Polyembryony

 

Occurence of more than one embryo in a seed is called as polyembryony.
The first case of polyembryony was reported in certain orange seeds by Anton Von Leuwenhoek
Polyembryony in angiosperms may arise by :
1. Cleavage of Embryo
2. Formation of embryos by cells of Embryo Sac other than the egg
3. Development of more than one embryo sac within same ovule
4. Activation of some sporophytic cells of the ovule

I. Cleavage polyembryony
Cleavage and proliferation of zygote or it's derivatives leading to the establishment of seperate primordia is widespread among gymnosperms.
Swamy (1943) recorded three modes of supernumerary embryo formation:
1. Zygote divides irregularly to form a mass of cells of which those lying towards the chalazal end grow simultaneously and give rise to many embryos,
2. The proembryo gives out small buds and outgrowths which may themselves function as embryos,
3. The filamentous embryo becomes branched, and each branch gives rise to an embryo

II. Embryos from cells of the embryo sac other than the egg:
In this category the most common source of additional embryos are the synergids
Depending on whether it arises from fertilized synergid or unfertilized synergid, the embryo may be diploid or haploid.
In Aristolochia bracteata, Poa alpina besides the egg and the polar nuclei, one or both the synergids may be fertilized.
This can be brought about by the entry of more than one pollen tube into the embryo sac or by the presence of additional male gametes in the same pollen tube.
Embryos arising from unfertilized synergids are known in Argemone mexicana, Phaseolus vulgaris.
Formation of embryos from antipodals is rather rare. It has been observed in Paspalum, Ulmus.

III. More than one embryo sac in the same ovule
Multiple Embryo Sac in an ovule may arise from:
1. Derivatives of the same megaspore mother cell,
2. Derivatives of two or more megaspore mother cells,
3. Nucellar cells, eg., Casaurina, Poa, Citrus, Loranthus

IV. Activation of some sporophytic cells of the ovule or Adventive Embryony
The embryos arising from the maternal sporophytic tissue ( outside the embryo sac) are called adventive embryos.
The only maternal tissues which are known to form adventive embryos are the nucellus and the integuments. Eg., Citrus, Mangifera, Optunia, Trillium
Nucellar embryos can be distinguished from the zygotic embryos by their lateral position in the Embryo Sac, irregular shape and lack of suspensor.

Causes of polyembryony:
Many theories have been advanced to explain the occurrence of the polyembryony:
Haberlandt proposed the " necrohormone theory". He regards the degenerating cells of the nucellus as source of stimulus for the adjacent cells to divide and form adventive embryos.

Haberlandt attempted to induce adventive polyembryony in Oenothera by damaging cells by pricking the ovules with a fine needle and by gently squeezing the ovary.

Frusato etal showed the embryo number in Citrus seeds may be influenced by the following factors:

1.     Age of the tree; increasing in older trees,

2.     Fruit-set; being higher in years of higher fruit set,

3.     Nutritional status of the plant

4.     Orientation of the branch of the tree; being higher on Northern than on Southern branches.

Importance of Polyembryony:

Nucellar Adventive polyembryony is of great significance in Horticulture.

The adventive embryos provide uniform seedlings of the parental type, as obtained through vegetative propogation by cutting.

However, nucellar seedlings of Citrus furnish better clones than cutting; because:

1.     The nucellar seedlings have a tap root and, therefore develop a better root system than do the cuttings. The latter have only a small lateral root system.

2.     The nucellar seedling show a restoration of the vigour after repeated  propagation by cutting.

3.     Free from disease.

Apomixis

 

The normal sexual cycle called amphimixis involves two important processes: a) Meiosis - through which a diploid sporophytic cell divides and forms four haploid gametophytic cell, 2) Fertilization - in which two haploid gametes of opposite sex fuse and form a zygote re-establishing the diploid sporophytic generation.
This, in a sexual cycle a diploid generation sporophyte alternates with a haploid generation gametophyte in angiosperms. The gametophytic generations are very short and are represented by embryo sac on the female side and microspore pollen grain on the male side. The remaining part of the cycle belongs to the sporophyte generation.

Plants where the usual reproduction has been completely replaced by a type of asexual reproduction are called apomictic and the phenomena is known as apomixis.

 Following Winkler, who introduced the term apomixis may be defined as the substitution of the usual sexual reproduction by a form of reproduction with does not involve meiosis and syngamy.  A species may include sexually as well as a apomitically reproducing individuals.

There are two main categories of apomixis: 1. Vegetative propagation and 2. Agamospermy

Agamospermy: The phenomenon in which the plants have retained seeds as the agent of propagation but the embryo is formed by some process in which normal meiosis and syngamy have been eliminated.

Three different types of agamospermy are recognised: a. Adventive embryony, b. Diplospory, 3. Apospory.

Adventive Embryony: in this type of agamospermy, the embryos arise from diploid sporophytic cells of the ovule lying outside the embryo sac, either from the nucellus or integuments.
In this the gametophytic generation is completely eliminated. The sexual embryo sac develops in normal way and the zygotic embryo either degenerates or competes with the apomictic embryos.
The embryo that is formed shows true morphological features i.e., presence of cotyledons, radicle, plumule, epicotyl and hypocotyl.
A favourite example of adventive Embryony is that of Citrus, in which 3-5 embryos are common. Also occurs in Cactaceae, Euphorbiaceae, Orchidaceae.

Diplospory:
The phenomenon where a diploid embryo sac is formed from a megaspore mother cell without regular meiotic division is called as diplospory.
In this type an archesporium differentiates, but the megaspore mother cell develops into an unreduced embryo sac. Eg., Areva tomentosa, Ixeris

Apospory:
Apospory was first reported by Rosenberg in the Hieracium spp.
A somatic cell in the nucellus directly forms an unreduced embryo sac, and the diploid egg parthenogenetically develops into an embryo.
The MMC goes through the usual meiotic division, but just about this stage a somatic cell situated in the chalazal region begins to enlarge and become vacuolated.
This cell gradually increases in volume, encroaching upon the megaspores and finally crushing them.
Eg., grasses, Crepis, Cliftomia, Malus, Ranunculus.

