Modern Trends in Relation to Taxonomy

In this article we will discuss about:- 1. Introduction to Modern Trends 2. Morphology in Relation to Taxonomy 3. Anatomy 4. Embryology 5. Palynology 6. Cytology 7. Ecology.

Contents:

1. Introduction to Modern Trends in Relation to Taxonomy
2. Morphology in Relation to Taxonomy
3. Anatomy in Relation to Taxonomy
4. Embryology in Relation to Taxonomy
5. Palynology in Relation to Taxonomy
6. Cytology in Relation to Taxonomy
7. Ecology in Relation to Taxonomy

1. Introduction to Modern Trends in Relation to Taxonomy:

Modern taxonomists consider that the gross morphological characters are not always sufficient to provide means of differentiation in determining the genetically and evolutionary relationship between taxa. To achieve this the taxonomical evidences from anatomy, embryology, palynology, cytology, palaeobotany, ecology, biochemistry etc. are discussed.

Dr. V. Puri has said “One of the most significant modern trends in plant taxonomy is towards a synthesis between the older methods, outlook and more recent developments in our knowledge of plants”.

Bailey (1949) has said, if a truly natural classification is to be attained, it must be based upon “the analysis and the harmonization of evidence from all organs, tissues and parts”.

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2. Morphology in Relation to Taxonomy:

Gross morphology has no doubt provided the foundation and framework for taxonomy, but it has become increasingly clear that contributions to systematics may come from almost any branch of biology. The modern taxonomist has, therefore, to have a broader outlook than his predecessor a few years ago.

Morphology is the study of structure and form of plants, usually dealing with the organism and its component organs:

Morphology has been the most widely used tool in the classification of higher plants from a very long time. Morphological features have been so extensively studied by botanists and taxonomists in various classes of plants that it might be said that there is little left to learn.

According to modern concepts, however it is not correct. Practically all the herbaria systems of classifications, botanical manuals are based on comparative morphology and anatomy.

Morphological characters are traditionally useful as evidence at all taxonomic levels, but particularly at the specific and generic ranks. Morphological evidence provides the basic language for plant characterizations, identification, classification, and relationships. Generally, morphological data are easily observable and obtainable, and thus most frequently used in taxonomic studies.

Morphological characters are plants habit, root structural types, stem habit, stem structural types, bud structural types, leaf structural types, inflorescence types, flower types, perianth structural types, androecial types, stamen types, gynoecial types, carpel types, ovule types, fruit types and seed types.

Modification of flowers in stamen number, anther position, ovary position, style length, stigma shape, number of carpels, number and fusion of perianth parts etc., contribute to the reproductive success of the species. The growth habit (herbaceous or woody) of plants may be of primary usefulness in classification. Brassicaceae and herbaceous; Asteraceae have both woody and herbaceous members.

Examples of morphological characters in taxonomy:

Growth habit – Herb, Shrub and Tree:

Hutchinson has divided dicotyledons into two major divisions Lignosae and Herbaceae on the basis of their woody and herbaceous habit. The Lignosae being a fundamentally woody group, with some herbs derived from woody plants and Herbaceae being a fundamentally herbaceous group with a few shrubby plants derived from herbaeous. Herb – members of Ochidaceae; woody members of Fagaceae.

Root:

Adventitious – Monocots, Tap – Dicots.

Stem:

Bulb – Allium, Rhizome – Iris.

Leaf:

Pitcher Leaves in Sarracenia (pither plant); tenticular leaves in Drosera (sundew).

Inflorescence:

Umbel in Apiaceae.

Corolla:

Cruciform in Brassicaceae, rosaceous in Rosaceae, papillionaceous in Papilionaceae.

The features of floral morphology are most important in the classification of flowering plants.

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3. Anatomy in Relation to Taxonomy:

It is believed that internal structure of plants can provide more information than external morphology. However, anatomical features cannot by themselves constitute the basis of classification but can be used with advantage to supplement those morphological attributes on which classification has been built. The anatomical studies of organs of flowering plants can serve as an integral part of taxonomy.

Swamy and Bailey (1949) remarked that “before attempting to arrange surviving Angiosperms in phylogenetic series, it is essential to obtain reliable evidence regarding salient trends of evolutionary specialisation in the various organs and internal structures of these plants. Such evidence can be acquired only by comprehensive and time consuming investigations of the dicotyledons and monocotyledons as a whole”.

Vegetative anatomy:

Stomata:

Stomatal study 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 (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 (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 (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 (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 occur 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.

