Biology

Structure of Flower

A complete flower typically has four verticils or whorls. They are as follows –

1. Calyx – The members that make up this whorl are called sepals. Sepals are green leaf-like structures that protect a flower in its early bud stage. Also, it is the outermost whorl of a flower.
2. Corolla – The members that make up this whorl are called petals. These structures are brightly coloured and aid in pollination by attracting insects. Also, they can be seen in different shapes like tubular, wheel-shaped, funnel-shaped or bell-shaped. The corolla and calyx together make up a non-reproductive structure called the perianth.
3. Androecium – They are the male reproductive structures composed of stamens. Each stamen consists of a filament or stalk and an anther. Each anther has pollen sacs that aid in the production of pollen grains.
4. Gynoecium – They are female reproductive structures composed of carpels. A carpel is composed of 3 parts namely ovary, style and stigma

Androecium: 

It is a male reproductive part. It consists of stamens. Each stamen is made up of filament and anthers.

• The stamens may either be free (polyandrous) or remain fused together. Epipetalous- stamens are attached to petals.

The cohesion of stamens fused conditions are as follows.

• Monadelphous: Stamens are united into a single group by union of filaments. The anthers remain free.

• Diadelphous: Stamens are united, and arranged in two bundle s.  (e.g. Pisum)

• Polyadelphous: Stamens are united and grouped onto several bundles. (e.g. Citrus)

• Synandrous: Stamens are united their whole length. Both filaments and anthers will be fused. (e.g. Cucurbita)

• Syngenesious: Stamens are united by the joining of anthers. The filaments remain free. (e.g. Tridax)

The stamens can also be found fused with other whorls. This condition is called adhesion. It may be of different types.

1. Epipetalous: Stamens are found attached (wholly or partially) to the corolla, by means of their filaments. Anthers would be free. (e.g. Ixora)

2. Epiphyllous: When stamens are found attached to the perianth of a flower, it is called epiphyllous condition. (e.g. Gloriosa)

3. Gyandrous: Stamens are attached to the carpel (either by whole length, or by the anthers.  (e.g. Calotropis)

Gynoecium :

The gynoecium of a flower is the female reproductive structure that is most essential for sexual reproduction. The gynoecium has ovaries that produce ovules which eventually develop into seeds and fruit.  Carpel is the main component of gynoecium that consists of ovary, style and stigma.

1. Stigma is a sticky portion at the distal end of a tubular structure called style. It is the primary receptive site where the pollen grains are captured.
2. Style is the major structure that aids in the process of fertilization. The pollen tube reaches the ovary via this tubular style.
3. Ovary is located at the swollen base of the carpel. Style is the elongated tube that links the ovary and the stigma. The ovary produces ovules which are present within chambers known as locules. Based on the number of locules, the ovary can also be classified as multilocular, unilocular, bilocular, etc.
4. pistil : Stigma, ovary and style are together termed as the pistil. A pistil is considered to be a collection of one or more carpels.

Placentation:

It is defined as the arrangement and attachment of ovules within the plant’s ovary.

The part of the ovary where ovules are attached is the placenta.

Types of placentation:

• Axile placentation: When the placenta is placed axially as in a lemon, then such type of placentation is termed axile placentation.

• Marginal placentation: When the placenta forms a ridge along the ventral suture as in pea, then this is known as marginal placentation.

• Free central placentation: When the ovules are born on the central axis, it is called free central as in primrose and Dianthus.

• Basal placentation: In this type of placentation, a single ovule is attached to the placenta and placed at the base of the ovary. Eg. Marigold.

Parietal placentation: In this type of placentation, the ovules develop on the inner wall or periphery of the ovary. Eg. Sunflower, kiwi, cucumber, cantaloupe.

Superficial: The placenta is formed all around the inner surface of the ovary including the septa.

1. Marginal- e.g. pea
2. Axile- e.g. lemon, china rose
3. Parietal- e.g. Argemone, mustard
4. Free central- e.g. Primrose, Dianthus
5. Basal- e.g. marigold, sunflower

The Flower

A flower has four whorls; calyx, corolla, androecium and gynoecium. These are attached to the swollen terminal of pedicel called the thalamus

Flower symmetry:

1. Actinomorphic- radially symmetrical flowers, e.g. chilli, datura, mustard
   1. Zygomorphic- when a flower can be divided into two equal parts in only one vertical plane, e.g. Cassia, pea, etc.