Parthenogeneis:

Diplospory as well as apospory produce diploid embryosacs. Now, to complete the apomitic cycle without altering the chromosome number of the sporophyte generation, the diploid egg must develop into a embryo without the participation of the male nucleus.

Formation of embryo from an unfertilised egg is called parthenogenesis.

In autonomous apomitics (Compositae and Rubiaceae) the development of embryo is independent of the pollination stimulus.

Howerver, in many apomitic species, the embryo develops only after pollination; the phenomenon is known as pseudogamy, eg., grasses.

Heslop-Harrison has suggested three possible roles of pollination in pseudogamy;

i.  To activate the growth of ovary and ovule,

ii. To supply the male nucleus for endosperm development, and

iii.   To stimulate parthenogenesis.

Importance of Apomixis:

Offers the possibility of indefinite multiplication of favourable biotypes without any variation due to segregation or recombination.

Structure of Megasporangium or Ovule

 

     The megasporangium together with its protective coats the integuments is called ovule. It is attached to the placenta on the minor wall of the ovary by a stalk called funiculus. An ovule ready for fertilization consists of nucellar tissue enveloped almost completely by one or two integuments leaving a small opening at the alrical end. This opening called micropyle, is the main passage for the entry of pollen tube into the ovule

    The basal region of the ovule where funiculus in attached is called chalaza in the nucleus is present the female gametophytes, also called embryo sac

INTEGUMENTS:

    The protective coverings of nucleus are called integuments. Mostly an ovule has either one or two integuments ovules with one integument are called unitegmic those with two integuments are known as bitegmic

       The sympetalae predominantly show unitegmic condition. Bitegmic ovules occurs in polypetalae,  moncots in some members of Olacaceae, Crinum, Viscum,  Loranthus,  Santalum, Balanophora etc. The ovules that lacks  integuments are calld ategmic

    Ontogenetically, the ovule arises as a small mound of homogenous tissue on the placenta in the ovary. Integuments arises close to the base of this tissue which forms the nucleus in the nature ovule

  Except for Euphorbiaceae where the inner integuments is initiated sub dermally in all others it is dermal in origin . The outer integuments is initiated either dermally or sub dermally . Although the integuments initiated later they grow faster than the nucleus soon surround it almost completely except in the region of the micropyle

 The unitegmic condition of ovules is considered to the derived from fusion of the two integuments as in some Myrtaceae, or  by suppression of one integuments.

Various degrees of fusion among parts is a common feature of ovule; the two integuments in a bitegmic ovule may be fused along their length or the inner integuments may be fused with the nucleus upto various lengths. In anatropous ovules very often the outer integuments on the side of the funiculus is almost indistinguishable from the funiculus because of their congenital fusion.

In some taxa especially belonging to the family Cactaceae, a prominent air space is present between the two integuments in these chalazal region. This feature is also shown by Basia, Petragonia tetragonoides and Tricanthema acquatius.

 Occurence of stomata on the outer integument has been reported in Cleome, Canna, Nelumbium, Isomeris. In Gossypium stomata differentiate into chalazal region of the outer integument two days before anthesis. Such stomata are suggested to serve in respiration rather than in a transpiration or in photosynthesis.

In addition to stomata abundant chlorophyll is present in the integuments of Hymenocallis, Amaryllis, Gladiolus, Lilium, Moringa

 Endothelium:

In most plants belonging to sympetalae with unitegumic ,tenuinucellate ovules, the nucellus degenerates at as early stage of ovule development and the inner most layer of the integument becomes specialised to perform the nutritive function from the embryo sac. This specialised tissue present around the embryo sac is called endothelium. So far the occurrence of endothelium has been observed in 65 families of dicots. The endothelium is usually single layered, in Compositae it may become multi-layered, ten to twelve layered endothelium is known in sunflower.

 An interesting feature of the endothelial cell is the development of adventitive embryos. Maheshwari Devi and Pullaiah have reported that in Melampodiun divaricatum sometimes the endothecial cells look like egg and undergo divisions forming structure resembling zygotic embryos. Such embryos lack suspensor and in this respect duffer from the zygotic embryos.

In Begoniacea, Droseraceae, Elamtinaceac, Papilionaceace the persistent nucellar cells from endothelium like tissue because of different origin has been called false endothelium

MICROPHYLE:

Depending upon the presence or absence of integuments the microphyle may or may not organized. In the bitegemic ovules the microphyle is generally formed by either both the integuments only the inner integuments. Only rarely does the outer integuments alone constitute the microphyle.

When both the integument are involved the passage formed by outer integument is called Exostone and that by the inner  integument is called Endostone. In post fertilisation stages a plug is formed that  occludes the microphyle. The plug probably has a protective role against dessication and pathogen invasion.

OBTURATOR

Any ocular structure associated with directing the growth of pollen tube toward the microphyle is generally referred to as obturator

Obturator exhibit great variation in their origin, morphology, anatomy and extent of development. They may originate from placenta or funicullus or both.

The most common type of obturator is one formed by local swelling if the funiculus(Anacardiaceae, Labiatae). In Crinum the funiculus simply becomes knee shaped function as obturator. In Tetragonia tetragonoides the obturator comprising glandular epidermal hairs, arises from both side if the long funiculus.

  Placental obturator occurs in the Euplurbiaceace, Cuscutaceae. In Aegle some epidermal cells of funiculus as well as placenta elongate into richly cytoplasmic multicellular hairs reaching upto microphyle.

The pollen tube grows along the obturator. The cells of the obturator produce a surface exudate and provide nutrition and mechanical and chemical guidance to the pollen tube

NUCELLUS:

It represents the wall of megasporangium. Each ovule had only one nucellus. As an abnormality however twin nucelli may occur in a common fold of integuments this had been observed in Aegle marmelos Hydrocleis etc..

