Types of Embryo Sac Development in Angiosperms

 

Megaspore is the first cell of female gametophyte or embryo sac. Megaspores in turn are formed by the megaspore mother cell after its meiotic division. The development of female gametophyte in angiosperm is completely endosporous i.e., within the megaspore.

The formation of the female gametophyte from one or more megaspore nuclei is called megagametogenesis.

Maheshwari formulated  a classification of different types of embryo sac development based on the following characters: i) the number of megaspores or megaspore nuclei which participate in the formation of embryo sac, ii) the total number of divisions which are found during the formation of the megaspore and the female gametophyte and iii) the number, arrangement of the nuclei and their chromosome number in the mature embryo sac.

Depending on the number of megaspores taking part in the development, the embryo sac of angiosperms may be classified into three main categories - monosporic, bisporic and tetrasporic. Each main group has more than one type of embryo sac named after the genus in which it was first described.

In monosporic type, only one of the four megaspores takes part in the development of the embryo sac. In bisporic type, two megaspore nuclei take part in the development of the embryo sac. In tetrasporic type, all the four megaspore nuclei take part in the development of the embryo sac.

Monosporic Embryo Sac

The embryo sac in which only one out of four megaspores is involved in the development of the embryo sac is called as Monosporic Embryo Sac. Since all the nuclei in such embryo sac are derived through mitoses of a single nucleus, hence all nuclei are genetically identical.

On the basis of number of nuclei in the mature embryo sac, Maheshwari recognised two types - 8 - nucleate and 4- nucleate.

8- nucleate or Polygonum type:

This type of embryo sac development was first discovered in Polygonum divaricatum for the first time by Strasburger, hence it is called as polygonum type. Since, this is common in Angiosperms(81%), it is called as normal type.

Of the four megaspores formed from the megaspore mother cell, only one megaspore situated towards chalazal end remains functional and the remaining three situated towards micropylar end degenerate.

The functional megaspore divides and gives rise to 2 nuclei, one of which moves to the micropylar pole and called as primary micropylar nuclei and the other moves to the chalazal pole and is called as primary chalazal nuclei.

The second division produces one pair of nuclei at micropylar and chalazal end. The third division results in two groups of 4 nuclei at the opposite poles of the elongated embryo sac.

The 4 nuclei towards the micropylar end differentiates into a three celled egg apparatus and the upper polar nucleus. The 4 cells towards the chalazal end differentiates into three antipodal cells and the lower polar nucleus.

The two polar nuclei fuse together somewhere in the middle of the embryo sac to give rise to a secondary nucleus.

The egg apparatus at the micropylar endconsists of two synergids and an egg cell. The other three nuclei at the chalazal end form the antipodal cells.

This type of embryo sac is most common and generally known as the normal type of embryo sac.

4 – nucleate or Oenother Type:

Greets in 1908 first discovered a 4-nucleate monosporic type of embryo sac in Oenothera lamarckiana. Hence it is called as Oenothera type.

This type of embryo sac is formed by the micropylar megaspore of the tetrad. The megaspore nucleus undergo mitotic division and form two nuclei which remain at the micropylar end. These two nuclei undergo one mitotic division and form 4 nuclei. Thus megaspore nuclei undergo only two nuclear divisions instead of the three occurring in the Polygonum type of embryo sac.

Thus, 4 nuclei are produced which organize to form 4-nucleate embryo sac. Three nuclei remain at the micropylar end and form an egg apparatus consisting of one egg cell and two synergids. The fourth nucleus acts as polar nuclei. Since, the third division is omitted and all the nuclei are situated in the micryopylar end of the embryo sac, there is neither a lower polar nuclei nor any antipodal cells.

This type of embryo sac development is found to be a characteristic and constant feature of family Onagraceae.

 

Bisporic Embryo Sac

In plants bearing the bisporic embryo sacs the first meiotic division in the megaspore mother cell is accompanied by wall formation, so that a dyad is formed. The upper cell of the dyad is much smaller and soon degenerates.

In the functional dyad cell, the nucleus undergoes second meiotic division to form 2 megaspore nuclei. Wall formation does not occur after the second division, and both the megaspore nuclei contribute to the formation of the embryo sac.

Since a bisporic embryo sac is derived from two meiotic division their nuclei belong to different genetic constitution.