Flowers can be trimerous, tetramerous or pentamerous depending on the multiple of floral appendages present 3, 4 or 5

Types of flowers depending on the presence or absence of bracts (reduced leaf present at the base of pedicel); Bracteate or Ebracteate

Types of flowers based on the position of the ovary:

1. Hypogynous- gynoecium occupies the highest place, above all the other parts. The ovary is known as superior. The sepals, petals and stamens arise from below the ovary.This is the commonest type of ovary. The flower is technically Hypogynous where hypo means flower parts start ‘below’ the ovary. e.g. brinjal, china rose, mustard

• Perigynous- gynoecium is present at the same level as the rest of the parts of a flower. The ovary is known as half inferior, The flower parts arise from the rim of a hypanthium – a cup shaped extension of the base which encloses the ovary. Imagine the hypanthium to be a flat ring and you can see that the flower parts actually arise from below the ovary. The flower is technically Perigynous where peri means ‘around’.  e.g. peach, plum, rose

• Epigynous- thalamus encloses the ovary completely and other parts are present above it. The ovary is known as inferior.  Inferior ovary. The sepals, corolla tube and stamens (filaments plus anthers) arise from above the ovary. The flower is technically Epigynous where epi means ‘above’. e.g. ray florets of a sunflower, guava, cucumber

Aestivation:

The mode of arrangement of sepals or petals in floral bud with respect to the other members of the same whorl is known as aestivation.

The main types of aestivations are valvate, twisted, imbricate and vexillary.

Valvate:

When sepals or petals in a whorl just touch one another at the margin, without overlapping, as in Calotropis, it is said to be valvate.

Twisted:

If one margin of the appendage overlaps that of the next one and so on as in china rose, lady’s finger and cotton, it is called twisted.

Imbricate:

If the margins of sepals or petals overlap one another but not in any particular direction as in Cassia and gulmohur, the aestivation is called imbricate.

Vexillary:

In pea and bean flowers, there are five petals, the largest (standard) overlaps the two lateral petals (wings) which in turn overlap the two smallest anterior petals (keel); this type of aestivation is known as vexillary or papilionaceous.

Families of flowering plants:

Main Difference between Fabaceae, Solanaceae, and Liliaceae

1. Fabaceae is a legume family, Solanaceae is a potato family, and Liliaceae is the lily family.
2. Fabaceae contains peas, chickpeas, and soybean, Solanaceae contain potatoes, tomatoes, and bell pepper, while Liliaceae contain onion, lily, and tulip.
3. Fabaceae is perennials or annuals, Solanaceae is annuals or biennials, while Liliaceae is perennials.
4. Fabaceae is monocarpellary; Solanaceae is bicarpellary, while Liliaceae is tricarpellary.
5. Fabaceae contain pods, while Solanaceae and Liliaceae contain berries or capsules.

Refer Flower formula library notes for more information

Fabaceae	Solanaceae	Liliaceae
It is a large family of trees, shrubs, vines, and herbs.	It is a nightshade or potato family of flowering plants	These are monocotyledonous, perennial, and herbaceous plants.
Legume family	Potato family	Lily family
Dicot	Dicot	Monocot
Peas, chickpeas, and soybean	Potato, tomato, and bell pepper.	Onion, lily, and tulip
Exhibits marginal placentation.	Exhibits axial placentation.	Exhibits axial placentation.
Perennial or annuals	Perennials, biennials, and annuals.	Perennials
Trees, shrubs, vines, and herbs	Herbs, shrubs, small trees	Herbs and shrubs
Monocarpellary	Bicarpellary	Tricarpellary
Contain pods	Contain berry or capsule	Contain berry or capsule
Fix atmospheric nitrogen to improve soil fertility	Contain plant varieties with agricultural importance	Contain decorative and ornamental plants.

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Floral Formula

Definition of Floral Formula

A floral formula can defined as the numeric and symbolic expression, which reveals the flower morphological characteristics by employing different symbols, letters, and figures. It is the conventional method accustomed to formulating the structure of the flower. It elucidates the information about the number of whorls and a relative relationship between each other.

Components

The floral formula uses discrete letters, signs and figures to represent the specific feature of the flower.

Letters used in Floral Formula

1.K: This letter denotes the sepals that form an outermost whorl called the calyx.

2. C: This letter represents the group of petals that constitute the second whorl called the corolla.

3. P: It is used to denote the tepals, which indicates the undifferentiated condition of the perianth members (sepals and petals).

4. A: It specifies the male reproduction part or stamens, which includes the filament and anther that all together makes up the third floral whorl known as androecium.

5. G: It denotes the female reproductive part, i.e. carpel, which includes the stigma, style and ovary that colloquially forms the innermost whorl called gynoecium.

6. Br: It represents the bracteate condition of the flower.

A plant having bracts is referred to as bracteate or bracteolate, while one that lacks them is referred to as ebracteate and ebracteolate, without bracts.
bracteate flowers: Flowers with bracts (a reduced leaf at the base of the pedicel) are called bracteate flowers. Bracts are small leaf-like structures found at a flower’s base. China rose, tulip, lily

7. Ebr: It indicates the ebracteate condition, in which a flower lacks bract.

8. Brl: It indicates the presence of bracteoles or represents the bracteolate condition.

9. Epik: It represents the presence of a secondary whorl surrounding the calyx called epicalyx.

10. Ebrl: It is used to indicate the absence of bracteoles or to represent the ebracteolate condition.

Symbols used in Floral Formula

1. 0: It indicates the absence of a particular member in flower.
2. ∞: It is generally used when the number of specific flower parts is more than 10.
3. ⊕: It indicates an actinomorphic condition.
4. %: It denotes a zygomorphic condition.

