Here is an essay on the ‘Branches of Botany’ for class 11 and 12. Find paragraphs, long and short essays on the ‘Branches of Botany’ especially written for school and college students.
Importance of Different Branches of Botany
Essay Contents:
- Essay on External Morphology
- Essay on Micromorphology
- Essay on Anatomy
- Essay on Cytology
- Essay on Embryology
- Essay on Palynology
- Essay on Phytochemistry
- Essay on Ecology
1. Essay on External Morphology:
The taxonomists still rely to a great extent on the morphological characteristics, because they are easily visible and can conventionally use in the classification of plants.
In addition to the conventional morphological characteristics that are now popular to the students, other characteristics like habit, underground organs, leaves, seedling morphology, stipules, non-conventional characteristics of floral parts, seeds etc. are used in the identification of various groups of plants.
In recent years, due to thorough and careful studies some characteristics neglected in the past came in focus and help in taxonomic practices:
A. Habit:
Size, branching pattern, spread, density etc. of a plant are treated commonly in taxonomic description. The shape of a tree i.e., bushy, umbrella- shaped, flat topped, cylindric, oblong etc. are used for the recognition of a tree. It is Excurrent in Polyalthea longifolia, Caudex in Cocos nucifera, Deliquescent in Mangifera indica, Culm in Bambusa tulda, etc. The characteristics of bark (colour, thickness, fissuring, texture etc.) are used to distinguish different species of Betula and Pinus.
B. Underground Organs:
Generally the underground parts are not collected during the preparation of a Herbarium. In some cases, they have been shown to be very useful in the identification of different taxa. Chouard (1936) have used the characteristics of bulbs and underground parts in the classification of Scilleae of Liliaceae. In Liliaceae two species of Chlorophytum viz. C. glaucoides Blatt. and C. glaucum Dalz. are morphologically alike except the characteristics of underground parts.
In C. glaucoides the root-fibres are slender and end in ellipsoid tubers, while in C. glaucum the root fibres are thick and without terminal tubers. The structure and morphology of root-tubers are used in the differentiation of large number of species in Dioscorea of Dioscoreaceae.
C. Leaves:
Leaf characteristics are extensively used in the differentiation of species of Betula, Ulmus and in many other genera. In Papilionaceae, different species of Dalbergia viz. D. sissoo Roxb., D. latifolia Roxb. and D. sympathetica Nimmo are distinguished by shape, size and arrangement of the leaflet on the rachis. Venation is also useful in differentiation of species. Numerous venation patterns in fossil genus Clossopteris was described by Pant (1958).
D. Stipules:
The stipule characteristic is also very useful in the differentiation of species. In Rosaceae the characteristics of stipule is variable, but almost constant within a species. The morphology of stipule is useful to differentiate the different species of Viola and Trifolium.
E. Seedling Morphology:
The characteristics of seedling have also been used in tracing phylogenetic relationships. Sampathkumar (1982) found that characteristics of cotyledons are of taxonomic importance in some Convolvulaceae. In recent years, the seedling morphology is also useful for keying out different genera and species.
F. Non-Conventional Characteristics of Floral Parts:
The characteristics of flowers and inflorescences are commonly used in the classification of plants. But more careful observations on floral parts show some non-conventional characteristics which become very useful in the identification of different taxa and their differentiation with others.
Some of them are:
a. The degree of branching of inflorescence is useful in the differentiation of various species of Pinus.
b. The configuration of floral disc and also nectaries have great value in the diagnosis of Cruciferae (Brassicaceae) and Tamarix of Tamaricaceae.
c. Staminal appendages contribute useful data to diagnose the species of Alyssum of Cruciferae.
d. Degree of pubescence on young ovaries and the type of papillae on stylar branches are useful in the classification of Compositae (Asteraceae).
G. Seeds:
The characteristics of seed are also useful in the differentiation of taxa at specific level. Duke (1961) reported that the characteristics of seed shape, colour and sculpturing contribute a critical indication of the systematic positions of species in Drymaria of Caryophyllaceae.
In Liliaceae, the two genera Antherium and Chlorophytum can be differentiated only by the shape and number of seeds. Similarly, other characteristics like hairy growth on testa, their colour and length contribute useful data to differentiate the genera and species of different families like Acanthaceae, Malvaceae, Convolvulaceae, etc.
2. Essay on Micromorphology:
A. Epidermis:
The characteristics of epidermis, such as shape, thickness, nature of sculpturing and inclusions of epidermal cells provide important data useful in taxonomic consideration. Sometimes the divisions of epidermal cell also consider in taxonomic analysis.
Narrow epidermal cells are the characteristics of the family Stylidiaceae and sclerification of the epidermal cell wall is of taxonomic importance in the tribe Mutisieae of Compositae.
In Cyperaceae, the distribution of silica-body on the surface of epidermis provide an important data of taxonomic importance:
a. Spherical and hemispherical warty bodies found in Scleria, Acriulars and Bisboeckelera,
b. Spherical and hemispherical echinulate bodies found in Rhynchospora,
c. Hemispherical smooth bodies in Lophoschoenus,
d. Wedge-shaped silica bodies are restricted in Ptilianthelium, Scirpodendron, and Neesenbeckia etc.
e. Bridge-shaped bodies in Mapania etc.
Based on epidermal characteristics like outline of epidermal cells in surface view, type of hairs, structure of stomata; Jain and Singh (1974) and Singh and Jain (1975) differentiated the species of Pyrus and Prunus. Jain and Singh (1974) also distinguished Himalayan species of oaks, based on epidermal characteristics.
B. Trichomes:
The trichomes show great variation in the structure and are used in taxonomy and phylogeny of angiosperms. It has been successfully used in the classification of genera and species. The plant parts may bear one or more types of trichomes. The anatomical characteristics of trichomes are also used in this respect.