Allium Type:

The chalazal dyad cell remains functional and gives rise to a 8 nucleate embryo sac. Strasburger described for the first time a bisporic embryo sac in Allium fistulosum.  Each megaspore nucleus undergoes two mitotic divisions forming eight nuclei.

The eight nuclei formed organize similar to the polygonum type.

Endymion Type:

In this type, the micropylar dyad cell remains functional and gives rise to a 8 nucleate embryo sac.

 

Tetrasporic Embryo Sac

In this type of Embryo sac development, the megaspore mother cell undergoes two meiotic division forming four nuclei. Neither of the meiotic divisions is accompanied by wall formation so that at the end of meiosis all the four haploid nuclei remain in a common cytoplasm forming a coenomegaspore. 

All the four nuclei of the coenomegaspore take part in the formation of the embryo sac. A tetrasporic embryo sac is more heterogenous than a bisporic embryo sac because the four nuclei of meiosis are involved in the formation of the embryo sac are genetically different.

Before the onset of postmeiotic mitosis, the manner in which the four nuclei in the coenomegaspore are arranged is of three types:

1)     1+1+1+1 arrangement: one nucleus toward micropyle, one nucleus toward chalaza and other two placed laterally (one on each side), eg., Peperomia, Penaea, Plumbago type.

2)     2+2 arrangement: two nuclei toward micropyle and two  towards chalaza, eg., Adoxa type

3)     1+3 arrangement: one nucleus toward micropylar end and three towards the chalaza, eg., Dursa, Fritillaria and Plumbagella type.

16-nucleate tetrasporic type:

Peperomia type:

In this type, as a result of two meiotic divisions in the megaspore mother cell 4 nuclei are formed. These for nuclei undergo two mitotic division and form 16 nuclei.

Of these 16 nuclei, two nuclei at the micropylar end become cellular and form egg apparatus consisting of one egg cell and one synergid. 6 nuclei at the chalazal end become cellular and form 6 antipodals. The remaining 8 nuclei fuse in the centre of embryo sac and form secondary nucleus with 8n condition.

Penaea type:

In this type, as a result of two meiotic divisions in the megaspore mother cell 4 nuclei are formed. These for nuclei undergo two mitotic division and form 16 nuclei.

These 16 nuclei organise into 4 groups; one group toward micropyle, one group toward chalaza and two groups lateral (4+4+4+4) arrangement.

One nucleus from each group migrate towards the centre of the embryo sac. They give rise to tetraploid secondary nucleus. The three nuclei at the micropylar side form egg apparatus by the formation of walls. It consists one egg cell and two synergids.

The remaining 9 nuclei organize into 3 groups of antipodal cells by formation of septa which are situated on 3 sides. These are called as triads.

 Drusa type:

In this type, as a result of two meiotic divisions in the megaspore mother cell 4 nuclei are formed. These are arranged in 1+3 manner. One nucleus at the micropylar end and the remaining 3 nuclei at chalazal end. These four nuclei undergo two mitotic division and form 16 nuclei.

 4 nuclei are present at the micropylar end and 12 nuclei at the chalazal end. Of the 4 nuclei at the micropylar end 3 nuclei organize and form egg apparatus. The fourth nuclei function as upper polar nucleus.

Among the 12 nuclei at the chalazal end one function as lower polar nuclei. Both upper and lower polar nuclei move in the centre of embryo sac and form diploid secondary nucleus.

The remaining 11 nuclei at the chalazal end organise into antipodals.

8 -  Nucleate Tetrasporic Embryo sac:

Adoxa Type:

It was first studies in Adoxa moschatellina by Jonsson. Embryo sac is 8 nucleate formed after a single post meiotic mitosis and has Polygonum type of arrangement. It is also found in Sambucus, Tulipa, Ulmus etc.

In this type, as a result of two meiotic divisions in the megaspore mother cell 4 nuclei are formed, of these two nuclei are present at each pole.

The four nuclei undergo one mitotic division and form 8 nuclei. Of these 4 nuclei from the micropylar end 3 nuclei organsie into egg apparatus by the formation of cell wall and one function as upper polar nuclei.

The four nuclei from the chalazal end 3 nuclei organize into antipodals and one remaining nucleus function as lower polar nuclei. The two polar nuclei migrate towards the centre and give rise to a secondary nucleus.

Fritillaria type:

This type of embryo sac was first observed in Fritillaria bulbiferum.

