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