Some important observations are:
i. Cruciferae:
Rollins (1941) observed that in Arabis the trichomes are commonly unicellular and eglandular, but in some species they are simple and in some others they are branched, either star-shaped or dendritic type (tree-like). In some species, they are terete, flattened or with swollen base.
ii. Compositae:
Rollins (1944, 1949) observed that the two species of Parthenium viz. P. argentatum and P. incanum can be distinguished by trichomes. The former is T-shaped and the latter whip-like with a long thread. The hybrids between the species show intermediate type of trichomes. Ramayya (1969) develop the generic key of Indian members of Compositae, based on trichome characteristics.
iii. Ericaceae:
Cowan (1950) observed the trichomes of Rhododendron that are useful in the taxonomic separation in infrageneric and specific levels.
iv. Oleaceae:
Imandar (1967) studied the structure and ontogeny of trichome and confirm the position of Nyctanthes in Oleaceae.
Ramayya (1972) thoroughly observed the trichomes of flowering plants and explained their phylogeny. According to him, unicellular stigmatic papillae are the most primitive among angiosperms. Thus the order Magnoliales is a heterogenous group and the angiosperms are a polyphyletic group.
Based on the morphology and ontogeny of trichomes in the tribe Heliantheae of Compositae (Asteraceae), Ramayya (1962, 1969) supported the view that Heliantheae is the most primitive tribe of Asteraceae.
Characteristics of trichomes have also been found valuable in the classification of Indian species of Malvastrum, in some members of Capparidaceae and in Caesalpiniaceae. Gornall (1986) utilised the anatomical characteristics in the systematics of the genus, Saxifraga.
C. Stomata:
The development and morphology of stomata may be helpful in assigning taxa of uncertain affinity to their true position and also to find out the evolutionary trends among the families of Angiosperms.
Some evidences are:
i. Structure and Distribution:
These play important roles in the study of taxonomic and phylogenetic significance as discussed by Shah (1968) and Kothari and Shah (1974, 1975) in Papilionaceae. The anomoperigenous and aniso-mesoperigenous types i.e., the primitive types are more common in the tribes Sophoreae, Podalyrieae and Gensiteae, whereas the advanced types occur more frequently in Phaseoleae, Dalbergieae and also in some members of Hedysareae.
ii. Ontogeny:
Both Caricaceae and Cucurbitaceae possess aperigenous and aniso-mesoperigenous types indicating close affinity between them.
a. Bentham, Hooker, and Engler placed the families Umbelliferae, Araliaceae and Cornaceae under the order Umbellales. Later, Cronquist (1965) segregated them and placed the first two families under the order Umbellales and the last one Cornaceae under a separate order, Cornales, along with Alangiaceae.
The ontogenic study of stomata support the placing of the above families. The former two families have anisocytic and paramesogenous types, while the latter two possess aperigenous stomata.
b. Bremekamp (1953) suggested the transfer of the genus Elytraria from Acanthaceae to Scrophulariaceae. Paliwal (1963) studied ontogeny of stomata and found that the genus Elytraria has diacytic stomata and syndetochelic mode of development, clearly indicating its close relationship with Acanthaceae than Scrophulariaceae.
c. The transfer of Nyctanthes is suggested from Oleaceae to Verbenaceae. The ontogenic studies of stomata indicate that, like other members of Oleaceae, it possesses tetramesoperigenous and occasionally aperigenous stomata which differ from the diamesogenous stomata of Verbenaceae.
3. Essay on Anatomy:
A. Leaf Anatomy:
Carlquist (1961) has asserted that “the leaf is perhaps anatomically most varied organ of angiosperms and its anatomical variations often concur closely with generic and specific and occasionally familial lines”.
The important structures of leaves considered useful in taxonomy are:
i. Structure of mesophyll.
ii. Structure of vein sheath.
iii. Stomatal crypt and time of formation.
iv. Suberisation and location of abscission layer.
v. Presence and absence of hydathodes, etc.
Duval-Jouve (1875) realised that the anatomy of leaf is one of the most reliable characteristics useful in grass systematics.
After that, many workers have contributed a lot in this line:
a. Brown (1958) has recognised 6 different types of tissue arrangement in grass leaf and correlated it with the taxonomic groups. Based on leaf anatomy, he also discussed the evolutionary tendencies in different groups of grasses.
b. Culter (1965) studied on the genus Thurnia and found inverted bundles unique in its leaf. This gives support to the view that Thurnia should be placed in a separate family Thurniaceae separating from Rapateaceae and Juncaceae — as was placed previously.
c. Govindarajalu (1969) made a key based on leaf anatomy to separate the different species of Cyperus.
d. Ayensu (1974) reported that the genera Barbacenia and Vellozia of Velloziaceae can be differentiated on the basis of form of sclerenchyma in leaf.
e. Many botanists have also worked on different taxa and made important contributions, e.g., Morly (1953) on Mouriri, Hagerup (1953) on Ericaceae, Tomlinson (1956, 59) on Musaceae and Zingiberaceae etc.
B. Petiole Anatomy:
The petiole shows various types of vascular structure as illustrated by Metcalfe and Chalk (1950). Vascular structure of dicotyledonous petioles also shows taxonomic importance.
Azizian and Culter (1982) reported the importance of vascular structure of petiole in the diagnostic significance of Phlomis and Eremostachys of Labiatae (Lamiaceae). The petioles of Phiomis show two distinct groups.
In one type, 1-2 median arcs are noted in Phlomis and species of Eremostachys and, in other type, numerous separate bundles are present and often form a complete ring in some Phlomis. The above observation establishes the distinction of the two groups in Phlomis and its close relation with Eremostachys.
C. Nodal Anatomy:
Sinnott (1914) distinguished three fundamental types of nodal anatomy in dicotyledons. These are unilacunar, trilacunar and multi- lacunar. According to him, the trilacunar node is primitive and the other two types are more advanced.
Many years later, Marsden and Bailey (1955) reported the 4th type as unilacunar two trace, which is now considered as the basic type for angiosperms. Sinnott and Bailey (1914) made a comparative study of nodal anatomy of angiosperms and showed the relationship and distinction among different genera and species.
a. Canright (1955) recognised the apparent phylogenetic sequence in the mature unilacunar node of the family Monimiaceae. The different genera of the family show the number of traces i.e., 2 in Trimenia, Piptocalyx etc., followed by 3, 5 or 7 in Anthobembix, Hortonia etc. and finally 1- broad arc-shaped leaf trace in Siparuneae.
b. Bailey and Howard (1941) divided the subfamily lcacinoideae of Icacinaceae into two different sections. One section shows unilacunar nodes and the other trilacunar nodes.