There are 31 known types of arrangement of subsidiary cells in the mature stomatal complex of vascular plants. At higher taxonomic levels, these distinct patterns play a big part in classification.

Within the Combretaceae, the stomata are anomocytic in the Combretoideae and paracytic in the Strephonematoideae. While the stomata are anomocytic in the Scrophulariaceae, they are diacytic in the closely related Acanthaceae. However, stomatal traits are not always reliable, e.g. anisocytic, anomocytic, diacytic and paracytic exist together in the same leaf of Lippia nodiflora (Verbenaceae).

Crystal:

Crystals of calcium oxalate are found in many families Apocynaceae, Begoniaceae, Cactaceae, Caricaceae and Euphorbiaceae. In these families sphaeraphides are common. Raphides are very common in the monocotyledons like Araceae and Musaceae.

According to Nair  (1977), the size and shape of crystals in the abaxial epidermis of the leaves of Myristica fragrans can enable us to differentiate between male and female trees even at the sapling phase. In general, simple crystals dominate in male trees and compound ones (druses) in female trees.

Latex tissue:

Laticiferous tissue is found in a number of families. Butomaceae, Araceae, Musaceae, Urticaceae, Podostamanaceae, Nymphaeaecae, Papaveraceae, Fumariaceae, Resedaceae, Euphorbiaceae, Aceraceae Caricaceae, Cactaceae, Sapotaceae, Apocynaceae, Asclepiadaceae, Convolvulaceae and Companulaceae possess milky or watery latex.

Cystoliths:

Crystals of calcium carbonate are found in the families Acanthaceae, Boraginaceae and Urticaceae. In the former two families the stalk of the cystolith is not conspicuous where as in the latter it is characteristic.

Secretory cells:

Possession of lysigenous or scizogenous cavities with oils, resins or mucilages provide taxonomic character in families like Rutaceae, Myrtaceae and Apiaceae.

Vascular bundle:

Types:

A. Scattered vascular bundles:

These are found in monocot stems.

B. Vascular bundles in ring:

It is a characteristic of Dicot stem anatomy.

Bicollateral vascular bundle:

These are found in Cucurbitaceae and Gentianaceae.

Cortical bundles:

They are found in a number of families e.g., Araliaceae (Panax), Bombacaceae (Bombax), Cactaceae (Rhipsalis), Ficoideae (Mesembyanthemum), Oleaceae (Nyctanthus) and Asteraceae (Vernonia).

Medullary phloem:

Araliaceae (Aralia racemosa), Amaranthaceae, Chenopodiaceae, Crassulaceae (Echevaria), Euphorbiaceae (Ricinus, amphivasal bundles), Melastomaceae (Melastoma), Nyctaginaceae, Polygonaceae, Piperaceae, Asteraceae (Compositate, Dahlia, Lactuca and Sonchus).

Internal phloem:

It occurs in Apocynaceae, Asclepiadaceae, Convolvulaceae, Cucurbitaceae, Lythraceae, Melastomaceae, Myrtaceae, Onagraceae, Punicaceae, Solanaceae, Thymelaeaceae, Papilionaceae (Mucuna).

Interxylary phloem – forminate type:

Salvadoraceae, Apocynaceae (Lysonia), Asclepiadaceae (Leptadenia lancifolia, Asclepias obtusifolia and Ceropegia), Combretaceae (Calycopteris).

Interxylary phloem-concentric type:

Chenopodiaceae (Majority of genera), Icacinaceae (Phytocrene), Loranthaceae (Nuytsia floribunda) Menispermaceae (Anomospermum), Onagraceae (Epilobium and Oenothera), Papilionaceae (Wistaria).

Successive ring of cambium:

Amarantaceae, Chenopodiaceae, Menispermaceae and Nyctaginaceae.

Hard bast:

The pericycle opposite to the vascular bundles is made up of fibres of sclerenchyma but it may be parenchymatous throughout as in many members of Ranunculaceae. It may form a continuous ring as in Cucurbitaceae, Piperaceae and Dioscoraceae. The pericycle may be made up of the phloem fibres with a thickening of cellulose as in Nerium and Vinca.

Wood Anatomy:

The evidentiary characters are wood cell type, wood cell size, wood cell shape, wood cell wall sculpture, wood cell patterns, stelar patterns, xylem maturation types, vascular bundle types; types of wood rays ground tissue parenchyma epidermis, mesophyll, stomata, sclereids, trichomes, crystals, nodes, petiole vasculation, venation, phloem cells etc.