In this type, as a result of two meiotic divisions in the megaspore mother cell 4 nuclei are formed, which are arranged in 1+3 manner i.e., one nuclei at micropylar end and 3 towards the chalazal end.

The three nuclei at chazal end fuse together and form a triploid nucleus. Now the embryo sac shows only 2 nuclei one haploid nuclei at the micropylar end and one triploid nuclei at chalazal end.

Each nuclei undergo one mitotic division forming 4 nuclei out of which 2 are haploid nuclei and two are triploid nuclei.

These four nuclei undergo second mitotic division and form 8 nuclei – 4 towards micropylar end are haploid and 4 towards chalazal end are triploid.

Of the four nuclei at  micropylar end, 3 organize into egg apparatus and one functions as upper polar nuclei. At the chalazal end, 3 nuclei organize into triploid antipodals and one functions as triploid lower polar nuclei.

The two polar nuclei( one haploid and one triploid move towards the centre of the embryo sac and form a tetraploid secondary nuclei.

  Plumbago type:

This type of embryo sac was first observed in Plumbago capensis.

In this type, as a result of two meiotic divisions in the megaspore mother cell 4  haploid  nuclei are formed, which are arranged in 1+1+1+1 manner i.e., one nuclei at micropylar end and 1 towards the chalazal end and remaining two on lateral side, one at each side.

These 4 nuclei undergo mitotic division and form 8 nuclei. Out of 2 nuclei at micropylar end, 1 nuclei function as egg cell. One nucleus from each side migrate towards the centre and form a tetraploid secondary nucleus. The remaining 3 nuclei which lie on the sides forms cell and later degenerate.

 4- nucleate Embryosac or Plumbagella type:

It was discovered in Plumbagella micrantha.

Four megaspore nuclei are formed as a result of meiosis in megaspore mother cell.

These 4 nuclei are arranged in 1+3 manner – one nucleus at the micropylar end and the other 3 at the chalazal end.

The three nuclei at chalazal end fuse and form a triploid nucleus.

So, there are two nuclei – one haploid nuclei at micropylar end and one triploid nuclei at chalazal end.

Now, these two nuclei present at opposite poles undergo mitotic division and form 2 haploid nuclei at micropylar end and two triploid nuclei at chalazal end.

Of the two haploid nuclei at the micropylar end one nucleus forms egg cell and other nucleus acts as upper polar nucleus. Of the two triploid nucluei at the chalazal end one acts as lower polar nucleus and other forms an antipodal cell.

In the centre of embryo sac the two polar nuclei fuse and form an tetraploid (4n) secondary nucleus. The important characteristic of this type of embryo sac are the presence of one triploid antipodal cell, one egg cell and one tetraploid secondary nucleus. The synergids are absent.

Structure of Embryosac or Female Gametophyte of Angiosperms

 

The Female gametophyte or embryo sac in angiosperms is eight nucleated and seven celled. It is called Polygonum type, since it was first studied in Polygonum divaricatum by Strasburger.

After the last nuclear division in the female gametophyte, the cleavages are such that all the cells of the embryo sac are formed within the wall of the parent megaspore.

The embryo sac has three main parts:

1.     Egg apparatus             2. Antipodals              3. Central Cell

Egg Apparatus:

Three nuclei of the embryo sac towards micropyle develop into an egg apparatus. As these nuclei are covered by thin walls, they are considered as ‘cells’.

The egg apparatus consist of one egg cell and two synergids. The three cells of the egg apparatus an egg cell and two synergids are arranged in triangular fusion.

EGG CELL:

The middle cell of the egg apparatus is big and acts as an egg cell. The egg shows common wall with the two synergids the central cell.

  The wall is thicker in the micropylar region but becomes thinner towards the chalazalside.It is absent at the chalazal end in cotton maize.  At the micropylar end the lateral walls of the egg cell appear to join the central cell wall.

 The egg cell becomes highly polarised early in its development. The polarity is expressed by the aggregation of cytoplasmic elements at the chalazal end of the cell. The micropylar end of the cell is occupied by a large vacuoles.

SYNGERGIDS:

They are elongated cells present at the micropylar end of the embryosac. When two synergies are present they lie in contact with each other and partly embrace the egg. They are pointed or hooked toward the micropyle..

  The wall around the synergies is incomplete. There is a distinct wall around the micropyle one third of the cell which thins towards the chalazal end and finally disappears. As a result the chalazal one third of the cell lacks a wall.

