear, more closure and more reasonable criteria and principles.
Taxometrics or numerical taxonomy is mainly concerned with procedural and operational problems, i.e., with the actual methods we take up in classifications.
This science is based on evidence of similarities exhibited by observed and recorded characters of taxa, not on phylogenetic probabilities. It is not concerned with producing new data but methods of handling them by means of electronic computers so as to reduce subjective element involved in comparing sets of data.
The necessity of mathematical applications in taxonomy has been realized by J.P.M. Brenon (1968) in Royal Botanic Garden, Kew (England). He has given the example of Kew herbarium with nearly four to five million specimens. The proper arrangement of such a huge number of plants is only possible by computerization of the arranging system.
J. Cullen (1968) has precisely revised the botanical problems of numerical taxonomy and has compared the numerical taxonomical methods with the orthodox taxonomical methods. In the orthodox taxonomy, the meaning of “character” is quite different from the meaning of “character” in numerical taxonomy.
In the former the character stands for any attribute referring to form, structure and behaviour which a taxonomist separates from the whole organism for a particular purpose such as comparison or interpretation, while in latter the character means a taxonomic character of two or more states, which within this study cannot be sub-divided any further.
This is known as unit character (operational taxonomic unit, OTU), which makes a basic unit of numerical plant taxonomy. The character concept is combined with the use of average individuals or some other forms of selecting samples to represent the basic taxa (OTU) of the group under consideration.
Another botanical problem of numerical taxonomy is the use of weighing technique. On the basis of numerical taxonomy, the new reasonable phylogenetic classification may be worked out.
The logical steps involved in numerical taxonomy are as follows:
1. Choice of units to be studied:
The first task is to decide what kinds of units are to be studied. These units may be comprised of individuals, strains, species, etc. The units should be as representative as possible of the organisms being considered. The entities of the lowest rank in particular study used are known as operational taxonomic units, (OTU’s).
2. Character selection:
A wide selection of the characters of OTU’s is required. At least 50 and preferably 100 or more characters are needed to produce fairly stable classification.
3. Measurement of similarity:
Overall similarity(s) is calculated by comparing OTU with every other and expressed as percentage, 100% similarity(s) for identity and 0% similarity(s) for non-resemblance. A similarity table may be worked out tabulating similarity coefficients and one for each operational taxonomic unit (OTU).
4. Cluster analysis:
The similarity in matrix is now arranged, so as to bring together into clusters OTU’S whose members have highest similarity.
5. Discrimination:
This is being done in the last, according to the degree of similarity and tabulated thereon.
In numerical taxonomy, the high-speed electronic computers and the powerful techniques of mathematics are employed to bring out a method of taxonomic analysis. These methods provide results that agree with established classification of plants. These computerized techniques have highly relevant and increasing repercussions on many of the procedures and concepts of various fields of taxonomy.
Phylogeny and Taxonomy:
Phylogeny is the evolutionary history of a taxon, and attempts to account for its origin and development. The term phylogeny is the autonym of ontogeny. Ontogeny differs from phylogeny in that it accounts for the life history of the individual plant from its development from the zygote to the production of its own gametes.
A primary objective of phylogenetic studies in botany is the determination of origins and relationships of all taxa of both extinct and living plants.
Significance of Phylogeny to Taxonomy:
Phylogeny deals with the evolutionary history of all taxa. It is a function of taxonomic research at all levels of classification. The ultimate goal of phylogenetic research is the production of a phylogenetic system of classification. The phylogenetic system shows the genetic and time relationship of any one taxon to another According to Turril (1942), “taxonomy is based on characters, phylogeny on changes of characters.”
A phylogenetic system of classification for plants would provide the answer to questions of (i) Their origin (ii) to their modes of evolution, (iii) to problems of monophyleticism vs. polyphyleticism, (iv) the identity of primitive and advanced characters, etc. It would conclude in a single stable classification of relationships.
Diversity of Phyletic Concepts:
The construction of the ultimate phylogenetic classification must be based on established facts regarding the characteristic features of the ancestors of every taxon level. These ancestors existed in remote prehistoric geological time, and because of their relative simplicity their characteristics are known as primitive which those of their present day descendants are known as advanced.
For example, Eichler, Wettstein, Rendle and others considered the unisexual apetalous cyclic flower to be primitive, whereas Bessey, Hutchinson and others treated it as advanced and considered the bisexual polypetalous flower with spiral arrangement of parts to be primitive.
The formulation of phylogenetic classifications requires the teamwork and collaboration of botanists of all disciplines and the considered evaluation of data.
“Opinions have differed as to whether the monocotyledonous plants are more advanced or more primitive than the dicotyledonous plants. If the view is accepted that woody habit preceded the herbaceous, then, since the vast majority of ancient and modern monocots are herbaceous, it would point to the monocots been derived from one or more dicot ancestors. Botanists accepting this view treat the dicots as the more primitive. Ms. Arber has held that the monocots preceded the dicots. Current criteria from morphological and anatomical studies of both vegetative and reproductive structures would place the monocots as probably having been derived from the dicots. There is evidence for the belief that the monocots are more likely to be monophyletic in origin than is true for the dicots.”