D. Wood Anatomy:
Wood anatomy is found to be very much useful in solving some taxonomic and phylogenetic dispute. Carlquist (1961) observed that “the well-established trends of xylem evolution should be taken into account by any systematist interested in formulating relationships and evolutionary status of angiosperm taxa”.
Some important characteristics are:
i. Vessel Elements:
Characteristics like distribution, diameter and frequency of vessels, perforation and thickening and presence of tyloses have been found useful in taxonomic and phylogenetic syudies. The non- porous wood without vessels is more primitive, as in Gymnosperms; compared to porous wood found in angiosperms. Plants with diffused porous wood are considered more primitive than ring porous wood.
ii. Vascular Rays:
The different characteristics of vascular rays offer important taxonomic significance. These include cellular composition and width of rays, dimensions of rays (in tangential section), degree of wall- thickness of ray cells etc. Heterogeneous rays i.e., rays with both radially and vertically elongated cells are considered more primitive, than the homogenous rays — rays with only radially elongated cells.
iii. Axial Parenchyma:
The characteristics and distribution of axial parenchyma cells are the distinctive features of secondary wood. These characteristics are used in systematics and phylogeny of angiosperms. Based on distribution, they are divided into two main groups: apotracheal type i.e., parenchyma distributed without any specific relation to the vessels, and paratracheal type i.e., parenchyma distributed in close association with the vessels.
Absence of parenchyma is considered primitive, as found in some members of Winteraceae. The diffuse arrangement of cells of parenchyma are considered more primitive than the diffuse-in-aggregates. The various aggregate arrangements (apotracheal banded with many paratracheal types) are considered as advanced types.
iv. Storied Wood:
It is the pattern of arrangement of cells or tissues in horizontal series as seen in tangential section. Storied woods are considered as advanced over the non- storied woods.
Wood anatomy has played an important role to solve certain taxonomic problems:
a. Parietales:
The anatomical findings indicate that the order Parietales of Engler and Prantl (1889) is a heterogeneous assemblage and the segregation of the order into Parietales and Guttiferales as done by Wettstein is justified.
b. Degeneria:
The genus Degeneria was placed under Magnoliaceae by Hutchinson (1959). Wood anatomy supported the segregation, placing it in a separate family Degeneriaceae. Thus the Degeneriaceae, Magnoliaceae and Eupomatiaceae are the closely related families.
c. Illicium:
As lllicium contains vessels, so it should be removed from Winteraceae (a vessel less family).
d. Annonaceae:
The family Annonaceae is placed under the order Annonales by Hutchinson (1959) and under Magnoliales by Takhtajan (1969). Studies on wood anatomy of the family by Purkayastha (1980) suggested that the Annonaceae have much advanced type of wood — this supported the view of Hutchinson.
e. Rhoipteleaceae:
The family has been placed either in Urticales or in Juglandales (Takhtajan). The anatomical studies of Withner (1941) supported the systematics of Rhoipteleaceae as done by Takhtajan. The scalariform perforation plates are found both in Juglandales and Rhoipteleaceae, but are absent in Urticales.
E. Floral Anatomy:
The floral anatomy also proved useful to solve the problems related to taxonomic and phylogenetic studies. This has been discussed in detail by different scientists like Eames, 1935; Puri, 1952, 62 and Murthy and Puri, 1980. A flower has typical arrangements i.e., sepals with 3, petals, stamens with 1 and carpels with 3 traces.
During the course of evolution, the vascular plane undergoes various modifications through amplification, reduction, cohesion and adhesion of vascular bundles. Such modifications are used as important tools in systematic studies.
Some of the examples are:
i. Paeonia:
The genus Paeonia has been separated from the family Ranunculaceae and placed in a separate family, Paeoniaceae, by the modern taxonomists based on floral anatomy.
The anatomical study shows:
a. Other genera of Ranunculaceae have sepals with 3 and petals with 1 trace, but Paeonia have few to several traces.
b. In other genera, stamen traces are derived independently, but in Paeonia stamens are supplied by few large trunk bundles.
c. In other genera the development of stamen is centripetal, but it is centrifugal in Paeonia.
d. In other genera the carpels have 3- traces, but Paeonia receives several traces.
The above differences justify the separation of Paeonia from Ranunculaceae and placed in a separate family of its own.
ii. Trapa:
The genus Trapa has been included in Trapaceae, after removing it from Onagraceae.
This has been justified by the following data based on vascular anatomy:
a. The vascular plan of flowers in Trapa differs from the other genera of Onagraceae.
b. The inferior ovary of Trapa has been considered to be receptacular due to down turning of the receptacular bundles. On the other hand, in other genera of Onagraceae, the ovary has been regarded as appendicular due to fusion of the bundles present in the same radii.
iii. Hydrocotyle Asiatica:
The Hydrocotyle asiatica L. has been transferred to the genus Centella by Urban and named it as Centella asiatica (L.) Urban. This was confirmed by Mittal (1955) based on floral anatomy
The observations are:
a. The inflorescence of Hydrocotyle is umbellose raceme, but in H. asiatica it is a cyme like Centella.
b. In the other species of Hydrocotyle, the ovular traces are derived from the placental strands, but in H. asiatica they are derived from the alternate bundles in each carpel as in the genus Centella.
4. Essay on Cytology:
Cytology plays an important role in the study of evolution, phylogeny and classification of plants. Classification of angiosperms in general and monocotyledons in particular has been a subject of considerable debate. Contradictory views have been held not only with broader subdivisions but also with the position of minor taxa. So the subject cytotaxonomy develops which utilises the cytological characteristics and phenomenon for the elucidation of taxonomic problems regarding the position of taxa at different level.