The archesporium differentiates immediately below the nucellar epidermis. In sympetalae the acchesporial cell directly function as the megaspore mother cell so that the sporogenous cell is also hypodermal.

Such ovules where the sporogenous cell is hypodermal the nucellar tissue around it remains single layered are called tenuinucellate.

In some other families the hypodermal archesporial cell divides transversely cutting an outer parietal cell an inner sporogenous cell. The parietal cell may either remain undivided or undergo periclinal and anticlinal divisions so that the sporogenous cells becomes embedded in the massive nucleus. The sporogenous cell may also become embedded in the nucellar tissue by divisions in the nucellae epidermis.

All such ovules where the sporogenous cells become sub hypodermal either due to the formation of parietal cells or due to division in the nucellar epidermis or both  called crassinucellate..

Davis (1966) has however suggested that only these ovules should be referred to as crassinucellate where the sporogenous cells become sub hypodermal due to the occurrence of parietal cells..

She has proposed the term pseudo crassinucellate for all those ovules where divisions in the nucellar epidermis are a responsible for the sub hypodermal nature of the sporogenous cell..

According to Davis of the 314 families for which informal is available 179 families show crassinucellate ovules, 105 families bear tenuinucellate ovules and 11 families posses pseudo-crassinuccelate ovules.

Generally the nucellus remains within the confines of the inner integuments rarely however it may project into the microphyte or beyond if forming a nucellar beak. Nucellar beak has been reported from Euphorbiaaceae, cucurbitaceae, Nyctaginaceae, polygonaceae, Salicaceae of cells may store starch protein crystals

The nucleus is mostly consumed by the developing embryosac or endosperm. In some plants it persists in the mature seed as a nutritive tissue. The persistent nucelllus is called perisperm.

There is other extreme where the nucellar tissue breaks down precocious consequently a large cavity called pseudo embryo sac is formed around the embryo sac.  This feature is unique to the family Podostemaceae. In absence of endosperm in the Podostemaceae, the pseudo embryosac which contains cytoplasm for nuclei, nourishes the developing embryo.

HYPOTASE:

It refers to a group of cells present below the embryosac and above the vascular supply to the funiculus. They become thick walled due to lignification and are poor in cytoplasmic contents.

Hypotase occurs in many families such as Amaryllludaceae, liliaceae, zingiberaceae, Euphorbiaceae, Theaceae, Umbellifareaceae, in the horanthacea a Hypotase is present below the archesporium.

EPITASE:

Refers to a group of cells present above the embryosac. This tissue persists as a hood over the apex of the embryosac for a long time in Costus and Castalia. It forms a cup like structure of cutirized cells and is distinguishable even during advance stages of embryo development.

Types of Ovules

Orthotropous

1. The ovule is straight, without any curvature

2. Micropyle, chalaza, funicle and embryo sac lie in a straight line. 

3. Ex – Polygonum, Piper

Anatropous

1.  Body of ovule becomes completely inverted to 1800.

2. The micropyle lies close to funicle

3. Micropyle, Embryo sac and chalaza lie on the same line.

4. Ex – Helianthus, Ricinus

Campylotropous

1. The body of the ovule is placed at right angles to the funiculus.

2. The body of ovule bends in such a way that micropyle comes towards funiculus. 

3. Micropyle and chalaza do not lie on the same straight line.

4. Ex – Pisum, Mustard

Dic

Secondary Wood or Secondary Xylem

 

The wood is a product of secondary growth. It is essentially composed of secondary xylem elements namely Trachery elements (Vessel and tracheids), Wood fibres (Fibre tracheids, libriform fibres and gelatinous fibres) and wood parenchyma (axial parenchyma).

Study of wood comes under separate discipline Xylatomy. The wood of Gymnosperms is generally designated as softwood, while that of Angiosperms as hardwood. This distinction is based upon the presence or absence of one important component in the wood i.e., fibres. In gymnosperms, the fibres are absent and have only tracheids, hence the wood of gymnosperm is called as softwood. In angiosperms, the ground tissue is made up of fibres between tracheids and vessels. Fibres are more stronger than tracheids and impart a great mechanical strength. Hence, the wood of angiosperms is called as hardwood.

Further, the wood of gymnosperms is designated as non-porous wood because it lacks vessels, while that of angiosperms is porous wood because of presence of vessels. The gymnosperm wood is more homogenous when compared to angiosperms wood because there are less number of kinds of wood elements in the former than later.

Typically gymnosperm wood consists of following elements- a) Tracheids, b) Xylem rays, c) xylem parenchyma and d) Resin ducts

Elements of angiosperm wood : a) Vessel, b) tracheids, c) fibres, d) xylem rays, e) parenchyma, d) gum ducts, e) Resin ducts.

The wood elements are disposed in two ways 1. Horizontal system and 2. Vertical system .

The elements of vertical system are aligned to the vertical axis of trunk consisting of vessels and tracheids, wood fibres, axial parenchyma, vertical resin canal, etc. 

The horizontal system consists of xylem rays, ray parenchyma and horizontal gum ducts.

The secondary xylem of dicots is more complex than that of the gymnosperms. The arrangement of the vessels in the secondary xylem of dicots is a characteristic feature and is used in the identification of species.

Wood anatomist refers to a vessel in cross section as a pore. Two principal types of woods are recognized on the basis of distribution of pores in a growth layer – diffused porous wood and ring porous wood.

Arrangement of the vessels or pores in the secondary xylem of dicots is a characteristic feature and is used in the identification of species.

When the vessels are more or less equal in diameter and uniformly distributed throughout the growth ring, the wood is termed as diffuse porous wood. Example – Acer, Populus, Betula, Acacia, Olea, Eucalyptus.