Based on wood or wood anatomy, Amborella, Tetracentrom and Trochodendron are transferred to independent families. A new family Degeneriaceae is created. Rapateaceae differs from Xyridiaceae in having silica bodies, tannin cells and type of chlorenchyma. Magnoliales are considered primitive on the basis of wood anatomy.

Floral Anatomy:

Floral anatomy for delimiting taxa is used in Asclepiadaceae, Opuntia, Cuscuta and Evolvulus. According to him, Cactaceae is closely related to Calycanthaceae. V. Singh (1977) found primary pattern of floral development in all the taxa in Alismatales as trimerous.

Eames (1953) suggested the removal of Paeonia from Ranunculaceae and elevation of a new family Paeoniaceae on the basis of floral anatomy.

The role of floral anatomy has been emphasised in solving taxonomic problems. According to Puri (1958), the gynoecium structure in the Capparidaceae, Cruciferae, Moringaceae and Papaveraceae is essentially alike. Though the placentation is parietal, the placental strands are inverted and occur on the inner side of the secondary marginal bundles which have normal orientation.

This supports the view that the Moringaceae should be included in the Rhoeadales and that the parietal placentation has been derived from that axile condition. As a similar phenomenon, takes place in the Cucurbitaceae and Passifloraceae, it lends support to the suggested alliance between the Moringaceae and Passifloraceae.

From a study of floral anatomy of four species of the Onagraceae, Roy (1949) observed significant variations in the vascular supply of the ovary of Trapa, justifying its removal to a separate family (Trapaceae). The separation of Hydrocotyle asiatica as Centella asiatica has been confirmed on anatomical grounds by Mitra (1995).

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4. Embryology in Relation to Taxonomy:

A recognition of the value of Embryology in taxonomy was delayed because of the time and trouble involved in collecting embryological data.

According to Maheshwari (1964) and Bhojwani and Bhatnagar (1978), the characters of taxonomic value in delimiting plant groups include the:

(a) Anther;

(b) Quadripartition of the microspore mother cell;

(c) Development and organisation of the pollen-grain,

(d) Development and structure of the ovule;

(e) Origin and extent of the sporogenous tissue in the ovule;

(f) Megasporogenesis and development of the embryosac;

(g) Form and organisation of the mature embryosac;

(h) Fertilisation;

(i) Endosperm;

(j) Embryo and

(k) Seed-coat.

Cyperaceae:

In the family Cyperaceae, while all four microspore nuclei are produced after meiosis, three of them are cut off on one side of the pollen grain and only the fourth develops to form the generative cell and then the male gametes.

All genera and species of Cyperaceae studied in Europe and Japan (and at Delhi by Mr. C.K. Shah) show this character and it is possible to identify a member of this family just as definitely by a microscopic study of its anthers as by other floral characters.

Further, the simultaneous type of microspore formation and the functioning of all the four microspores in the Juncaceae indicate that it is this family from which the Cyperaceae have probably been derived.

Centrospermales:

The Cactaceae agrees with the rest of the Centrospermales in having the following embryological characters:

(a) Glandular anther tapetum whose cells become two-to four-nucleate;

(b) Microspore-mother cells is which two meiotic divisions are succeeded by a simultaneous quadripartition into the microspores;

(c) Trinucleate pollen-grains;

(d) Campylotropous ovules with strongly curved funiculi and massive nucellic;

(e) A hypodermal archesporial cell which cuts of a wall cell;

(f) A micropyle formed by the swollen tips of the inner integument which protrude out and approach the functions;

(g) Formation of a nucellar cap originating from, periclinal division of cells of the nucellar epidermis;

(h) Functioning of the chalazal megaspore of the tetrad:

(i) Formation of a monosporic eight-nucleate embryosac;

(j) Functioning of the perisperm as the main storage region;

(k) Disappearance of most of the endosperm in the mature seed generally leaving merely a single-layered cap over the radicle.

Loranthaceae:

The studies of Johri and associates (1957) on the Loranthaceae show that the Loranthoideae is embryologically different from the Viscoideae as regards mode of development of embryosac, endosperm, embryo and in the location of the viscid zone of the fruit and that the subfamilies should be raised to the status of families.

Onagraceae and Trapaceae:

A monosporic tetranucleate embryo-sac is characteristic of all members of the Onagraceae and is not found in any other family of angiosperms. The genus Trapa having an eight-nucleate embryo-sac, which was once placed in the Onagraceae, has since been removed and assigned to a new family Trapaceae.