• ⚥: It represents the bisexuality of flowers.
• ⚦: It represents the unisexual, staminate flower.
• ♀️: This represents the unisexual, pistillate flower.

Combination of Letters, Symbols and Numbers

• K5: It shows the aposepalous condition, in which the five sepals are free.

• K(5): It shows the gamosepalous condition, in which the five sepals are united.

• C5: It represents the apopetalous condition or the presence of the five free petals.

• C(5): It represents the gamopetalous condition or the presence of five fused petals.

• Cx: It indicates corolla cruciform.
• A curve drawn over the letters P and A: It represents the epiphyllous stamens.
• A curve drawn over the letters C and A: It shows the epipetalous stamens.
• A3: It indicates the presence of three free stamens.
• A2+2: It indicates the presence of  4 free stamens, two in each whorl.
• A(9)+1: It represents the presence of diadelphous stamens (10 in number), in which nine stamens are fused in one whorl, and one stamen remains free.

• A0: Sterile stamen (staminode).
• G0: Sterile carpel (pistillode).
• G-: It represents the semi inferior ovary.
• A line over the letter G: It represents the inferior position of an ovary.
• The line below the letter G: It represents the superior position of an ovary.
• A curve over the letters G and A: It shows the gynostagium condition.

Floral Formula:

Symbol	Description / Full form
Br	Bracteate condition
Ebr	Ebracteate condition
Brl	Bracteolate
Epik	Epicalyx
Ebrl	Ebracteolate
0 (zero)	Absence of a particular whorl
∞	Indefinite number of floral parts in a whorl
⊕	Actinomorphic condition (radially symmetric) flowers.
%	Zygomorphic condition (bilaterally symmetric)
K	Calyx (Sepals)
C	Corolla (petals)
P	Perianth (tepals)
A	Androecium (stamens)
G	Gynoecium (carpels)
⚥	Bisexual flower
⚦	Unisexual, staminate flower
♀️	Unisexual, pistillate flower
K5	Five sepals, aposepalous
K(5)	Five sepals gamosepalous
C5	Five petals, apopetalous
C(5)	Five petals, gamopetalous
	Epiphyllous stamens
	Epipetalous stamens
A3	Three stamens free
A2+2	Stamens 4, 2 whorls
A10+1	Stamens 10, diadelphous – 9 stamens unite to form one bundle and 1 other bundle
A0	Sterile stamen (staminode)
G0	Sterile carpel (pistillode)
G-	Semi inferior ovary
	Inferior ovary
	Superior ovary

How to Use Floral Formula?

Firstly, the presence or absence of bract and bracteoles in the flowers of different families is observed and noted by using particular letters.

Then the symmetry of the flower is written according to the position of floral whorls relative to the mother axis. Flowers can be polysymmetric (actinomorphic), dissymmetric, monosymmetric (zygomorphic), asymmetric or spirally arranged. Therefore, the flowers showing different symmetry is indicated by specific symbols in the floral formula.

After that, the sex of the flower is written, i.e. whether a plant is hermaphrodite (bisexual) or unisexual. Bisexual flowers possess the male plus female reproductive parts, while the unisexual flowers either contain stamens or carpels.

Then the number of floral members starting with calyx, corolla, androecium to gynoecium is written. Here each member is denoted by using specific letters like ‘K’ represents calyx, ‘C’ represents corolla, ‘A’ indicates androecium and ‘G’ represents gynoecium. The number of each whorl is written after the following letters starting from K, C, A to G.

The number used after the letter is placed inside bracket ( ), If the particular whorl is united. But, if the floral members remain free, then the bracket is not applied. Adnation between the floral whorls is represented by drawing a curve from the top of the letters.

The position of the ovary can be shown by drawing a horizontal line above, below or in front of the letter ‘G’, which represents the inferior, superior or half inferior position of the ovary.

Examples

The floral formula can be used to describe the flowers of the particular family or the different species of flower. Let us take few examples of the floral formula to study the floral characteristics.

Floral Formula of Fabaceae Family

(%) shows that the symmetry of the flower is monosymmetric or zygomorphic. (⚥) indicates that the flower is perfect or hermaphrodite (includes stamen plus pistil). A letter ‘K’ indicates the outermost whorl, i.e. calyx and (5) shows that the number of sepals is five that are united with each other.

‘C’ indicates the second floral whorl, i.e. corolla or petals and the number after this, i.e. 1+2+(2), represents vexillary aestivation. ‘A’ means the male reproductive part or androecium, and the number after it (9)+1 shows the presence of diadelphous stamens (includes 10 stamens).