The Chromosome number, morphology and behaviour at meiosis are the parameteristics considered in this respect. In addition, cytochemistry and banding patterns are also used. A. Chromosome number: Chromosome number is constant in many taxa (species) which is considered as an important taxonomic characteristic. Phanerogams with lowest chromosome number (n=2) is found in Haplopappus gracilis of Compositae and the highest number (n= 263-265) is found in Poa litorosa of Poaceae.
In higher groups of plants, three main cytological types are found — diploid, aneuploid, and polyploids. In addition, haploid type is also reported in some species like Cocos nucifera, Prunus persica and Theobroma cacao. The chromosome number may or may not help identify the different taxa.
a. Not Help to Identify:
Due to uniformity in different species, the chromosome number does not make any help to identify the taxa, but it is useful to differentiate them from other taxa. All the species of Pinus have n=12. Similarly Quercus and most other members of Fagaceae also have n=12.
In the above cases chromosome number does not make any help to differentiate the species. But in both the cases, the species are easily distinguishable by morphological characteristics. The species differentiation has come through gene mutation without differentiation of their karyotype.
b. Help to Identify:
The polyploid plants are not recognised as species unless they are morphologically distinct. Only when the diploid and polyploids are clearly distinct, they can be recognised as different species, such as in Saxifraga hyperborea (2n=26) and S. rivularis (2n=52). Thus it is not accepted in Galium aparine of Rubiaceae, having the numbers 2n= 22, 44, 66 and even 88. In Salix, different species are considered to the different polyploidy members.
The basic number in Salix is 19 and the different diploid and polyploidy species that have been recognised are:
i. Salix viminalis is diploid (2n=38),
ii. S. atrocinerea is tetraploid (2n=76),
iii. S. phylicifolia is hexaploid (2n=114) and
iv. S. myrsinites is octaploid (2n=152).
In aneuploidy, the diploid monosomies are inviable normally and in trisomics (2n+1), the extra chromosomes produce certain minor variations, thus limiting their taxonomic significance. Several disomic (2n) species have been recognized in Taraxacum.
Chromosome number may help to maintain the actual position of taxon at generic level.
In Tephrosia of Fabaceae all the species have 2n=22, except T. constricta where 2n=16, justifying its separation as a new genus Sphinctospermum.
Both the plants Cicendia filiformis and Mixcrocala pusilla of Gentianaceae were placed under Cicendia. The former have the basic chromosome number 13 and the latter with 10 justifying their separation into different genera.
B. Chromosome Morphology:
Delaunay (1926) was the first to formulate the concept of karyotype as ‘a group of species resembling each other in the number, size and form of their chromosomes’. Karyotype studies mainly concerned with morphological aspects and are represented by figures termed as ‘Idiograms’ or ‘Karyograms’.
The principle parameters of such study to get the differences are:
a. The number of chromosome,
b. The relative size and form of chromosomes of the same set,
c. The size and the number of secondary constrictions and satellites,
d. Absolute size of the chromosomes, and
e. The distribution of eu- and hetero- chromatin.
The absolute size of the chromosomes in a karyotype is fairly constant and is considered as species-specific characteristic.
It has been noted in Chrysanthemum and Dianthus that the diploid have longer chromosomes than the polyploids. In Crepis, the annual species have smaller chromosomes than the perennials. The length of chromosomes alone always does not show taxonomic significance.
Another type of supernumery chromosome found in plants of many families (at least 250) is the B-chromosomes. These chromosomes are small and heterochromatic in nature. Variation occurs in their number. They do not pair with A-chromosomes but can pair with each other.
C. Behaviour of Chromosome at Meiosis:
Behaviour of chromosome at meiosis helps to find out the relationship of different species and the population as a whole. Two important aspects of chromosome behaviour are considered in respect of phylogenetic significance. These are: i. Synopsis during meiotic prophase, and ii. Frequency of chiasma.
Cytological studies in the above aspects help to solve the different taxonomic problems. Before dealing with the actual problem one should know the characteristics considered as criteria for deciding the status of a taxon.
a. Symmetrical karyotype (when all the chromosomes are of equal size with the centromere at the middle) indicates primitive characteristic and asymmetrical karyotype (chromosomes differ in size and are acrocentric with the centromere almost near the tip) indicates advanced characteristic.
b. Large chromosomes represent a primitive characteristic from which through gradual deletion small chromosomes have been evolved. (The reduction of size may involve in all or in some chromosomes of a genome. The condition further leads to asymmetry in the nature of karyotype).
c. Plants with low chromosome number indicate primitive and high chromosome number indicates as an advanced condition.
Briefly, it can be considered that the symmetrical karyotype with low chromosomes numbers and long metacentric chromosomes indicate primitive characteristics, whereas asymmetrical karyotype with high chromosome number and short acrocentric chromosomes are the advanced characteristics.
Some taxonomic problems can be explained at the different level of taxa.
Some examples are:
a. Pandanales:
Engler considered Pandanales, including 3 principal genera Pandanus, Typha and Sparganium as the ancient stock from which the monocotyledons have been evolved. On the other hand, Hutchinson considered Pandanales as an advanced group and the aquatic family Alismataceae as the progenitor of Monocots.
Cytological studies show that the basic chromosome number of all the three genera, Pandanus (n=30), Typha (n=15) and Sparganium (n=15) of Pandanales is x=15 and the chromosomes are small and acrocentric. Thus it shows that such a high number of small-sized chromosome undoubtedly represent an advanced level in evolution.
From the above data, it can be concluded that the consideration of Hutchinson as an advanced group for Pandanales is justified and the Engler’s idea about the primitiveness is not acceptable.
Based on habitat, Hutchinson separated Engler’s Pandanales into two orders, Pandanales (includes Pandanus, nearly terrestrial) and Typhales (includes Typha and Sparganium, both are aquatic).
Cytological studies show that in all the three genera the basic number is x=15 and they show chromosome homogeneity. All the genera are characterised by very small chromosomes with nearly identical types of constriction. Based on karyotype, they look very similar to each other indicating the homogeneity of Pandanales.
Thus the splitting of the order as done by Hutchinson is not acceptable and the idea of placing of all the three genera in a single order Pandanales as done by Engler is justified.
b. Alismatales and Butomales Complex:
John Hutchinson considered the Alismatales and Butomales complex as the primitive group of Monocotyledons because of the aquatic habit, and thought that all other members of Monocotyledons have been evolved from it.