When the wood contains vessels or pores of different diameters and in which those produced at the beginning of the season are distinctly larger than those of the late wood, and they are arranged in the form of conspicuous ring at the beginning of the growth ring, the wood is said to be ring porous wood. Example – Fraxinus, Quercus, Pisticia, Morus.

Ring porous wood is thought to be more advanced than diffuse porous wood from the phylogenetic point of view. The former is found only in relatively few species and mainly in plants from Northern Hemisphere.

Parenchyma cells are frequently met within the xylem tissue of most of the plants and are referred as xylem or wood parenchyma. These cells are more or less elongated, placed end to end and may be thick or thin walled.

Ontogenetically, development of xylem parenchyma cells is supposed to be from fusiform initials. In secondary xylem, the xylem parenchyma is of two types: i. Axial parenchyma: parenchyma cells are arranged end to end in vertical rows among the trachery elements. This cells are rectangular to elongated with horizontal end walls. Ii. Radial parenchyma: aligned horizontally or radially.

Parenchyma associated with the vessel is called as paratracheal parenchyma. Parenchyma not associated with the vessel is called as apotracheal parenchyma.

Tectona Grandis

Family: Verbenaceae

Vernacular Name: Teku, Sagwan, Teak

General Characters:

Sap wood is distinct from the heart wood. Sap wood white to pale yellowish-brown. Heart wood golden-brown with darker streaks, turning deep-brown on exposure to air.

Wood with oil feel, strongly and characteristically scented.

Wood moderately hard, moderately heavy. Average weight 650 kg/m³. Medium to coarse texture with straight grain.

Growth rings distinct. Ring porous wood (possess pores or vessles of different diameter with distinctly large pores in the early wood than in the late wood).            

Soft tissue or parenchyma predominantly vasicentric i.e., forming thin sheath around the pores and also delimiting growth rings.

Fibres non libriform, gelatinous, coarse, septate.

Rays distinct, visible to naked eye.

 

Dalbergia latifolia

Family: Fabaceae

Vernacular Names: Shisham, Jitregi, Rosewood

General Characters of the wood:

Sapwood is distinct from heart wood. Wood with pale-yellowish-white with pinkish tinge.

Heartwood purplish-brown to purple with darker streaks.

Wood with faint pleasant odour.

Wood hard, heavy, average weight – 815kg/m³ at 12% moisture content.

Texture coarse, grains straight to shallow inter-locked grains.

Diffuse porous wood (pores that are more or less equal in diameter and uniformly distributed throughout the growth ring).

Growth ring distinct to fairly distinct, pores large to small visible to naked eye..

Growth ring distinct to fairly distinct, pores large to small visible to naked eye.

Soft tissue (parenchyma) mostly around the pores, forming eye-lets, with lateral extensions often connecting the adjacent pores by narrow, wavy, tangential bands.

Rays fine to very fine, distinct only under the lens.

Ripple marks distinct, seen only under the lens.

Wood is categorized as first-class wood and yields the ‘most handsome Indian black-wood or Rose wood’.

Pterocarpus santalinus

Family: Fabaceae

Vernacular names : Rakta-chandanam, Red-sanders, Lal Chandan.

General Characters of the wood:

Sap wood distinct from heart wood.

Sap wood orange-red to claret purple.

Heart wood purplish-black and yields red dye (santalin)

Wood very hard to heavy with inter locked grains with lot of fibres, coarse textured.

Diffuse porous wood.

Pores occluded with reddish brown gum deposits.

Soft tissue (parenchyma) paratracheal.

Rays not visible to naked eye, very fine, closely placed separated by rows of parenchyma.

Fibres present, abundant, semi-libriform to libriform.

Ripple marks present, not visible to naked eyes.

A valuable timber of class with heavy demand.

Termenalia tomentosa

Family: Combretaceae

Vernaular names: Nalla maddi

General Characters of wood:

Soft wood distinct from heart wood.

Soft wood pinkish-white to pinkish grey.

Heart wood walnut-brown to deep brown with darker streaks.

Wood heavy to very heavy with coarse texture with interlocked grains. Average weight – 880 kg/m³ at 12% moisture content.

Growth rings present but visible under the lens, delimited by a fine-line of parenchyma.

Soft tissue (parenchyma) predominantly aliform, forming light-coloured eye-lets around the pores.

Rays fine to very fine, seen only under the lens as numerous closely spaced lines.

 

Pterocarpus marsupium

Family: Fabaceae

Vernacular names: Bijasal, Pedda-egisa.

General Characters of wood:

Sap wood distinct from heart wood.

Sap wood pale-yellowish or nearly white.

Heartwood golden-brown with darker streaks, turning brown with age.

Wood moderately hard to very hard, moderately heavy to heavy, average weight – 800 kg/m³ at 12% moisture content.

Medium to coarse-textured.

A diffuse porous wood contains a yellow dye.

Growth rings distinct or indistinct.

Soft wood (parenchyma) predominantly narrow, wavy, partially enclosing the pores.

Ray very fine, not visible to the eye.

Ripple marks present and visible to eye.

 

Azardirachta indica

Family: Meliaceae

Vernacular names: Vepa, Neem.

Sap wood distinct from heartwood.

Sap wood greyish-white.

Heartwood red first, when exposed turn to reddish brown..

Wood with characteristic taste and aroma, moderately heavy, with narrowly inter-locked grains, medium  to coarse textured.

Growth rings distinct, sharply delimited by narrow brown, concentric lines of terminal parenchyma.

Wood parenchyma paratracheal.

Fibres non-libriform to semi-libriform in radial rows forming extensive tracts between vessels to rays.

Rays visible to naked eye, medium to fine, heterogenous

Plant Taxonomy - Principles of Classification

 

Taxonomy is the oldest disciplines of biology. The ideas of taxonomy were there on the earth, right from the beginning of human civilization. Man classified the plants and animals around him, on the basis of their usefulness. The scientific classification of organisms started from the time of Aristotle, a great Greek Philosopher. The credit for classification of plants goes to Theophrastus (370 285 BC), a contemporary of Aristotle.