Manasi Ram’s (1956) work on Trapa bispinosa fully confirms this view. Earlier, Eames (1953) expressed the view that on anatomical evidence also Trapa does not belong to the Onagraceae and is not even closely related to it. Table I presents the embryological differences between the families Onagraceae and Trapaceae.

Santalaceae:

Gagnepain & Boureau (1946, 1947), raised doubts about the position of Exocarpus and stated that instead of being regarded as an angiosperm it should be assigned to the gymnosperms and given a place somewhere near the Taxaceae. Lam (1948) commented as follows: “At any rate, Exocarpus seems an interesting case and probably represents a transition between the protangiospermous gymnosperms and the Monochlamydeae”.

The embryological studies of Manasi Ram (1958) have clearly shown, however, that Exocarpus is a perfectly valid angiosperm with an archesporial cell functioning as a megaspore mother cell, an embryo-sac of the Polygonum type, a cellular endosperm with a chalazal haustorium, and a pericarp derived from the wall of the ovary. Its correct position, therefore, lies in the Santalaceae to which it was assigned by previous systematists.

Other taxonomal cases settled by embryological data:

Paeonia was previously kept in Ranunculaceae but later on it was found that by anatomy and pollen characters, this genus differs from other members of Ranunculaceae. Embryological studies found that embryogeny of Paeonia is very different from other members of Ranunculaceae and this genus was then shifted from Ranunculaceae.

In Butomaceae Butomus is unique in having a monosporic 8-nucleate embryo sac; Butomopsis, Limnocharis and Hydrocleis has bisporic, 5-nucleate embryo sacs. Butomus is retained in Butomaceae while all the others are transferred to Alismataceae.

Embryological evidences supported Hutchinson’s (1959) treatment of Alismataceae and Butomaceae as separate families and the placement of the latter along with Hydrocharitaceae in the same order.

The separation of Hippocratea from Celastraceae into a separate family Hippocrateaceae.

The genus Sphenoclea is placed under Campanulaceae based on embryological studies.

The splitting of Gentianaceae into Menyanthaceae and Gentianaceae; Malvaceae into Bombacaceae and Malvaceae.

Embryogeny of Lemna, Wolffia and Arisaema confirms older views of an intimate relationship of the duck weeds with the aroids.

It may be emphasized that while the embryologist lays no claim to erect a phylogenetic system of its own, embryological data need to be considered along with information from other sources in order to approach a natural system of classification.

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5. Palynology in Relation to Taxonomy:

Pollen characters:

Polynology is the science of pollen and spores and its applications. It is derived from the Greek word palynein meaning to scatter. The significance of pollen attributes in taxonomy has been realised during the last three decades.

The outer wall of pollen-grains is endowed with unique structural traits which are broadly categorised in order of their importance in phylogeny into the apertures, exine ornamentation, exine strata, shape and size.

According to Bailey and Nast (1943), “there are families of dicotyledons in which the pollen is of very considerable taxonomic significance not only in the differentiation of subfamilies and tribes but also of genera and species”. A few examples will illustrate the statement.

The Caryophyllales is recognised by centrospermous type of pollen with a spinulose and punctate-perforate tectum. The Malvaceae and the Compositae contain typically spinulose exine, the Plumbaginaceae verrucate pollen and the Gramineae smooth, sulcate ones.

NPC system:

The pollen classification is based on number-position character analysis, called NPC system. As a rule, the number of apertures is only one in the cryptogams and gymnosperms whenever the grains are aperturate but the position is distal in the latter and proximal in the former and the apertures are non-trichotomous (e.g. monolete) or trichotomous (e.g. trilete) in character.

The apertural conditions of pollen-grains have been looked upon as strong characters in solving taxonomic problems. It has been recommended that taxa with the same general NPC formula be grouped together and those showing aberrant NPC separately. For example, the Parietales of Engler-Diels has been probed by means of ‘palynological compass needle’.

The NPC formula for the order is 345, i.e. pollen-grains are 3-termate (N₃), zonotreme (P₄) and colporate (C₅). A significant variant is the Canellaceae where the pollen has one distal colpus; here NPC is 133, hence Hutchinson (1959) proposed shifting of Canellaceae from Parietales.