In 10 stamens, 9 filaments unite to constitute one bundle, and the remaining one filament makes up another one. The line below the letter ‘G’ indicates the superior position of an ovary, and digit 1 shows the monocarpellary ovary.

Floral Formula of Liliaceae Family

Through this floral formula, we can identify many floral characteristics. This formula indicates that the flower is bracteate, bisexual or hermaphrodite. A letter ‘P’ indicates that the sepals and petals are undifferentiated, i.e. there are three tepals in every two whorls.

Letter ‘A’ denotes androecium, and ‘3+3’ suggests there are three free stamens in every two whorls. A line below a letter ‘G’ specifies the hypogynous condition of an ovary, and the number ‘(3)’ represents the presence of three fused carpels.

Floral Formula of Solanacae Family

The general floral formula of Solanaceae family is as follows:

Here the symbols represent:

⊕	Actinomorphic (radial symmetry)
⚥	Bisexual
K(5)	Calyx – 5 sepals, gamosepalous (united)
C(5)	Corolla – 5 petals, gamopetalous
A5	Androecium – 5 stamens, polyandrous (free), epipetalous (attached to petals)
G(2)	Gynoecium – bicarpellary, syncarpous (united), superior ovary

• Calyx (K): Composed of 5 sepals, which are united (gamosepalous).
• Corolla ©: Consists of 5 petals, which are also united (gamopetalous).
• Androecium (A): Contains 5 stamens, which are free (polyandrous) and attached to the petals (epipetalous).
• Gynoecium (G): Bicarpellary (two carpels), syncarpous (united), and has a superior ovary.

The Solanaceae family includes a variety of plants, such as potatoes, tomatoes, bell peppers, eggplants, and tobacco. These plants exhibit diverse characteristics and are widely distributed across tropical, subtropical, and temperate zones.

Fabaceae floral formula

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Gametogenesis is defined as the process through which gametes are formed in reproducing organisms. In mammals, the gametogenesis process in males is called spermatogenesis (formation of sperms) and in females, it is called oogenesis (formation of ovum).

Gametogenesis can be of two types – 

• Spermatogenesis – formation of sperm in the testes of males.
• Oogenesis – Formation of ovum in the ovary of females.

Gametogenesis

Oogenesis

We have already discussed that oogenesis is the process of formation of ovum within the ovaries. Did you know that in mammals, oogenesis begins even before a female is born? Oogenesis begins in the ovary of a developing female foetus as early as the 25th week of pregnancy. Let us now get into the details of the process.

Oogenesis is completed in three stages – multiplication stage, growth phase and maturation phase.

Multiplication phase

During foetal development, as the process of oogenesis begins, few cells of the germinal epithelium enlarge and undergo repeated mitotic divisions to form a group of germ cells known as oogonia or egg mother cells. Around two to eight million oogonia are formed in the foetal ovary and no more oogonia is added or formed in the ovary after birth. Each oogonia undergoes repeated mitotic division and forms a group of cells which project into the stroma of the ovary in the form of a cord known as the egg tube of Pfluger. This tube soon takes the form of a round egg nest.

Growth phase

Formation of primary follicle

Only one of the cells within the round egg nest enlarges to become the primary oocyte. The rest of the cells, known as granulosa cells, form a layer around the primary oocyte which is known as the follicular epithelium. The primary oocyte surrounded by the follicular epithelium forms the primary follicle.

The growth phase of the primary oocyte is very long and elaborate and involves a number of changes. The oocyte increases in size many-fold due to accumulation of reserve food materials. The number of mitochondria increases along with the activity of the endoplasmic reticulum and Golgi complex. A vitelline membrane forms around the oocyte. The nucleus of the oocyte increases in size due to increase in nucleic acid content and the number of nucleoli. rRNA genes amplify to synthesise more rRNA which would eventually help in synthesising more ribosomes for protein synthesis.

Maturation phase

The maturation phase begins when the nucleus of the primary follicle starts dividing meiotically but is arrested at the diplotene stage of prophase I at the foetal stage. Further nuclear division occurs only at the age of puberty.

A human female is born with around two million primary oocytes of which many degenerate till she reaches puberty. Only 60,000-80,000 primary follicles remain at the age of puberty, of which only 400-500 mature into ova during a woman’s lifetime. 

At puberty, the primary follicles start transforming into secondary follicles and get surrounded by several layers of granulosa cells and a new theca.

The secondary follicle gradually transforms into a tertiary follicle by the appearance of a fluid filled cavity known as the antrum. The fluid is known as the liquor folliculi and provides nutrition to the growing primary oocyte. The formation of the antrum causes the follicular cells of the theca to be organised into an outer theca externa and an inner theca interna.