The term ‘ taxonomy ‘ introduced in 1813, by Augustin Pyramus de Candolle in his book “ Theories Elementaire de la Botanique.” It is a combination of two Greek words Taxis + Nomos, where Taxis means orderly arrangement and nomos means  discourse / study. In biological classification, the taxonomic group of any rank is known as a taxon (pl.taxa). Herman J. Lam (1948), proposed the term ‘taxon’.

Till the end of 19 century, the systems of classification proposed were based on external morphological features, mainly of the flower. Such a classical approach known as “Alpha taxonomy”. In later years the information from other disciplines also taken into consideration, particularly in proposing Phylogenetic classification. Various branches like Anatomy, Cytology, Embryology, Chemistry and Palynology, in addition to morphology are taken into consideration.  Thus the multidisciplinary, synthetic approach is an attempt to achieve “Omega taxonomy”.

Taaxonomy deals with 3 aspects. 1. Identification 2. Nomenclature and 3. Classification

1.     Identification

It is the determination of taxon, whether or not it is similar to the known which has been recorded earlier. It is fulfilled by consulting the botanical literature or material in the herbaria. The botanical literature refers to Flora or Monographs. The plant life in a given geographical area is known as flora and the enumeration, description and means of identification contained in a book form as Flora.

 

Based on description of the taxa, artificial keys are provided in the Flora. The plants are identified easily, by descriptions and keys. Herbarium is another source of identification. It may be international (Royal Botanical Garden Herbarium – Kew), national (Central National Herbarium – CAL, Kolkata), or regional (Herbarium Hyderabadensis – Osmania University, Hyderabad).

 

2.     Nomenclature

It is concerned with allocation and determination of names, also concerned with construction, application of rules of naming. The vernacular names are not acceptable for usage in botanical literature as there is no universality. The polynomials are in vogue centuries ago is replaced by Binomial nomenclature introduced by Gaspard Bauhin. Later Caroli Linnaei (1753), used binomial nomenclature, more methodically in his book ‘ Species Plantarum’.

 

3.      Classification

Classification is the orderly arrangement of plants. Grouping of like organisms is made. Then the units so made, are named on the basis of hierarchy of categories.

 

 Types of classification

Broadly classificatory types are divided into 3 main types       A) Artificial Classification   B) Natural Classification    C) Phylogenetic Classification

 

A)     Artificial Classification 

In this type one / two characters are given importance, on the account of medicinal and commercial considerations.  Theophrastus “ the grandfather of modern botany”  classified plants into herbs, sub shrubs, shrubs and trees is an example to artificial classification. Linnaeus also made sexual system of classification, in which he recognised 24 classes, mainly on the basis of number, length, union and separation of stamens and carpels.

 

B)     Natural classification

This is the type of classification in which, use of as many characters as possible from natural habitats is made to group the taxa which are similar are placed together. The first scheme of classification based on natural characters was presented by Antoine Laurent de Jussieu of France in 1789. Contemporary to A.L. de Jussieu,  Augustin Pyramus de Candolle (1778 – 1841), presented a new classification of plants and put all alike plants together.

The latest, the best and highly recognised natural system of classification was proposed by George Bentham (1800 – 1884) and Joseph Dalton Hooker  (1817 – 1911). They classified plants strictly on the basis of natural scheme, and published in their book               “ Genera Plantarum.”

 

C)     Phylogenetic classification

This classifications are based on Darwin’s theory of organic evolution in 1859 in the book “ On the origin of Species”. Botanists started working on the concept of evolution regarding the development of a classification system of plants. S. Endlicher and A.W.Eichler, two German botanists, were first to start along this line of thought. Their schemes were later modified and developed by Adolf Englar (1844 – 1930) and Karl Prantl (1849 – 1893), which was published in “Die Naturlichen Pflanzenfamilien.”

 

 

 

 

 

 

 

 

 

Bentham and Hooker’s system of Classification

George Bentham  (1800 – 1884) and Joseph Dalton Hooker (1817 – 1911) proposed  the most accepted natural system of classification in the three volumed Genera Plantarum in Latin.

Bentham and Hooker’s system of classification is still used and followed in several herbaria of the world. Most of the Indian herbaria are also arranged according to this system of classification. It is the best system to identify plants in the laboratories. The generic descriptions of the plants prepared from their own observations. In all they described 97,205 species belonging to 200 families of flowering plants.

 

The Bentham and Hooker’s system of classification is clearly derived from the systems of de Jussieu and de Candolle. They divided all Phanerogams or seed plants into Dicotyledons, Gymnosperms and Monocotyledons. Ranales were placed in the beginning and the grasses at the end in this classification. A summary outline of their classification is mentioned below.

 

A)     Dicotyledons ( reticulate venation, 2 cotyledons, pentamerous flowers )

1.      Polypetalae (Corolla of separate petals)

 

Series  I : Thalamiflorae ( stamens many, hypogynous, disc absent)

 

Order 1. Ranales : Ranunculaceae, Magnoliaceae, Annonaceae, Nymphaeaceae and 4 more families.

 

Order 2. Parietales : Papaveraceae, Capparidaceae, Brassicaceae, Violaceae and5 more families.

 

Order 3. Polygalineae : Polygaleae and 3 more families.

 

Order 4. Caryophyllineae : Caryophyllaceae, Portulacaceae and 2 more families.

 

Order 5. Guttiferales: Guttiferae (Clusiaceae) and 5 more families.

 

Order 6. Malvales : Malvaceae, Tiliaceae and Sterculiaceae.

 

 

Series II : Disciflorae (hypogynous flowers, disc present)

 

Order 1. Geraniales : Geraniaceae, Rutaceae, Meliaceae and 8 more families.