The genus Alisma of Alismataceae has the characteristic feature of an ancient genus Alisma plantago-aquatica.
Because, it has:
i. Low chromosome number starting from x=5.
ii. Long chromosomes.
iii. Symmetrical karyotype.
The above characteristics indicate the potentiality of this complex to be the ancestral stock. But this is not true for all the members of Alismatales and Butomales complex. The genera like Hydrocharis of Hydrocharitaceae and Butomus and Limnocharis of Butomaceae and also others show asymmetrical karyotype with high chromosome number and are shorter in size.
So the above genera must not be recognised as primitive and the homogeneity of the complex group cannot be considered. Lastly, it can be concluded that the group should be broken up into several groups and should not be considered as the progenitors of all other groups of Monocotyledons. Only Alisma might have given rise to some of the groups.
c. Liliaceae and Amaryllidaceae:
Liliaceate have superior ovary that can be distinguished from Amaryllidaceae having inferior ovary. Engler placed both the families under the order Liliiflorae. Similarly, Hutchinson also placed the families Liliaceae and Amaryllidaceae into two separate orders: Liliales and Amaryllidales based mainly on the characteristics of inflorescence, but he constructed a new order Agavales. In his Agavales, he included several genera like Agave, Yucca, Dracaena etc. from Liliaceae and Polianthes from Amaryllidaceae.
The three tribes like Agapantheae, Allieae and Gillesieae were transferred to Amaryllidaceae by Hutchinson from Engler’s Liliaceae due to the presence of umbellate scapose cyme like Amaryllidaceae.
Cytological studies with the members of Agapantheae, Allieae and Gillesieae show that the chromosomes are long and look almost alike with the members of Amaryllidaceae. Though the karyotypes do not fully match, even in that manner they do not show drastic difference to make any obstacle in the inclusion of the above three tribes under the family Amaryllidaceae.
The five genera — Agave, Yucca, Dracaena, Sauservia and Cordyline — of Liliaceae and Polianthes of Amaryllidaceae were placed under Agavales. Funkia was not taken out by Hutchinson as was done by the modern cytotaxo- nomists.
Cytological studies on the three genera Agave, Yucca and Funkia show that they have chromosome no. x =30, with 25S (short) and 5L (long). The genus Polianthes also shows same result. The above result proves the justification to include Polianthes along with the other genera under Agavales.
On the other hand, the genera like Dracaena, Sauservia and Cordyline show x=18-21 and are of graded type. The above three genera do not fit with Agave and others and should go back to the family Liliaceae. Thus it can be said that the creation of Agavaceae with genera being taken out from two different families by Hutchinson was no doubt a bold approach.
It can be said that chromosomal characteristics alone are not always a reliable taxonomic criteria, nor they are considered as good phylogenetic markers.
5. Essay on Embryology:
Embryology plays a very significant role in systematic consideration and phylogeny. The role of embryology in the taxonomy of flowering plants has been discussed by Maheswari (1958, 1964), Johri (1963), Kapil and Bhatnagar (1980) and Cocucci (1983).
The embryology includes not only the characteristics of embryogeny and mature embryo, but also the characteristics of micro- and megasporogenesis.
Maheswari (1950, 1963) considered the following characteristics to discuss the taxonomic problems:
a. Arrangement and number of anther chambers, shape and thickening of endothecium and nature of tapetum.
b. Development and arrangement of the pollen grain.
c. Types of quadripartition in pollen mother cell.
d. Structure and development of the ovule.
e. Nature of nucellus.
f. Origin and extent of sporogenous tissue inside the ovule.
g. Arrangement of megaspores and mode of development of embryo sac.
h. Types of mature embryo sac.
i. Path of pollen tube growth and the time taken between pollination and fertilisation.
j. Presence and absence of endosperm, endosperm haustoria and also the types of endosperm.
k. Structure of mature embryo, etc.
Some taxonomic problems can be explained at the different level of taxa.
Some examples are:
a. Paeonia:
In earlier classifications, the genus was included in Ranunculaceae. Later, Hutchinson (1959) placed it in a separate family Paeoniaceae.
The embryological studies on Paeonia and other members of the Ranunculaceae show:
i. In Paeonia, the generative cell is large and elongated, the embryo sac is narrow and long, the seed coat is massive and seeds germinate by hypogeal means. The above characteristics contrast with the other genera of Ranunculaceae.
ii. In Paeonia, the embryogeny is unique which is not available in other members of Ranunculaceae.
The above embryological data support the thinking of Hutchinson (1959) in the separation (from Ranunculaceae) and placing of Paeonia in a separate family Paeoniaceae.
b. Butomus:
Bentham and Hooker divided the family Alismataceae into two tribes, Alismeae and Butomeae. Engler and Prantl treated the above two tribes in the rank of family, Alismataceae and Butomaceae (includes Butomus and other 5 genera Ostenia, Hydrocleis, Limnocharis, Tenagocharis and Elattosis). Hutchinson placed the families under separate orders Alismatales and Butomales, respectively.
The embryo- logical studies indicate:
i. The genus Butomus having anatropous ovules and straight embryo can be separated from the other five genera showing campylotropous ovules and ‘horse-shoe’- shaped embryo.
ii. Butomus has a Polygonum type of embryo sac, but the other genera have Oenothera type of embryo sac.
Therefore, Pichon (1946) suggested that all the five genera should be transferred to Alismaceae, although the placentation of the carpel is laminar.
Later, both Cronquist (1968, 1981) and Takhtajan (1969, 1980) included only Butomus under the family Butomaceae and the rest five genera under a separate family Limnocharitaceae and placed the families Butomaceae (unigenic), Limnocharitaceae and Alismaceae under the order Alismales (Alismatales).
c. Trapa:
The genus Trapa was placed in the family Onagraceae by Bentham and Hooker. Later, Engler segregated out the genus Trapa and placed it in a separate family Trapaceae for its perigynous flower, bilocular ovary and spiny fruit which ha§ been accepted by most of the recent taxonomists.