   A prominent structure called filiform apparatus(FA)is present at the micropylar end of each synergid. Recent electron microscopic works have revealed that the filiform apparatus is a mass of finger-like projections of the cell into the cytoplasm..

  The cytoplasm of the synergid is strongly polarised. They are ephemeral structures. In embryosac with two synergid one degenerates before the entry of the pollen tube into the  embryo sac, whereas the other one often called the persistent synergid degenerates shortly after the embryo sac has received the pollen tube discharge.

FUCTIONS:

Looking at the structure and concentration of cell organelles the synergids appear highly active metabolically three function have been ascribed to synergids..

 1) They play an important role in directing the pollen tube growth by secreting some chemotropically active substances..

2) The degenerating synergids forms the seat for pollen tube discharge in the embryosac..

3) Jenser suggested that the filiform apparatus may be aiding the synergid in the absorption and​transportation of materials into the embryosac form the nucellus.

ANTIPODAL CELLS:

The three nuclei arranged at the posterior side of the embryo sac are called the antipodal cells. The antipodal cells exhibit the greatest variation amongst all the cells of the embryosac. Usually they degenerate before or soon after fertilisation without any appreciable enlargement.

  In many plants the antipodals are persistent and show some structural cytological features suggesting their possible role in the nutrition of the embryosac. In the Caltha palustin they persist upto the octant stage of pro embryo. In grasses they undergo a series of mitosis divisions leading to the formation of a large number of antipodal cells. This highest number of antipodal cells known is 300 recorded in Sasa paniculata.

In Zea mays, during additional divisions in antipodals, the walls of many cells remain incomplete leaving protoplasmic continuities between adjacent cells. This results in the formation of multinucleate protoplasm or syncytium.

Haustorial behaviour of antipodal cells is known in many plants. In Argemone mexicana the antipodal cells are much larger than the either the egg or the synergids. After fertilisation they continue to enlarge and persistupto heart shaped stage of the embryo.

  Three main functions have been attributed to the antipodal cells. Often nutritive role had been proposed for the antipodal cells especially where they are persistent. Formation of wall projections in antipodal cells of Maize, Rice, Poppy gives them the appearance of the so called transfer cells and support the suggestion that these cells any be associated with the nutrition of embryosac​.

  The antipodal cells may also store large quantities of starch, lipids, and proteins which are utilised by the developing endosperm embryo. Another role ascribed to antipodals is to produce and secrete substances that control the growth development of endosperm..

CENTRAL CELL:

It is the largest cell of the embryosac and the mother cell of the endosperm.  The enlargement of the embryosac after the last nuclear division is largely due to the inflation of the large central vacuole of the central cell.

  The nuclei of the central cell called polar nuclei are very large and each possesses a conspicuous nucleolus.They are present either in the centre of the cell suspended by cytoplasmic strands, or in the cytoplasm close to the egg apparatus.

 Unlike the egg cell the cytoplasm if the central cell is rich in all cell organelles appears to be the centre of intense synthetic activity. There are plasmids containing starch and sometimes proteins and phytoferrtin. In Capsella the central cell possess numerous sphaerosomes associated with glyoxysomes that probably convert fat into sugar. The central cells contains sufficient food reserves that are available for use during fertilisation and early stages of endosperm development..

   The presence of cell wall projections in the micropylar or chalazal region shows that central cell draws nutrition from the surrounding nucellus or integuments

Anomalous Secondary Growth

In most of the angiosperms normal secondary growth takes place. But many dicotyledons show deviation from the normal type of secondary growth. The growth which is in contrast to the normal secondary growth is known as the abnormal or anomalous secondary growth.
Anomalous secondary growth is commonly seen in tropical plants than in temperate plants. Haberlandt recongnized two types of anomalous secondary growths – adaptive and non-adaptive.
The anomalous secondary growth that takes place due to its functional requirements is called adaptive secondary growth. It takes place to fulfil the mechanical requirements of the plants as per their morphological requirements as per their morphological structure e.g., Aristolochia, Bignonia, Draceana.
The anomalous secondary growth that takes place due to environmental requirements and has no functional significance to the plants is called as non-adaptive secondary growth. Eg., Amaranthus, Achyranthes, Boerhaavia.
Usually, secondary growth does not take place in monocotyledonous plants. However, certain monocots are arborescent plants. To suit to their woody habit and to produce a strong stem, anomalous secondary growth occurs in these plants; e.g., Yucca, Agave, Dracaena.
In storage roots to produce the required tissue to store the food, adaptive type of anomalous secondary growth takes place; e.g., Beta vulgaris, Daucus carota etc.