Order 2. Olacales : Olacineae and 2 more families.

Order 3. Celastrales : Rhamnaceae and 3 more families.

Order 4. Sapindales : Sapindaceae, Anacardiaceae and Sabiaceae.

 

Series  III : Calyciflorae ( perigynous or epigynous flowers, ovary inferior )

Order 1. Rosales: Leguminosae, Rosaceae and 7 more families.

Order 2. Myrtales : Combretaceae, Myrtaceae, Lythraceae and 3 more families.

Order 3. Passiflorales : Cucurbitaceae, Begoniaceae and 5 more families.

Order 4. Ficoidales : Cactaceae and Ficoideae.

Order 5. Umbellales : Umbelliferae and 2 more families.

 

2.      Gamopetalae ( Petals of corolla partially / completely fused )

 

Series I. Inferae ( inferior ovary )

thOrder 1. Rubiales : Rubiaceae and Caprifoliaceae

Order 2. Asterales : Compositae and 3 more families

Order 3. Campanales : Campanulaceae and 3 more families

 

Series II. Heteromerae ( ovary superior, androecium of one / two series, carpels more than 2)

Order 1. Ericales : Ericaceae and 5 more families

Order 2. Primulales : Primulaceae and 2 more families

Order 3. Ebenales : Sapotaceae and 2 more families

 

Series III. Bicarpellatae : ( ovary superior, androecium of 1 series, carpels 2)

Order 1. Gentianales : Oleaceae, Apocyanaceae, Asclepiadaceae and 3 more families

Order 2. Polemoniales : Convolvulaceae, Solanaceae and 3 more families

 Order 3. Personales : Scropulariaceae, Pedaliaceae, Bignoniaceae, Acanthaceae and 4 more families.

Order 4. Lamiales : Labiatae, Verbenaceae and 2 more families

 

3.      Monochlamydeae (Petals absent )

Series I. Curvemryeae ( embryo coiled, ovule generally 1) : Amaranthaceae, Chenopodiaceae, Polygonaceae and 4 more families

 

Series II. Multiovulate aquaticae (ovules many, immersed aquatics): Podostemaceae.

Series III. Multiovulatae terrestris ( ovules many, plants terrestrial ) : Nepenthaceea and 2 more families

Series IV. Microembryeae ( embryo very minute) : Piperaceae and 3 more families

Series V. Daphnales (ovary with 1 carpel and 1 ovule) : Proteaceae and 3 more families

Series VI. Achlamydosporeae (usually inferior ovary, 1 locule with 1-3 ovules) : Loranthaceae, Santalaceae and Balanophoreae

Series VII. Unisexuales ( Flowers unisexual) : Euphorbiaceae, Urticaceae and 7 more families

Series VIII. Ordines anomali (families of uncertain relationship) : Ceratophyllaceae and 3 more families.

B)      Gymnospermae (naked seed plants) : Gnetaceae, Coniferae and Cycadaceae

 

C)     Monocotyledons (Parellel venation, one cotyledon and trimerous flowers)

 

Series I. Microspermae (inferior ovary, minute seeds ): Orchidaceae and 2 more families

Series II. Epigynae (inferior ovary, large seeds) : Iridaceae, Amaryllidaceae and 5 more families

Series III. Coronarieae (superior ovary, coloured perianth) : Liliaceae, Commelinaceae and 6 more families

 Series IV. Calycineae (superior ovary, green perianth) :  Juncaceae, Palmae and Flagellariaceae

 Series V. Nudiflorae (perianth usually absent, superior ovary) : Typhaceae, Araceae and 3 more families

Series VI. Apocarpae (carpels free) : Alismaceae and 2 more families

Series VII. Glumaceae (reduced perianth, bracts large, scaly) : Cyperaceae, Gramineae and 3 more families.

 

Merits of Bentham and Hooker’s classification

 

1.      It is the first great natural system of classification.

2.      It is very easy to follow for all practical purposes, and that’s why Kew Herbarium and several other Herbaria of the world, including India, are arranged according to this system.

3.      The classification starts with Ranales (primitive) and ends with Glumaceae (advanced) and the same is in consonance with the present concept. Arber, Parkin and Hutchinson were expressedsimilar view with regard to the primitive flower.

4.      In this system the monocots are derived from dicots. Several recent taxonomic findings support this view.

 

                            Demerits

1.      The position of Gymnosperms in between dicots and monocots is foremost demerit.

2.      Several important floral characters have been neglected.

3.      Some closely related families have been separated and placed under different orders (cohorts). In the same way, a number of unrelated families have been grouped nearer.

4.      Advanced families such as Orchidaceae, have been considered primitive by placing them in the beginning.

5.      The entire arrangement of monocots is unnatural and unphylogenetic in this system.

 

Englar & Prantl’s system of classification

Adolf Englar (1844 – 1930) & Karl Prantl (1849 – 1893) of Germany  proposed a phylogenetic system of classification. This system of classification was based on Eichler. They published their work in 23 volumes of Die Naturlichen Pflanzenfamilien. This work consists of well illustrations, provides keys and description of all the plant families known to them at that time. They classified all the plants from algae to angiosperms. This system is used in most of the non – British herbaria of the world.

The followers of Englar and Prantl published revised classification in several successive editions of syllabus der Pflanzenfamilien. The twelfth edition of Syllabus, dealing angiosperms, was edited by Melchior in 1964.  

The most noteworthy features of Englar & Prantl’s system of classification are that they (i) placed monocots before dicots (ii) considered orchids to be more evolved than grasses, and (iii) considered apetalous and catkin – bearing dicots primitive to the dicots bearing petals and simple unisexual flowers.