The embryological study shows:
i. In Trapa, the embryo sac is Polygonum type, but other genera show Oenothera type of embryo sac.
ii. In Trapa, the endosperm is lacking, but others have nuclear endosperm.
iii. In Trapa, the development of embryo is Solanad type, but it is Onagrad type in Onagraceae.
iv. In Trapa, the suspensor is well developed showing haustorial type, but it is short and inconspicuous in the family Onagraceae.
v. In Trapa, the cotyledons are unequal, but they are equal in other members of Onagraceae.
The above embryological studies support the Engler’s separation of Trapa from Onagraceae to a separate family Trapaceae.
The embryological studies are also useful to solve some problems at family level:
a. Saxifragaceae and Crassulaceae:
Both the families are placed very close under the order Rosales by the earlier taxonomists.
Presence of the following embryological characteristics of both the families justifies their consideration:
i. Multilayered anther wall.
ii. Bitegmic ovules.
iii. Polygonum type of embryo sac development.
iv. Endosperm cellular.
v. Embryo development Caryophyllid type.
b. Gramineae and Cyperaceae:
The earlier taxonomists like Bentham, Hooker and Engler and others placed the two families very close to each other.
The embryological studies show that the following characteristics are common in both the families justifying their consideration:
i. In Gramineae, the microsporogenesis is of successive type and the tetrad members are separated, whereas, in Cyperaceae, it is of simultaneous type and the tetrad members do not separate, where three degenerate and only one remains functional.
ii. In Gramineae, the archesporium is without parietal cell, but it is present in Cyperaceae.
iii. In Gramineae, the antipodals form a complex or become coenocytic, but it is ephemeral in Cyperaceae.
iv. In Gramineae, the testa and pericarp are fused and either only the inner integument form the testa or both are obliterated, but in Cyperaceae they are distinctly free and the testa consists of both the integuments.
v. In Gramineae, the embryogeny is of variable type, but it is of Onagrad type in Cyperaceae.
c. Lemnaceae:
The family is considered to have originated from Pistia of Araceae or from Helobiales. S. C. Maheshwari (1954, 1956. 1958) studied the embryological aspects and clearly indicates that the Lemnaceae are evolved from Araceae and are not related with the order Helobiales.
The Lemnaceae resemble with Araceae by the following characteristics:
i. Presence of bitegmic ovules with a micropylar cap.
ii. Cellular endosperm with a chalazal haustorium.
iii. Irregular sequence in the division of embryo development.
iv. Short and stocky suspensor.
v. Integumentary operculum in the seed.
The Helobiales having the following characteristics by which it differs:
i. The ovules do not have any micropylar cap.
ii. The endosperm is Helobial or nuclear type.
iii. Regular sequence in the division of embryo development.
iv. Prominent basal cell in the embryo.
6. Essay on Palynology:
G. Erdtman, published “Pollen Morphology and Plant Taxonomy of Angiosperms” in 1952, which opens a new phase in the role of palynology in taxonomy. Since then, it aggravated the use of pollen characteristics in taxonomic studies.
Later, Erdtman (1969) published an excellent review on the application of palynology in taxonomy. Investigations in this line become more aggravated by the use of Electron Microscope in pollen studies. The pollen characteristics are then being extensively used in taxonomic and evolutionary studies as mentioned in the publication of Blackmore, (1984).
The following pollen characteristics are used in taxonomic studies:
a. Size and Shape:
The sizes are very small, small, medium, large, very large and gigantic. The shape also varies with different views: in polar view it may be circular, triangular etc.; in equatorial view it may be prolate, perprolate, per- oblate, spheroidal etc. and in lateral view the bilateral pollens are planoconvex, biconvex or concavoconvex.
b. Exine Sculpturing:
The surface of the pollen may be smooth or they may be variously sculptured. It may be of two types: Excrescences type and Depression type.
Excrescences Type:
i. Spinulose (having spinules).
ii. Granulose (having minute granules).
iii. Gemmate (having rounded warts with concentric base).
iv. Tuberculate (having tubercule like excrescences).
v. Clavate (having club-shaped excrescences).
vi. Baculate (having rod-shaped excrescences), etc.
Depression Type:
i. Striate (lumina parallel).
ii. Rugulate (lumina anastomosing).
iii. Reticulate (lumina forms a network).
iv. Foveolate (lumina circular and closely placed).
v. Scrobiculate (lumina circular and distantly placed).
vi. Fossulate (lumina elongated), etc.
c. Apertures:
Based on the number, position and character (NPC) of aperture, the pollens are of different types.
i. Number: 1-many.
ii. Position: Proximal, distal, zonal or global.
iii. Characteristics: Colpate (furrow), porate (circular) and inaperturate.
d. Ultrafine Structures:
The ultrafine structures of sporoderm have been studied by Transmission Electron Microscopy which show considerable taxonomic value.
e. Pollen Association:
Pollens are generally associated in tetrads in angiosperms. Such association is common in dicots (Annonaceae, Ericaceae, Mimosaceae, Droseraceae etc.), less common in monocots (Cyperaceae, Eriocaulaceae and Juncaceae) and the extreme cohesion is found in Asclepiadaceae (dicot) and Orchidaceae (monocot), where the pollens remain in pollinia.
f. Nuclear Number in Pollen:
The nuclear condition of pollen in angiosperms has taxonomic importance. At the time of anthesis, pollens are 2- nucleate in monocotyledons, 2-nucleate in polypetalous dicotyledons and apetalous taxa, 3-nucleate in gamopetalous dicotyledons.
The characteristics of pollen are useful in the differentiation of various taxa and also to find out the phylogenetic relationship.
a. Family Level:
Araceae and Lemnaceae:
Hutchinson in his Arales included both Araceae and Lemnaceae. The family Araceae is eurypalynous with 1-2-4-colpate, 3-porate or inaperturate pollen with exine sculpturing and, on the other hand, Lemnaceae is stenopalynous with 1-porate and spinous pollen.