Anomalous secondary Growth in Achyranthes Stem
The young stem has a wavy outline with alternate ridges and furrows.
Epidermis: Made up of single row of tubular cells. The cells are closely arranged with thick outer walls coated with lignin, followed by cutin. Several multicellular hairs are present over the cells of the epidermis.
Cortex: The peripheral hypodermal region is made up of collenchymas below the ridges and chlorenchyma below the furrows.
Endoermis: consists of single row of tangentially elongated parenchyma cells.
Pericycle: Made up of sclerenchyma, parenchymatous cells are also present.
Vascular Bundles: arranged in form of a ring.  They are conjoint, collateral, endarch and open. Medullary rays are present between the vascular bundles.
Apart from the primary vascular bundles, two medullary vascular bundles are present in the pith region. The medullary vascular bundles are conjoint, collateral, endarch and closed. These two bundles lie and grow opposite to each other.
Secondary Growth and Medullary Bundles:
In the pericycle region, extrastelar cambium strips develop which produce secondary vascular bundles. Cambium also produces the conjunctive tissue  between the vascular bundles. Secondary vascular bundles and conjunctive tissues are present without any sharp limits. So phloem of the secondary vascular bundles appears in the form of patches. This phloem is the included phloem.

Anomalous secondary Growth in Boerhaavia
In the primary structure, the stem can be distinguished into epidermis, cortex and stele
Epidermis:
 Single-layered epidermis consists of small, radially elongated cells. Multicellular epidermal hair arise from some cells. A thick cuticle is present on the epidermis. Some stomata are also present.
Cortex:
 It  well-differentiated and consists of few- layered collenchymatous hypodermis followed by chlorenchyma. Collenchyma is 3 to 4 cells deep, but generally it is only one-layered near stomata.
Chlorenchymatous cells are thin-walled, oval, full of chloroplasts and enclose many intercellular spaces.
Endodermis is clearly developed and made up  many, tubular, thick-walled cells.
Pericycle:
 Inner to the endodermis is present parenchymatous pericycle but at some places it is represented by isolated patches of sclerenchyma.
Vascular Bundles:
Vascular bundles are present in three rings. In the innermost ring are present two large bundles; in the middle ring the number ranges from 6 to 14 while the outermost ring consists of 15 to 20 vascular bundles.
 Vascular bundles of innermost and middle rings are medullary bundles. All the Vascular bundles are conjoint, collateral, endarch and open.
Two vascular bundles of the innermost ring are large, oval and lie opposite to each other with their xylem facing towards centre and phloem outwards.
Vascular bundles of inner and middle rings may show a little secondary growth. The cambium produces only a little amount of secondary xylem to the inner side and secondary phloem to the outer side. As a result the vascular bundles of these rings become slightly enlarged. Interfascicular cambium never develops between these bundles.
Anomalous Secondary Growth:
In the vascular bundles of outer ring, strips of cambia are formed between the vascular bundles, which fuse with the strips of the fascicular cambium and thus form a continuous ring of cambium. The fascicular cambium produces secondary xylem to the inner side and secondary phloem to the outer side. The inter fascicular cambium give rise to the conjuctive parenchyma to the inner side and secondary parenchyma to the outer side.
Very soon the activity of this cambial ring stops.
Later a cambial ring develops secondarily from the pericycle and becomes active. It cuts secondary phloem towards outer side and secondary xylem towards inner side. In between them conjuctive tissue is produced. As a result, a ring of collateral bundles separated by conjuctive tissue is formed. After a period of activity, this cambium also stops to function. Another ring of cambium arises outside, which behaves in the same pattern.
Thus, in this stem several rings of cambia arise successively in a centrifugal manner. The abnormal cambial ring produces xylem and conjuctive tissue on the inner side and phloem and parenchyma on the outer side. The resulting tissue gives the appearance of concentric rings of vascular bundles embedded in the conjuctive tissue.