Formation of Graafian follicle

The tertiary follicle matures into a Graafian follicle. This is the mature mammalian follicle and is about 2.5 cm in diameter. The wall of the Graafian follicle is made up of several layers of granular follicular cells and is divided into theca interna and theca externa. A fluid filled cavity called the antrum is present which is filled with the fluid liquor folliculi secreted by the follicular cells. The oocyte is attached to the granulosa cells of the follicular wall with the help of a cluster of cells known as cumulus oophorus. The secondary oocyte in the Graafian follicle is surrounded by a thick, non-cellular layer known as the zona pellucida and a layer of follicular cells known as the corona radiata.

Completion of first meiotic division and formation of secondary oocyte

Within the Graafian follicle the primary oocyte grows in size and it completes the first meiotic division which leads to the formation of a haploid secondary oocyte and a haploid polar body. The division of ooplasm (cytoplasm of the oocyte) is unequal and the secondary oocyte retains most of the nutrient rich cytoplasm of the primary oocyte while the first polar body receives very less ooplasm. As for the fate of the first polar body, nothing is known. It is known to have no function. 

The nucleus of the secondary oocyte enters into the second meiotic division. At the time of ovulation, the mature graafian follicle rises to the surface of the ovary and ruptures to release the egg or ovum at the secondary oocyte stage with its nucleus arrested at the metaphase II stage of the second meiotic division. This process is known as ovulation. 

The secondary oocyte completes the second meiotic division only after it is fertilised by a sperm. The completion of the second meiotic division also results in the formation of an ovum and a second polar body. Meanwhile, the first polar body also completes the second meiotic division and forms two polar bodies. Thus, after the completion of meiosis of the primary oocyte, a haploid ovum and three haploid polar bodies are formed. The polar bodies have no function. If the fertilisation does not occur within 24 hours of ovulation, the oocyte degenerates remaining arrested in meiosis II of cell division.

Fig: Meiotic division during oogenesis

The ruptured Graafian follicle is left containing a blood clot known as the corpus haemorrhagic. The follicular cells of the ruptured follicle form a yellowish mass which is known as the corpus luteum. It is responsible for the secretion of the progesterone and relaxin hormones. In the absence of fertilisation, the corpus degenerates in about a week and appears as a white mass known as corpus albicans.

Ovulation

The mature Graafian follicle reaches the surface of the ovary and ruptures to release the secondary oocyte by the process of ovulation. It is caused by the increased turgidity in the follicle. Contraction of the smooth muscles around the follicle also helps in the process. In human females, only one egg is released from either one of the ovaries during ovulation of a menstrual cycle.

Hormones – Oogenesis

The hypothalamus releases the gonadotropin releasing hormone (GnRH) which stimulates the anterior pituitary to release two gonadotropic hormones named FSH (follicle stimulating hormone) and LH (Luteinising hormone). FSH stimulates follicular growth and oocyte maturation. The granulosa cells of the developing follicles also release a hormone named oestrogen. When the level of oestrogen and LH becomes very high, the Graafian follicle ruptures and releases the egg by the process of ovulation. High concentration of oestrogen prevents further release of GnRH and FSH. 

LH promotes the conversion of the ruptured Graafian follicle into corpus luteum which secretes progesterone. Progesterone promotes development of the uterus to prepare itself for receiving the fertilised zygote.

In case of fertilisation, the corpus luteum persists and as progesterone levels increase, it inhibits further secretion of GnRH and consequently the release of FSH, LH and oestrogen is also inhibited. This prevents further growth and maturation of follicles and hence no ovulation occurs as long as pregnancy is maintained.

In case there is no fertilisation, the corpus luteum degenerates and the uterine lining sheds. As the level of progesterone goes down, the release of GnRH increases and the body gets ready for the next menstrual cycle and development of primary follicles.

The role of hormones in oogenesis is to regulate the development and maturation of the female gametes, or eggs, in the ovaries. Hormones are chemical messengers that are produced by endocrine glands and travel through the bloodstream to target organs. The main hormones involved in oogenesis are:

• Gonadotropin-releasing hormone (GnRH): This hormone is released by the hypothalamus, a part of the brain that controls many bodily functions. GnRH stimulates the anterior pituitary gland, another endocrine gland in the brain, to secrete two other hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
• Follicle-stimulating hormone (FSH): This hormone stimulates the growth and development of the ovarian follicles, which are structures that contain the primary oocytes, or immature eggs. FSH also induces the production of estrogen by the follicular cells, which are the cells that surround the oocytes.
• Estrogen: This hormone is produced by the follicular cells and has several functions in oogenesis. Estrogen promotes the growth and maturation of the oocytes and the follicles, prepares the uterus for implantation of a fertilized egg, and inhibits the secretion of FSH by the anterior pituitary gland through a negative feedback mechanism. Estrogen also triggers a positive feedback mechanism that causes a surge of LH secretion by the anterior pituitary gland.
• Luteinizing hormone (LH): This hormone is responsible for inducing ovulation, which is the release of a secondary oocyte, or mature egg, from the ovary. LH also stimulates the transformation of the ruptured follicle into a structure called the corpus luteum, which produces another hormone: progesterone.
• Progesterone: This hormone is produced by the corpus luteum and has several functions in oogenesis. Progesterone maintains the thickness and blood supply of the uterine lining, supports the implantation and development of a fertilized egg, and inhibits the secretion of GnRH, FSH, and LH by the hypothalamus and the anterior pituitary gland through a negative feedback mechanism. If the egg is not fertilized, the corpus luteum degenerates and the levels of progesterone drop, leading to the shedding of the uterine lining, or menstruation.