Bombacaceae:
Bentham and Hooker divided the family into four subfamilies, Malveae, Ureneae, Hibisceae and Bombaceae. Later, Engler treated Bombaceae as a separate family Bombacaceae. The exine of most of the members of Malvaceae is spinous, but the exine of Bombaceae is reticulate. The above study supporting the separation of Bombaceae as a family is justified.
Berberidaceae:
The family Berberidaceae consists of 12 genera. Modern taxonomists removed the genus Podophyllum from Berberidaceae and placed it in a separate family Podophyllaceae. The pollen grains in Podophyllum remain united, but they are free in other genera, supporting the removal of Podophyllum.
Orchidaceae:
Vij and Kashyap (1978) studied pollens of 50 orchid species and found three groups:
i. Single pollen grain (monad) (Cypripedium cordigerum, Paphiopedilum, etc.)
ii. Tetrads (in tribes Epidendrieae and Neottieae), and
iii. Perfect massulae (in Orchideae and some members of Neottieae).
b. Genus Level:
Salicaceae:
The family Salicaceae consists of 2 genera, Salix and Populus that can be distinguished on the basis of pollen characteristics. The genus Populus has spherical pollen without distinct aperture, whereas there is long and narrowed 3-furrowed pollen in Salix.
Phytolaccaceae:
The family consists of 22 genera. The pollen studies on two genera Rivinia and Phytolacca indicate that they can be distinguished very easily on pollen characteristics. The pollen of Rivinia is pantocolpate, whereas it is 3-zonocolpate in Phytolacca.
c. Species Level:
The pollen characteristics help in differentiating the species within a single genus:
Anemone:
Based on germinal aperture, the different species of Anemone (Ranunculaceae) can be distinguished. The pollens are 3-zonocolpate in Anemone obtusiloba, pantocolpate in A. rivularis, pantoporate in A. alchemillaefolia and spiraperturate in A. fulgens.
Malva:
Based on pollen size, the different species of Malva (Malvaceae) can be distinguished. The size of pollen ranges from 105- 126μm in M. sylvestris, but in M. rotundifolia it is only 74-84μm.
Bauhinia. Based on the exine patterns, the different species of Bauhinia (Fabaceae) can be differentiated:
a. Pilate in B. acuminata.
b. Reticulate in B.racemosa.
c. Striate in B. krugii.
d. Verrucate in B. retusa.
e. Spinulate in B. malabarica and
f. Reticulate-tuberculate in B. purpurea.
The different species of Ranunculus can easily be identified by studying the pollens. Pollen characteristics also help to distinguish the cultivers of Canna, Bougainvillea etc. Different sizes of the areoles on the exine surface help to differentiate the cultivers and hybrids of Cajanus cajan (Fabaceae).
7. Essay on Phytochemistry:
The chemical characteristics of plants attract the plant taxonomists since the early part of 1960s and since then it demanded as promising subdiscipline of taxonomy. This subdiscipline is called as Chemotaxonomy or Biochemical systematics.
The phytochemical characteristics of taxonomic significance have been classified into three main groups:
i. Primary Constituents:
These are proteins, nucleic acids, chlorophylls and polysaccharides.
ii. Secondary Constituents:
These are simple phenolic compounds (benzoic, caffeic and nicotinic acid) and polyphenolic compounds (terpenes, flavonoids, coumarines and pigments).
iii. Miscellaneous Substances:
This group includes ellagitannins, iridoid compounds, etc.
The following content deals with some of the above compounds in relation to taxonomy:
a. Amino Acids:
In relation to the distribution of non-protein amino acids in seeds, Bell (1962) recognised 7 infra- generic groups in Lathyrus. Similarly 4 groups have been recognized in Vicia. The differentiation based only on protein characterise is very difficult.
Thus, Cronquist (1976) made the comment that “It is the phenotype, not individual genes, that characterises the organism and is subject to selection. This whole is greater than the sum of its parts, and it cannot be reconstructed by separate analysis of all those parts”.
The serological studies help in finding the relation among the different taxa. The genus Liriodendron had been found to be quite distinct from other members of the family Magnoliaceae and both the genera Michelia and Magnolia displayed closest affinity within the family.
Later, Pickering and Fairbrothers (1970), based on their finding, supported the classification of Umbelliferae into Hydrocotyloideae, Saniculoideae and Apioideae and concluded that the Apioideae is more closely related to Saniculoideae than the Hydrocotyloideae.
b. Phenolic Compounds:
The phenolic compounds are ellagic, caffeic, benzoic and nicotinic acids. The ellagic acid is present only in the tribe Kerrieae under the subfamily Rosoideae of Rosaceae, but it is absent in other tribes of the Rosoideae.
c. Polyphenolic Compounds:
The polyphenolic compounds are flavonoids, coumarins, terpenes, alkaloids etc. The flavonoides have been widely exploited in chemotaxonomic studies. Their role has been reviewed by Harborne (1975). Swain (1975) reported that the structural complexity of flavonoides increases along with evolution, thus the Chara has glycoflavones, whereas proanthocyanidines are present in higher pteridophytes (ferns), gymnosperms and in some angiosperm progenitors.
Flavonoides have been used to trace the species relationships in Chenopodium and to predict generic affinities among Ulmaceae. Giannasi and Chuang (1976) studied the flavonoides in Perideridia of Umbelliferae. Though the 16 species of Perideridia have same flavonoids, they are divided into three groups based on certain specific combinations.
These are:
i. Producing only flavonoides,
ii. Producing mainly flavonols along with a flavones, and
iii. Producing mainly flavones.
d. Alkaloids:
These are heterogeneous groups of organic nitrogen containing bases, often with a heterocyclic ring. Alkaloid content has been considered as a useful source of taxonomic consideration. The members of Fabaceae have lupin alkaloides, Papaveraceae have isoquinoline alkaloids and Solanaceae have tropane derivatives.
In some cases, their distribution is restricted to a particular species, such as conine to a few members of Apiaceae, strychnine to a few species of Strychnos and morphine is found only in Papaver somniferum.
In the subfamily Lotoideae of Fabaceae, three tribes Podalyrieae, Genisteae and Sophoreae are characterised by the presence of lupin alkaloids suggesting their origin from common ancestor.