 

Subdivision – Angiospermae

 Class 1. Monocotyledoneae

         Orders 1. Pandanales (Typhaceae)    

                       2. Helobiae (Alismataceae & 6 more)   

                      3. Triuridales (Triuridaceae)   

                      4. Glumiflorae (Cyperaceae & Gramineae)    

                      5. Principes (Palmae)   

                      6. Synanthae (Cyclanthaceae)   

                      7. Spathiflorae (Araceae, Lemnaceae)

        8. Farinosae (Commilinaceae and 12 more) 

        9. Liliflorae (Juncaceae., Liliaceae, Amaryllidaceae, Iridaceae and 5 more)  

       10. Scitamineae (Musaceae & 3 more)

       11. Microspermae (Orchicdaceae & Burmanniaceae)

 

Class 2. Dicotyledoneae

Sub class 1. Archichlamydeae

Orders 1.   Verticellatae (casuarinaceae)

2. Piperales (Piperaceae & 2 more)

3. Hydrostachyales (Hydrostachyaceae)

4. Salicales (Salicaceae)

5. Garryales (Garryaceae)

6. Myricales (Myricaceae)

7. Balanopsidales (Balanopsidaceae)

8. Leitneriales (Leitneriaceae)

9. Juglandales (Juglandaceae)

10. Julianiales (Julianiaceae)

11. Batidales (Batidaceae)

12. Fagales (Fagaceae, Butolaceae)

13. Urticales (Moraceae, Urticaceae, Ulmaceae)

14. Podostemonales (Podostemonaceae)

15. Proteales (proteaceae)

16. Santanales (Santalaceae, Loranthaceae & 5 more)

17.  Aristolochiales (Aristolochiaceae and 2 more)

18. Balanophorales (Balanophoraceae)

19. Polygonales (Polygonaceae)

20. Centrospermae (Chenopodiaceae, Amaranthaceae, Nyctaginaceae and 7 more)

21. Ranales (Ranunculaceae, Magnoliaceae, Annonaceae and 15 more)

22. Rhoeadales (Papavaraceae, Capparaceae, Cruciferae and 4 more)

23. Sarraceniales (3 families)

24. Rosales (Rosaceae, Leguminosae and 15 more)

25. Pandanales (Pandanaceae)

26. Geraniales (Geraniaceae, Rutaceae, Meliaceae, Euphorbiaceae and 17 more)

27. Sapindales (Anacardiaceae and 22 more)

28. Rhamnales (Rhamnaceae and Vitaceae)

29. Malvales (Malvaceae, Tiliaceae, Bombacaceae, sterculiaceae and 3 more)

30. Parietales (Violaceae and 30 more)

31. Opuntiales (Cactaceae)

32. Myrtiflorae (Myrtaceae, Combretaceae and 21 more)

33. Umbelliflorae (Umbelliferae and 2 more)

 

Subclass 2. Metachlamydeae (Sympetalae)

Orders 1. Diapensiales (Diapensiaceae)

               2. Ericales (Ericaceae and 3 more)

               3. Primulales (Primulaceae and 2 more)

               4. Plumbaginales (Plumbaginaceae)

               5. Ebenales (sapotaceae and 6 more)

               6. Contortae (Apocynaceae, Asclepiadaceae, Oleaceae and 3 more)

               7. Tubiflorae (Convolvulaceae, Borginaceae, Verbenaceae, Labiatae, Solanaceae,    Scrophulariaceae, Bignoniaceae, Pedaliaceae, Acanthaceae and 13 more)

               8. Plantaginales (Plantaginaceae)

               9. Rubiales (Rubiaceae and 4 more)

               10. Cucurbitales (Cucurbitaceae)

               11. Campanulatae (Campanulaceae, Compositae and 4 more)

 

Merits of Englar & Prant’s Classification

1.      This is a convenient and well known filing system of several herbaria of the world.

2.      Polypetalae and Monochlamydae of Bentham & Hooker were merged by Englar & Prantl into single subclass Archichlamydae.

3.      This system treated families such as Orchidaceae and Compositae as advanced families.

4.      In this system several closely related families (Liliaceae, Juncaceae, Iridaceae and Amaryllidaceae) are treated close to one another.

5.      Abundant illustrations are provided along with the description of the families.

6.      The system is provided with exhaustive keys of families and orders.

7.      The description of each family also contains a summary of its embryology, morphology, anatomyand geographical distribution.

 

Demerits

1.      Monocots have been placed before dicots in this system. But it has been revised  in 1964 edition.

2.      Naked flowers of Amentiferae have been treated as primitive in this system.

3.      Helobiae, consisting of primitive forms, have been placed between two advanced orders Glumiflorae and Pandanales.

4.      Araceae are derived from Liliaceae, but Englar & Prantl placed Araceae before Liliaceae.

5.      This system fails to recognize the significance of reduction, and becauseof this “ simple” were equated with “primitive” according to Cronquist (1965).

 

Angiosperm Phylogeny Group  (APG)

Angiosperms / flowering plants, traditionally have been divided into 2 primary groups based on the presence of a single cotyledon (Monocots) or two cotyledons (Dicots). This division has accounted for the long recognition of those groups in flowering plant classifications. However phylogenetic analysis based on nuclear, plastid and mitochondrial  DNA sequences and morphology do not support this dichotomy. Cladistic analysis of the families of dicotyledons revealed that their diagnostic characters represent simply the plesiomorphic ( = primitive characters within angiosperms. Thus, the dicots represent a paraphyletic group.

On the other hand, the monocots formed a monophyletic group, and this clade (= branch) is given the name Monocots. Within the dicots majority of the species share the characters traditionally attributed to dicots, such as flowers with 4/5 parts and tricolpate pollen, thus forming a monophyletic group. This group of flowering plants had been called eudicots or tricolpates.

During the 1900’s, reconstruction of flowering plant phylogeny took great step forward. Rapidly accumulating DNA sequences, in particular from the plastid gene rbc L, Cp DNA mat K gene, mitochondrial gene atp A and 18 Sr DNA provided new and informative sets of data. At the same time the development of effective PCR techniques made possible to apply cladistic methods of analysis.