These hormones work together to coordinate the process of oogenesis and the menstrual cycle, which are the main events of female reproduction.

Fig: Hormonal control of oogenesis

Spermatogenesis vs Oogenesis

Similarities

• Both the processes occur in the germ cells of the gonads.
• Both result in the formation of haploid gametes
• Both the processes involve a multiplication phase, a growth phase and maturation phase.
• In both the processes, changes occur in both the nucleus and the cytoplasm of the primary spermatocyte and primary oocyte.

Differences

• Spermatogenesis begins at puberty in males whereas the process of development of oocytes for oogenesis begins at the foetal stage during the 25th week of development.
• Spermatogenesis results in the formation of millions of sperm cells (male gametes) but oogenesis results in the formation of only one egg or ovum (female gamete) during each menstrual cycle.
• In spermatogenesis, the spermatogonia go through four more stages to form the sperm, namely, primary spermatocyte, secondary oocyte, spermatids and spermatozoa or sperm. In oogenesis, the oogonia go through three stages only, primary oocyte, secondary oocyte and finally form the ovum.
• During spermatogenesis, meiosis results in equal cell division and formation of four spermatids. During oogenesis, the meiotic divisions are unequal and result in the formation of a single large ovum and three small non-functional polar bodies.

Ploidy

Ploidy level of Nucellus, Endosperm, Polar nuclei, Megaspore mother cell, female gametophyte respectively are

• Nucellus, microspore mother cells (MMC), and megaspore mother cells are part of sporophyte so these are diploid cells and their ploidy is 2n. And female gametophytes are part of gametophyte and ploidy is n.
• The endosperm constitutes an organism separate from the growing embryo. About 70% of angiosperm species have polyploid endosperm cells. These are typically triploid but can vary widely from diploid (2n) to 15n.
• Sperm released fuses with the two polar nuclei forming a triploid cell that develops into the endosperm.
• A megaspore mother cell is a diploid cell in which meiosis occurs, resulting in the production of four haploid megaspores.
• Hence, Ploidy level of Nucellus, endosperm, polar nuclei, Megaspore mother cell, female gametophyte respectively are 2n,3n,n,2n,n.

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What is Pollination?

Pollination is a method where pollen grains are picked from an anther, which is the male part of a flower and transferred to the flower’s female part called the stigma, where seeds are produced.

Cleistogamous flower are bisexual flowers which never open and pollination occurs in the closed flower.

The chasmogamous flowers are open with their anther and stigma exposed for pollination.

In these flowers two types of pollinations take place:

• Self-Pollination
• Cross Pollination

Self-Pollination 

Self-pollination occurs when both the anther and the stigma mature simultaneously and come in contact with each other.

Autogamy is self-formation which occurs between anther and stigma of the same flower.

• In self- pollination, there is no involvement of external factors like wind, water, and other pollinating agents.
• Self-pollination ensures that even a smaller quantity of produced pollen grains from plants have a good success rate in pollination.
• The purity of the race is maintained, as there is no diversity in the genes.

Cross-Pollination 

This type of pollination occurs in self-incompatible plants. In this, the anther and the stigma mature at different times so cannot come in contact with each other. Cross-pollination is of two types:

• Geitonogamy
• Xenogamy

Cross-Pollination(Geitonogamy)– When the pollen grains from the anther transfer to the stigma of a different flower in the same plant, it is known as geitonogamy. It is seen in monoecious plants like maize. It is a cross pollination within the same plant. In this, pollens are transferred by insects or winds. 

Cross-Pollination(Xenogamy)– When the pollen grains from the anther of a flower get transferred to the stigma of a flower in different plant, it is known as xenogamy. This process carries genetically different pollen to the stigma.  e.g., papaya, maize.

Cross Pollination method leads to an increase in genetic diversity as different flowers will share and combine their genetic information to create unique offspring.

• Cross-pollination introduces new genes into a sequence of species, and this is mainly due to the fertilization between genetically different gametes.
• The recessive characters in the lineage are eliminated because of genetic recombination.
• It helps improve the immunity towards diseases.

Types of Cross-Pollination

The process of cross-pollination requires the help of biotic and abiotic agents like animals, birds, wind, insects, water and other agents as pollinators.

Zoophily

Pollination by Animals (Zoophily): Animals help in seed dispersal by eating the fruits of plants and the seeds get disposed through their waste and germinate in a new place where they are transferred.