Apertual conditions:

A general analysis of the apertural conditions in the plant kingdom reveals that apertures are ill-developed (primorphous) in thallophytes. In the archegoniates (comprising bryophytes, pteridophytes and gymnosperms), pollen-grains are trimorphous and in the angiosperms polymorphous.

The above morphological situation provided a logical base for classifying the plant kingdom into the Primorphosporatae (Syn. Thallophyta), Trimorphosporatae (Syn. Archegoniatae) and Polymorphosporatae (Syn. Angiospermae) by Nair (1974). This analysis has thrown light on the phylogeny and evolution of the primitive angiosperms.

Stenopaiynous and Eurypalynous taxa:

In angiospermic taxa, termed as “Stenopalynous”, the pollen type is constant; while in others, called “Eurypalynous”, there are different pollen types varying in size, shape, aperture, exine stratification, etc.

The stenopalynous taxa are, as a rule, very natural. The occurrence of a pollen type, representative of a stenopalynous taxon in a plant of doubtful affinity, may provide an important indication of its taxonomic position.

Amongst the eurypalynous taxa some may be natural and others quite heterogeneous. Different pollen types in eurypalynous taxa furnish important indications in classifying them into subgroups and then arranging these subgroups according to varying degrees of similarity between each other.

In certain cases this may even result in the splitting up of a taxon or submerging of others. Numerous examples of such rearrangements in families and genera have been given by Erdtman.

As pointed out by Bailey and Nast “There are families of dicotyledons in which the pollen is of very considerable taxonomic significance, not only in the differentiation of subfamilies and tribes, but also of genera and species”. For example, after the studies of Lindau and Bremekamp pollen structure has become an indispensable character in the taxonomy of Acanthaceae.

Number of nuclei in pollen:

The number of nuclei in the pollen at the time of dispersal has been used by taxonomists. The angiosperm pollen is either binucleate or trinucleate according to the precocity of division of the generative nucleus. The binucleate condition is considered as more primitive than the trinucleate.

In the Centrospermae, the pollen is uniformly trinucleate. The monocot (Liliaceae) is binucleate, the apetalous and polypetalous dicots are binucleate and gamopetalous members trinucleate.

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6. Cytology in Relation to Taxonomy:

The application of cytological data in elucidation of taxonomic problems, it is seen that various attributes of chromosomes like number, morphology, size, behaviour in crosses and aberrations in reproduction are all important.

Chromosome number:

The haploid number of chromosomes in angiosperms ranges from n = 12 in Halopappus gracilis (Asteraceae) to around n = 132 in Poa littoroa (Poaceae). Most of the angiosperms have chromosome numbers ranging between n = 7 and n = 12. About 35 to 40% per cent of the flowering plants are polyploids.

It is usually seen that closely related plants, like the different species of a genus, show chromosome numbers which reveal an arithmetic relation with one another – often in multiples of base number of characteristics of the genus. For example the different species of Piper show chromosome numbers in multiples of 26, like 2n = 52 in P. nigrum, 2n = 78 in P. betle, and 2n = 104 in wild species of Piper (Mathew, 1958).

In Morus nigra has the highest chromosome number (2n = 308), in M. cathyana there are forms with 2n = 56, 84; 112. Solanum nigrum is a good example of the existence of a species complex, comprising diploid (n = 12), tetraploid (n = 24) and hexaploid (n = 36) forms.

Chromosome morphology:

A study of chromosome morphology is informative to the taxonomist in assessing affinities and modes of origin of separate species. The important work of Babcock in Crepis show how chromosome morphology coupled with chromosome number is of considerable importance in the genetic and taxonomic phases of study.

In genera like Crepis and Plantago the large size and small number of their chromosomes have been of great value in this type of study.

Chromosome size:

It has already been discovered that evolutionary development involves in addition to alterations in chromosome number, their size, changes in structure etc., so that analysis of these cytological characters may also shed important light on species relationships. Recent observations on the Menispermaceae have shown that this aspect of cytology is sometimes valuable in taxonomic discussions.

In this family the genera Stephania, Cyclea and Cissampelos are grouped under the Cissampelideae by Hooker. While Cyclea and Cissampelos show reduction in the perianth parts of the female flower Stephania shows no such reduction.

Cytologically Cyclea and Cissampelos are seen to be based on 12, while Stephania shows n = 13. It is seen that the number n = 13 is characteristic of the tribe Cocculeae, which further shows chromosomes of small size.