Anomalous secondary Growth in Bignonia Stem
Bignonia is a woody climber or lianas. The mechanical requirements of woody lianas are different. The stems of lianes require flexibility to twin around the support. The secondary xylem formed through normal secondary growth is organized in the form of a cylindrical pole. This type of secondary growth is not useful to the lianeous plants. Hence, by the anomalous activity of the cambium, furrows are produced in the secondary xylem of these plants.
Primary Structure:
In T.S, the young stem exhibits the ridges and furrows in outline.
Epidermis:
 Single-layered epidermis consists of rectangular cells. A thick cuticle is present.  A few multicellular hair are also arising from some cells.
Cortex:
It is well-differentiated into collenchyma and parenchyma. Collenchyma is present below the epidermis in the ridges in young stem but at maturity there develops sclerenchyma.
 Parenchyma is present below the sclerenchyma or collenchyma in the ridges and directly below the epidermis in the grooves. Endodermis is undistinguishable from cortical cells. The cells lack casparian strips.
Pericycle:
 The pericycle shows alternate bands of sclerenchyma and parenchyma.
Conjoint, collateral, endarch and open vascular bundles are arranged around the pith in form of a ring.
Anomalous Secondary Growth
The interfascicular and intrafascicular cambium form the cambial ring. The activity of cambium is normal and it produces more secondary xylem to the inner side and little secondary phloem to the outside. But soon, at four places, the cambium produces more amount of secondary phloem on the outside and relatively small amount of secondary xylem on the inner side. As a result four deep furrows of phloem projecting into the secondary xylem are formed.
These furrows become deeper as the secondary growth advances. Each phloem wedge is provided with bars of sclerenchyma, which give them mechanical support and secondary phloem is not crushed. Although only four furrows are formed initially, their number increases gradually as the stem matures.
Though these stems appear to be strong, these are adaptive to bend and twine around due to the presence of furrows. A solid cylinder of xylem is likely to break under pressure of bending and twining. The furrows of the phloem present in the xylem act as shock absorbers and allow the stem to bend.
Hence, the anomalous secondary growth in the stem of Begnonia is a adaptive type.

Anomalous Secondary Growth in Dracaena
Monocotyledons normally do not show secondary growth due to absence of vascular cambium, but few plants like Dracaena, Yucca, Aloe etc show anomalous secondary growth.
Primary Structure:
Dracaena is an arborescent plant, belonging to the family Liliaceae. The young stem shows typical monocotyledonous structure.
Epidermis: Outer most layer made of single layer of cells with thin cuticle on the outside.
Cortex: Several rows of parechymatous cortex is present.
Stele: Several vascular bundles are scattered irregulary in the ground tissue. The vascular bundles are collateral, and closed.
Anomalous Secondary growth:
During the initiation of secondary growth, the parenchymatous cells of the cortex, external to the primary vascular bundles, become meristematic and forms the cambium in a ring. The cells of cambium divide and produce more cells towards the inner side and few cells towards outside. The cells produced on the inner side develop into vascular bundles and conjuctive tissue.
Each vascular bundle develops from a single cambial initial. The initial divides first by anticlinally to form a row of two or three cells. These cells undergo periclinal division, but the division soon become irregular. This results in the formation of a group of cells.
The peripheral cells develop into xylem elements, whereas the central cells differentiate into phloem elements. This leads to formation of amphivasal vascular bundles.
Formation of cork:
After certain period of secondary growth, the parenchymatous cells below the epidermis give rise to the meristematic initials. The initials divide several times periclinally. The cells thus formed become suberised and form the cork or phellem. This cork, with suberin coated cells and radial or storied arrangement is called as storied cork.