Presence of isoquinoline alkaloids in both the family Papaveraceae and Fumariaceae indicate their close relationship.
e. Betalains:
Betalains differ from rest of the phenolic compounds owing to the presence of nitrogen in them. These are red and yellow (betacyanins and betaxanthins) pigments and found in only 10 families of angiosperms that have been traditionally included under a single order Centrospermae. The Cactaceae was placed in an order of their own, the Cactales or Opuntiales. But according to phytochemists for the presence of betalains, they should be kept under Centrospermae instead of Cactales.
f. Terpenoids:
These are volatile compounds and are almost universal in their occurrence. Earlier, plants were characterised by smell due to the presence of various terpenes e.g., species of Cymbopogon of Poaceae, Ocimum of Lamiaceae, the tribe Tagitineae of Asteraceae and so on.
Mirov (1961) done the classical work in plant taxonomy on gum terpentines of Pinus. The terpenes such as monoterpenes, diterpenes and sesquiterpenes have been studied in relation to taxonomy. Irwin (1968) distinguished 3 sympatic taxa in Hedeoma under the family Lamiaceae by using their terpenoides. Flake and Turner (1973) studied the terpenoids in various populations of juniperus virginiana and tried to explain their affinity.
g. Other Substances:
Some visible chemical substances such as starch grains and raphides also provide taxonomic significance. Form of starch grains along with other characteristics have been used in the classification of Gramineae (Poaceae). Forms of the crystals of calcium oxalate on ovarian wall in Compositae (Asteraceae) have considerable taxonomic value. Similarly presence or absence of raphides provides an useful characteristic in the classification of Rubiaceae.
8. Essay on Ecology:
Ecological studies also play a very significant role in systematic consideration. The ecological variation in plants is the result of response to environment involving different factors like climatic, edaphic and biotic. These factors have direct effect on phenotype and indirect effect on the genotype. Thus the appearance of the individual plant is the result of their genetic constitution and environmental influences.
Turesson collected a series of population samples from different parts and grown in uniform condition in his experimental garden at Akarp in Sweden. He observed that in some cases the differences observed in the field disappeared in respect of height, habit flowering time, etc. These plants are called ecotypes. Others, which show persistent differences, appeared to be due to genetic differences.
Later, Clausen, Keck and Hiesey have researched on this between 1940 and 1950. They carried out the geneological studies with population of various species of Achillea millefolium of Asteraceae and Potentilla glandulosa of Rosaceae. Experimental gardens were established at varying altitudes at 30 to 500 metres and also at 3,300 metres and were maintained free from any other vegetation including weeds.
They observed the response of the above plants in different climatic conditions. Crossing between the ecotypes resulted into fully fertile hybrid and produced new genotype with new adaptive features. So, it indicated that the interaction between habitat of plant and its genotype could produce ecotype.
George et al. (1936) grown different races of Plantago maritima of Plantaginaceae in experimental garden under uniform environmental conditions and observed that the pattern of ecotype variation was more frequently continuous than discontinuous, corresponding to gradients shown in the natural habitat. The continuity of ecotype variation was also supported by Bocher (1943) with experiment on Plantago lanceolata.
Phenetics and Cladistics:
Systematics is the study of the nature, causes, patterns and trends in variation among taxa. The plant systematics encompasses plant species which can be recognised, classified, circumscribed, and named with a logical relationship based on evolution exist among these units.
These are two basic methods used in establishing relationship between the organisms:
These are:
(a) Phenetic method, and
(b) Phylogenetic or Cladistic method.
(a) Phenetic Method:
In this method, the classification is based on the overall similarity of taxa from all available sources such as morphology, anatomy, palynology, embryology, phyto- chemistry, cytology, ultrastructure, etc., along with all other field data.
Thus, in phenetic classification, the taxa are grouped by shared features, regardless of whether their similarities are symplesiomorphies (shared primitive characters) or synapomorphies (shared derived characters) in a phylogenetic sense. This classification is denoted by a phenogram (graphic presentation).
(b) Phylogenetic or Cladistic Method:
This classification is based on the phylogenetic studies, indicating the presumed ancestor/descendent relationship. Thus, only shared derived, characters (synapomorphies) are used to assess phylogenetic relationship, while the shared ancestral characters (symplesiomorphies) are of no value for such classification.
The grouping of taxa is made by a genealogical descent, as denoted by a genealogical tree or phylogenetic tree or cladogram. The approach of such study is called cladistics and the scientists using this method are known as cladists.
Phenetics Versus Cladistics:
The character states are of two basic types: homologous characters and analogous characters. The homologous characters are those which have been inherited from a common ancestor. The analogous characters are the derived characters which are the results of independent evolutionary events such as paralled or convergent evolution.
The ancestral characters are termed plesiomorphies and derived characters are called apomorphies. Thus, shared ancestral characters are called symplesimorphies, similarly shared derived characters are known as synapomorphies.
Only, synapomorphies are used to assess phylogenetic relationship (cladistics) and symplesiomorphies are used to interprete the relationship among the groups under investigations. Thus, synapomorphies alone are of no value for interpreting relationships among the groups (phenetics) to be studied, vis-a-vis symplesiomorphies are not used in cladistic analysis.
In phenogram and cladogram construction, the resultant groupings are quite different, even though the same data matrix is used. For example, in the data from the character x taxon matrix (Fig. 4.28), taxa B and C would be grouped together in a phenogram of phenetic classification (Fig. 4.28). Because, taxon B and taxon C share the states of more character (state 0 of characters 2 and 3) than either does with taxon A.
In cladistic approach, with the addition of a hypothetical ancestor XYZ (Fig. 4.29), it is apparent that taxa A and B share a derived common feature (state 1 of character 1) indicating their recent common ancestor than either does to taxon C. Therefore, taxa A and B would be grouped together in cladogram. The similarity between taxon A and taxon C are all symplesiomorphic (shared ancestral character states) and do not bear any information with regard to phylogeny.
The phenetics may be preferable in classifying taxa of low rank such as species. The cladistics rely on the principle of parsimony and assume the existence of past evolutionary events which can be detected today as a synapomorphy in defining a particular monophyletic group.