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Anthropophilly

Artificial method of pollination is performed by spreading pollen grains over the female flowers. Artificial pollination is done by humans, and it is called Anthropophilly.

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Hydrophily

Pollination by Water –Hydrophily – A few aquatic plants have their flowers growing in the air. They are pollinated by the insects. Other plants that have their flowers submerged in water release their pollen in the water that drifts in the water and are caught by the feathery stigma of female flowers.

The two methods are:

• Surface Pollination
• Submerged Pollination

Surface Pollination

As the name implies, the pollens float on the surface of the water and reach the stigmas of the female flowers. Water Thyme (Hydrilla), Ditch Grasses (Ruppia), Waterweeds (Elodea) and Water Starwort (Callitriche) are notable examples where this method of pollination is observed. In plants such as Eelgrass (Vallisneria), the male flowers detach from the plant and float on the surface of the water, passively flowing and coming in contact with the female flower’s stigmas.

Vallisneria (Fig above: Male flower bursts inside water, floats and reaches female flower)

Submerged Pollination

Submerged pollination occurs if the pollen grain

 is heavier than water – where it sinks down and is caught by the stigmas of female flowers. This form of pollination is seen in plants like the water-nymphs (Najas).

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Ornithophily

Bird pollinated plants (Ornithophily):

The bird pollinated flowers have funnel –shaped or tubular corollas which are brightly coloured.

The floral parts are commonly leathery, and they produce copious amount of nectar and large quantities of pollen which are sticky.

Most bird-pollinated flowers are red and have a lot of nectar. They also tend to be unscented.

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Chiropterophily

Bat pollinated plants (chiropterophily): The pollination of plants by bats is called chiropterophily.

Bats generally pollinate plants that bloom at night. They are attracted to large, white or pale-coloured blooms measuring 1 to 3 ½ inches in diameter.

Bats like nectar-rich, highly fragrant blooms with a musty, fruity aroma. Flowers are usually tube- or funnel-shaped.

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Anemophily

Wind pollinated plants (Anemophily):

Wind pollinated plants are those that rely on wind to transfer pollen from the male reproductive organs (anthers) to the female reproductive organs (stigma) of flowers. 

These plants release billions of pollen grains into the air so that a lucky few will hit their targets on other plants.  The stigma is often sticky and large to attract the pollen grains to it.

Features: Wind-pollinated flowers are usually green or dull-coloured and lack the sepals, sepals and nectar that most flowers have. 

These include wheat, rice, corn, rye, barley, and oats. Coconut, palm, maize, grasses and all gymnosperms are the best examples of wind-pollinated plants.

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Entomophily

Insect pollinated plants (Entomophily):

 Insect-pollinated plants rely on insects, birds, and animals to transfer pollen grains from the anther of one flower to the stigma of another.

 Examples of insect-pollinated flowers include sunflowers, orchids, and buddleia. 

Insect-pollinated flowers are usually large with brightly coloured petals . The pollen grains in insect-pollinated flowers are larger, sticky, and spiky to aid in transportation by the pollinator.

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Interactions and Relationship
Lessons from class 12
Commensalism, mutualism, and parasitism are the three main categories of symbiosis found in nature.

Commensalism + /0

In a commensal relationship, one species benefits and there is a neutral effect on the other—it neither benefits nor is harmed.

Sucker Fish and Shark

• Sucker fish attaches to shark fish with the help of dorsal fin feeding on the falling pieces of food and shark does not get any benefit from it therefore, this interaction is Commensalism.

Barnacles and whale

• Barnacles benefiting from having a place to settle and filter food, while the whales do not seem to suffer any negative consequences from their presence.

Other examples of commensalism are:

• Spiders spinning webs on plants.
• Hermit crabs that use discarded snail shells to protect themselves. When commensals dwell on the host’s body, using it as their habitat, it is called Inquilinism. 

Orchid and Mango tree

• An orchid growing on the branch of a mango tree is an epiphyte. Epiphytes are plants growing on other plants which, however, do not derive nutrition from them.
• Burdock seeds on animals- Burdock seeds get attached to the animals and are transported to various places on their back. This helps them to spread in the land. Burdock seeds are very light that animals barely experience their presence. 
• Cattle egrets eat the insects stirred by the cattle when they are grazing

Mutualism (+/+)

In this type of symbiosis, both organisms benefit from the relationship.

A classic example of this is the relationship between termites and the protists that live in their gut. The protists digest the cellulose contained in the wood, releasing nutrients for the benefit of the termite. In turn, the protists receive a steady supply of food and live in a protected environment.

Sea Anemone and clown fish

Other examples of mutualism are the algae that live in the tissues of coral in reefs, clownfish that live in the tentacles of sea anemones, in which the anemone protects the clownfish against predators, while the clownfish provides the anemone with excreted nutrients (ammonia, sulfur, and phosphorus).