Anomalous Secondary Growth in Beta Vulgaris Root:
In the root of Beta vulagaris adaptive type of anomalous secondary growth is found. In this root additional tissue are required to store food and these are produced through secondary growth.
In Beta root the primary vascular system is diarch and exarch. As in the normal roots the xylem and phloem are arranged radially and alternately.
Anomalous Secondary Growth
A normal cambial ring is formed from the pericycle cells external to the protoxylem and the cells of the conjuctive tissue below the phloem.
This primary cambial ring exhibits anomalous activity, since it produces secondary xylem and parenchyma alternately on the inner side, instead of producing only xylem on the inner side and phloem on the outer side.
It forms secondary phloem opposite to the secondary xylem and parenchyma opposite to the existing parenchyma on its outer side.
Thus the vascular bundles formed are collateral in nature and arranged in a ring. These bundles are separated by narrow strips of parenchyma. This cambium becomes inactive after some time.
Then later, the cells of the pericycle undergo periclinal divisions and one of the inner layers behaves as cambium. This can be recongnised as the first additional cambium. Due to the activity of the additional cambium a ring of secondary vascular bundle is developed. These bundles are separated by wide radial channels of storage tissue.
Very soon the second additional cambial or the third cambial ring is formed from the phloem parenchyma or pericycle and organizes the third vascular bundle ring.
In this way, several rings of additional cambia are formed successively and produce the vascular bundle rings. In all these, parenchyma is present between the vascular bundles. Several parenchymatous layers are formed. These cells store food material. Storage sugars and anthocyanin pigment are also present in these cells.
In this way the thickness in the Beet root is increased by the activity of the concentric rings of cambium.
In a well developed Beet root, a phellogen is differentiated in the peripheral region, which produces the suberin coated, dead cell of the cork to the outside and secondary parenchyma to the inner side. The latter is known as the secondary cortex.
These secondary cortex and cork with cork cambium constitute the protective cover known as periderm.

Interaction of Genes

According to Mendel’s Law of Independent Assortment, the segregation of one gene is independent of segregation of another gene. It is also important that these genes must also have been functioning (exerting their effects on the phenotype) independently of one another.

Usually a single gene controls one character. However, it should not be surprising that the expression of one gene sometimes alter the expression of one or more of the alleles of a second (non-allelic) gene. In such a situation two or more than two genes may interact to give rise to a completely novel phenotype.

The phenomenon of two or more genes affecting or influencing the expression of each other in development of a single character of an organism is known as gene interaction.

Gene interactions lead to variations in the classical Mendelian mono and di-hybrid ratios.

Supplementary Genes (9:3:4 ratio)

Supplementary genes are two independent pairs of dominant genes which interact in such a way that one dominant gene will produce its effect, irrespective of the presence or absence of the other gene, while the second gene can only produce its effect in the presence of the first gene.

This is known as the supplementary gene interaction and the phenotypic ratio becomes modified into 9:3:4.

In Mice, Black, albino and agouti patterns of coat colour are seen. The wild body colour is known as agouti, characterised by banding of individual hairs. It is characterised by colour banded hairs in which the part nearest the skin is gray, then yellow band and finally the distal part is either black or brown. The agouti colour is controlled by a gene ‘A’. The dominant allele ‘C’ in absence of gene ‘A’ gives coloured mice. However, in the presence of dominant allele ‘C’, ‘A’ gives rise to agouti. A

In absence of gene ‘C’ , ‘A’ is unable to express itself and mice with genotype ccAA and ccAa and ccaa are albinos. The albino lacks pigments

Complementary Genes (9:7 ratio)

A classical example of interaction of genes is the complementation between two genes meaning that both genes are necessary for the protection of a particular phenotype.

W. Basteson and R.C Punnet observed that , when two white flowered varieties of sweet pea, Lathyrus odoratus were crossed, F

On selfing, the white flowered plants were not true-breeding. Some of them produced purple and white flowered plants in the ratio of 3:1. Obviously, the results obtained were a modification of 9:3:3:1ratio in which the last three classes have the same phenotype, there by producing a phenotypic ratio of 9:7

The above results are easily explained if we assume that purple colour of flowers in Lathyrus odoratus is determined by two dominant genes C and P. If either or both of the dominant genes are absent the flowers becomes white. The phenotype of white parents, therefore were CCpp or ccPP. Purple colour is the result of a complementary effect of dominant alleles at two different loci which segregates independently of each other.

Metabolic process in living organisms takefunctional enzyme, pigmentation does not occur.

 Enzyme products of genes C and P are both necessary for the production of anthocyanins. When the genotype is cc, the first enzyme is not produced, consequently, reaction to the intermediate product (chromogen)is stopped. Likewise, if the genotype is pp, the lack of the second enzyme halts the second metabolic step. In other words, if the genotype is cc or pp, the synthesis pathway is blocked and no pigments are produced, resulting in a white flowers .