Stephania also shows chromosomes of small size. Stephania also shows chromosomes of small size while Cyclea and Cissampelos have much larger chromosomes. On cytological grounds therefore Stephania may be transferred to the tribe Cocculeae. This is an instance of how chromosome size coupled with chromosome number provides data of taxonomic importance.

Large chromosomes, low chromosome number and symmetrical karyotype represent a primitive status, while small chromosomes, high number and extreme asymmetry indicate advancement.

The application of these principles has yielded interesting results in the Pandanales, Alismataceae, Hydrocharitaceae, Liliaceae, Amaryllidaceae and Dioscoreaceae. Within the Geraniales, chromosome study does not justify the exclusion of the Batsaminaceae as proposed by Engler.

Chromosome behaviour in crosses:

The behaviour of chromosomes in crosses is a reliable factor in assessing relationships. Pathak (1940) made a careful karyotypic analysis of various species of Aegilops, Secale and Triticum, suggesting that the hexaploid T. spelta and T. vulgare were probably derived through hybridisation between a tetraploid wheat and Roy (1959) carried out detailed genome analysis of Aegilops longissima and A. sharonensis.

On the basis of chromosome pairing and fertility of F₁ hybrids and the derived amphidiploidy, he thought that the two species are closely related. From a study of the karyotypes of species of Aegilops, Cheenaveeraiah (1962) postulated that the section Sitopsis should be shifted from Aegilops to Triticum or given the rank of a new genus.

Sisymbrium irio, a polytypic species, where plants of various sizes are available, differing in the size and shape of leaves, flowers and fruits as well as branching pattern. According to Khoshoo (1960), variation and evolution within this Species is primarily due to hybridisation and polyploidy and secondarily to gene mutation and structural changes in chromosomes.

Often there exists a close parallelism between taxonomical and cytogenetical studies. In Prunus persica for example the systematist satisfactorily classified the variations; the geneticist found that these variations were inherited as per simple Mendelian laws and the cytologists found the plants to be ordinary diploids.

Again in Pyrus malus the systematist, in spite of all the care bestowed, found the classification difficult and involved, and the cytologist found them to be secondary polyploids while the geneticist found the analysis of the inheritance of characters extremely complicated on account of involved polyploid ratios.

In Boragineae there had been considerable disagreement regarding the position of Brunnera macrophylla. Johnston (1924) recognised Brunnera as a distinct genus and Smith’s (1932) cytological studies not only supported Johnston but also affirmed that segregation of Anchusa myostdiflora and A. sempervirens into Brunnera macrophylla and Caryolopha sempervirens.

Removal of Brunnera macrophylla from the genus Anchusa is substantialized on three grounds, viz.:

(1) Size of chromosomes,

(2) Form of chromosomes, and

(3) Number of chromosomes.

“Despite a healthy scepticism of the value of the chromosomes in easing taxonomic decision- making, the annual outpouring of cytological data continues to illuminate the taxonomic study of many groups. With increasing knowledge the chromosomes will further reflect the manifold ways in which evolution has proceeded and will thus become an increasing refined tool, among the many available to the discriminating taxonomist”.

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7. Ecology in Relation to Taxonomy:

The ecological criteria are of comparatively little direct importance in taxonomy, though ecological criteria at the interspecific level can not be neglected.

In flowering plants, tolerance and plasticity are widespread. The tolerance of a plant population is determined by its ability to survive and reproduce upon exposure to a range of environmental factors. The tolerance is greater when the range is wider. On the other hand, plasticity is ascertained by the degree to which the appearance of plants vary in moving from one set of factors to the other.

Ecotypes:

Ecological studies can help in determining the taxonomic status of a species. On the basis of developmental morphology, culture experiments and other analytical data, the two species of Lindenbergia, L. polyantha and L. urticaefolia, have been demonstrated to be ecotypes of the same species, the former being a calcicolous or a miniature form of the latter.

Euphorbia thymifolia exhibits ecotypic differentiation in response to the calcium content of the soil the red ecotype is a facultative calcicole and the green one an obligate calcifuge.

Pursuing the matter further, the red plant shows three physiologically distinct ecotypes. Depending on the soil exchangeable calcium, Boerhaavia diffusa, Gomphrena celosioides and Mecardonia dianthera give rise to appropriate calcicolous and calcifugous ecotypes.

Euphorbia hirta bears three forms: one upright form, growing in protected habitats, is a distinct ecotype from the other prostrate one; the latter also appears as a compact form of the footpath and a diffuse one of the grazed lands which are interconvertiable in reciprocal transplants.