Oxpecker bird and Buffalo

• The relationship between the Oxpecker bird and Buffalo where bird eats the insects on the skin of buffalo while the buffalo gets rid of the ticks and insects.

Algae and fungi

• Lichen representing mutual association between algae and fungi, in which algae is protected by fungi. The fungi provide nutrients for synthesis of food to the algae, while algae provide food to fungi, as they are incapable of synthesising their own food.

Aphids and Ants

• Aphids are little sap-sucking insects that secrete honeydew, a sugary liquid that is the waste product of their diet. Many aphid species are known to engage in a mutualistic relationship with ants that feed on the honeydew. In return, some species of ants will protect the aphids from predators and parasites.

• The corals and algae relationship. The coral provides shelter and essential nutrients for the zooxanthellae to use during photosynthesis, while the zooxanthellae produce synthetised sugars, which the coral feeds on, and oxygen as a by-product.

• Another example is monkeys and fruit-producing trees. The monkeys’ benefit by eating the fruit, and the trees benefit from the monkeys carrying the seeds away as a method of seed dispersal.
• Honeybees and Flowers: Honey bees get nectar from flowers and flowers are benefitted by pollination done by honeybees.

Parasitism (+/-)

Parasitism is a relationship where one symbiont benefits (the parasite) and the other (the host) is harmed in some way.

Parasitism is the type of symbiotic relationship or long-term relationship between any two species either plants or animals. Here the parasite gains benefits from the host which in turn harms the host without killing it.

• External Parasites (Ectoparasite) Leeches (Hirudinea), fleas, ticks, and lice are a few examples of parasites that don’t normally cause disease directly. They suck blood from the host without causing any harm to their host.
• There are different species of Plasmodium that are picked up by the mosquitoes and transmitted to different people causing Malaria.
• Tapeworm/ Teania solium can live in the human gastrointestinaltract for years. It spreads through under-cooked pork. These are more than three meters long.
• Fasciola Hepatica is an endoparasite that affects Liver. Internal parasites show more extreme specialization and have a less complex structure.

Brood parasitism is a breeding strategy in which the brood parasite female evades all parental care by laying its eggs in host nests of the same (conspecific brood parasitism, CBP) or different species (interspecific brood parasitism, IBP) relying on unrelated foster parents to care for their parasitic offspring. Eg. Cuckoo laying eggs in crow’s nest.

Amensalism (-/0)

Amensalism is a biological interaction between two species in which one is inhibited or destroyed and the other is unaffected.

There are two basic modes:

Competition, in which a larger or stronger organism excludes a smaller or weaker one from living space or deprives it of food. For example, the black walnut tree produces a chemical called juglone that inhibits the growth of many plants beneath it.

Antibiosis, in which one organism is destroyed by metabolic products of another. An example of antibiosis is the interaction between Penicillium and bacteria. The mould Penicillium creates the secretion known as penicillin, which is extremely toxic to bacteria. 

Amensalism:

Predation (+/-)

In this relationship, one organism kills and eats the other. Examples include a jaguar killing and eating a tapir as well as horses eating grass. Unlike parasitism, where the host remains alive while the parasite lives off it, predation results in the death of one of the organisms

Population of predator is generally small because if predator is too efficient or too many in number, they may over exploit its prey which may extinct and following it, predator will also extinct so, predator checks the population of prey but do not extinct them. 

The predator population works according to the prey and help in keeping the prey population in a check. In absence of predators, the population will increase immensely and will affect the normal ecological balance, so they respond functionally.

Competition (-/-)

Competition is when two organisms both want the same limited resource. Because they have to compete for the resource (often food, water, or territory), both organisms are negatively impacted.

Interspecific competition, in ecology, is a form of competition in which individuals of different species compete for the same resources in an ecosystem (e.g. food or living space).

Competition between members of the same species is called intraspecific competition.

Cheetahs and lions both attempting to hunt the same small herd of zebras is an example of competition. If there aren’t enough zebras for both animals to eat, they will both get less food than what they want and may suffer from hunger/starvation as a result.

In interspecies competition, two species use the same limited resource. Competition has a negative effect on both species. Mostly it occurs when there is a limited resource, and two species are trying to accommodate. This competition has adverse effects on all species utilizing a similar and limited resource at the same time and place making them smaller in population size.

Plant -animal interactions often involve co-evolution of the muralists, that is, the evolution of the flower and its pollinator species are closely linked, with one another. It occurs in mutualism and parasitism where one species is close relationship with the other.

• Gause’s law of competitive exclusion or just Gause’s law is a proposition that states that two species competing for the same resource cannot coexist at constant population values if other ecological factors remain constant.

• When two competing life forms attempt to occupy the same niche, only one outcome is possible: One life form will drive out the other.

Symbiosis interaction Chart

Interaction	Species A	Species B
Mutualism	+	+
Commensalism	+	0
Parasitism	+	–
Amensalism	–	0
Predation	+	–
Competition	–	–

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Now check what you have learnt !!!!!