Such an interaction of genes to jointly

Duplicate genes
When dominant alleles of two genes produce the same phenotype with out cumulative effect, the 9:3:3:1 ratio becomes modified into a 15:1 ratio. If different genes determine the same or nearly same phenotype, such genes are called as duplicate genes.
A classic example of duplicate genes occurs in shepherd's purse belonging to the genus Capsella (Capsella bursa pastoris).
Two kinds of fruit phenotypes with respect to the genus were known - (I) triangular capsules and (II) top shaped capsules.
When plants with these phenotypes were crossed, in F1 generation only triangular capsules were observed.
When such d individuals with triangular capsules were intercrossed among themselves, in F2 progeny,  plants with triangular capsules and top shaped capsules were obtained in 15:1 ratio.

Obviously the top shaped capsules results from double recessive genotype. If 'A' and'B' are two genes, top shaped capsules will be obtained on plants with the genotype aabb. Plants with triangular capsules can be AABB, AAbb or aaBB and other genotypes with heterozygosity.
It shows that even a single dominant gene is enough to give rise to triangular capsules.

The biochemical basis for the 15:1 ratio can be understood by examining one step pathway in which a dominant allele at either of two genes is enough to produce enzymes for the catalysis of a given reaction.

Only when there is a double recessive, say aabb, is the pathway blocked and the aparrent phenotype expressed.
When either of two genes can function to produce the dominant phenotype, this type of gene Interaction is called as duplicate gene interaction.
Pressumbly, the two different genes produce similar gene product, and one of them may have arisen by duplication from the other gene.

Epistasis
One important type of functional interaction between different genes occurs when an allele or genotype at one locus"masks" or " inhibits" the expression of a non-allele or genotype at a distinct locus, such an interaction is known as epistasis ( Gr. - standing upon).
Any gene that masks the expression of another non- allelic gene is known as epistatic gene. And the gene whose expression is prevented or masked is known as hypostatic gene.
Therefore, while dominance involved intragenic or inter- allelic gene suppression, the epistasis involves intergenic suppression.
Epistasis is of following types:- (I) Dominant epistasis and (II) Recessive epistasis.

Dominant Epistasis:
When the dominant allele of one gene (e.g. A) masks or inhibits the expression of alleles of another gene ( e.g. B) and expresses itself phenotypically, then gene A is said to be epistatic to the gene B.
Since a dominant gene exerts its influence by suppressing the expression of gene B or b, it is known as dominant epistasis.
Fruits of Cucubita pepo ( summer squash) can be white, yellow and green. White is dominant over both yellow and green. Yellow is dominant over green only.
White colour is determined by the dominant gene W and no other gene for fruit colour is expressed in its presence. Thus, the dominant W is epistasis to other fruit colour genes.
In the presence of homozygous recessive (ww) gene, another gene Y determines yellow colour of the fruit. Homozygous recessive for both genes ( wwyy) bear green fruits.
Plants with white fruits (WWYY) crossed to plants with green fruits (wwyy) produce F1 progeny which bears only white fruits. F2 progeny plants segregated in 12 white, 3 Yellow and 1 green i.e., 12:3:1 ratio.

Thus, fruit colour in Cucurbita pepo is determined by two different genes and the first two classes of a typical dihybrid F2 ratio(9:3:3:1) are phenotypically similar.

Recessive Epistasis
Sometimes the recessive alleles of one gene (aa) masks the phenotypic expression of alleles of another gene (BB, BB or bb alleles).
This type of epistasis due to a recessive gene is called recessive epistasis. Due to recessive epistasis the phenotypic ratio 9:3:3:1  becomes modified to 9:3:4 ratio.
Recessive epistasis is same as supplementary genes.

Recessive  Lethals:
In 1905, L Cuenot reported the inheritance of mouse body colour, which did not fit the expected Mendelian segregation pattern. It was shown that yellow body colour was dominant over normal brown colour.
Yellow colour was controlled by a single gene which is designated as Y, while y determine the normal brown colour.
It was found that yellow mice could never be obtained in homozygous condition. When yellow mice were crossed among themselves, segregation for yellow and brown body colour was obtained in 2:1 ratio. The brown individuals were pure, therefore, homozygous and yellow individuals were heterozygous.

These results could be explained if we assume that allele Y which is dominant for yellow body colour in heterzygous condition was recessive for lethality in homozygous condition since in the homozygous condition YY, it kills the individual in early embryonic state ( I.e., during gastrulation). Consequently, when ever a homozygous individual for Y is produced, the lethal character will express itself and the individual will die. Thus, a homozygous yellow will never be produced.