This new knowledge of phylogeny revealed relations in conflict with the widely used modern classifications ( Cronquist, 1981, Thorne 1992, Takthajan, 1997) which were based on selected similarities and differences in morphology. It became clear that none of the previous classifications accurately reflected the phylogenetic relationships of the flowering plants.

To solve this problem, a group of flowering plant systematic, calling themselves as the Angiosperm phylogeny Group (APG), proposed a new classification for the families of flowering plants.

This group is composed of B.Bremer (Royal Swedish Academy of Sciences, Sweden), K.Bremer (Uppsala University, Sweden), M.W.Chase (Royal Botanic Gardens, Kew, UK), J.L.Reveal (University of Maryland, Colorado, USA), D.E. Soltis (University of Florida, USA), P.S.Soltis (Florida Museum of Natural History, USA), and Peter F.Stevens (University of Missouri, USA).

The APG system of classification proposed in 1998 (APG I, 1998), comprised 462 families arranged in 40 monophyletic orders under small number of informal monophyletic higher groups : monocots, commelinoids, eudicots, core eudicots, rosids, eurosids I, eurosids II, asteroids, euasterids I and euasterids II.

Judd et al. (1999) presented some modifications in APG – I, by recognising a total of 51 orders and shifting some families to these orders from informal groups.

A recent revision of APG (APG – II, 2003) and continuous up gradation on Angiosperms phylogeny has resulted in considerable refinement.

A broad outline of APG – II (2003) classification is presented below.

 

Magnoliophyta

Group                                                order

                                                            Austrobaileyales

1. Monocots                                    1. Acorales

                                                            2. Alismatales

                                                            3. Asparagales

                                                            4. Dioscoreales

                                                            5. Liliales

                                                            6. Pandanales

 

2. Magnolids                                    1. Magnoliales

                                                            2. Laurales

                                                            3. Canellales

                                                            4. Piperales

                                                            5. Ceratophyllales

 

3.Commelinids                                1. Arecales

                                                            2. Poales

                                                            3. Commelinales

                                                            4. Zingiberales

4. Eudicots                                        1. Ranunculales

                                                            2. Proteales

 

5.Core Eudicots                               1. Gunnerales

                                                            2. Caryophyllales

                                                            3. Santalales

                                                            4. Saxifragales

 

6.Rosids                                             1. Crossosomatales

                                                            2. Geraniales

                                                            3.  Myrtales

7.Eurosids I                                       1. Celastrales

                                                            2. Malpighiales

                                                            3. Oxalidales

                                                            4. Fabales

                                                            5. Rosales

                                                            6. Cucurbitales

                                                            7. Fagales

8.Eurosids II                                     1. Brassicales

                                                            2. Malvales

                                                            3. Sapindales

9.Asterids                                          1. Cornales

                                                            2.Ericales

10.Euasterids I                                 1. Garryales

                                                            2. Gentianales

                                                            3. Lamiales

                                                            4. Solanales

11. Euasterids II                1. Aquifoliales

                                                            2. apiales

                                                            3. Asterales

                                                            4. Dipsacales

 

Unplaced families :  Amborellaceae, Cabombaceae, Chloranthaceae and Nymphaeaceae.

 

Current concepts of Angiosperm Taxonomy

Morphological characters of plants been used extensively both for producing classification and for identification purposes and still they are indispensable for the taxonomist. However morphological features alone are not adequate in proper assessment of the systematic status of a taxon and its phylogeny. Evidences from other disciplines like cytology, anatomy, embryology, Physiology, palynology, phenology, biochemistry and genetics etc, have been found to be useful in solving some of the taxonomic problems. The impact of the above disciplines on present day taxonomy has changed it from alpha (classical) to omega (modern) taxonomy.

Embryology in taxonomy

Embryology is the study of micro and megasporogenesis, gametophyte development, fertilization and development of endosperm, embryo and seed. Embryological evidences have been used in solving the taxonomical problems at all levels. Role of embryology in solving taxonomic problems was done by German embryologist, Schnarf in 1931. According to Jones and Luchsinger (1987), the embryological characters have proved to be significant help “in determining relationships within families, genera and species”.

Some examples of role of embryology in taxonomy

1.      Dicots and monocots:  Angiosperms are universally divided into dicotyledons and monocotyledons. This classification is based on number of cotyledons.

2.      Caryophyllales:  Trinucleate pollen, bitegmic  crassinucellate ovules which are campylotropous, seed with peripheral embryo and perisperm with little or no endosperm, are the characters found only in Caryophyllales, widely known as Centrospermae (Cronquist, 1968).

3.      Helobiae :  the monocotyledonous order, treated as a subclass in some recent systems of classification, by presence of Helobial type of endosperm.

4.      Orchidales : the distinguishing feature of the members of this order is presence of undifferentiated embryo and very little or no endosperm.

5.      Onagraceae :  this family is recognized by the presence of Onagrad type of embryosac.

6.      Cyperaceae : in flowering plants, 4 functional microspores developfrom each microspore mother cell (pollen mother cell). But in Cyperaceae, each microspore mother cell gives rise to only one pollen grain. Out of 4 nuclei, formed by meiosis, 3 are cut off on one side and do not form pollen grains.

7.       Exocarpus :  because of a naked ovule and pollen chamber, exocarpus was removed from Santalaceae of angiosperms and was treated as a member of the family Exocarpaceae near Taxaceae in gymnosperms. But the presence of a typical angiospermic flower, polygonum type of embryosac, cellular embryosac led Ram (1956) to confirm that Exocarpus belongs to the family Santalaceae of angiosperms, not with gymnosperms.

8.      Trapa : majority of the taxonomists treat trapa as a genus of Onagraceae, while others consider it to belongs to the family Hydrocaryaceae. But embryological details (polygonum type embryosac, absence of endosperm, well developed suspensor haustorium etc) suggest that Trapa should be treated under an independent family Trapaceae.