Notes-Part-2-Class-12-Biology-Chapter-1-Reproduction in Lower & Higher Plants-Maharashtra Board

Types of cross pollination :

(a) Chasmogamy : When flowers open and expose their sex organs.

(b) Homogamy : Condition when anther and stigma mature at same time.

(c) Cleistogamy : Condition when flowers remain closed, e.g. Viola, Commelina.

(d) Geitonogamy : Conditon where transfer of pollen grains to stigma of other flower produced on same plant, e.g. Unisexual flowers of Cucurbita.

Agents of pollination :

(1) Abiotic (Non-living)

• Wind-Anemophily
• Water-Hydrophily

Anemophily : The transfer of pollen grains through wind is called anemophily.

• Plants that are pollinated by wind are called anemophilous plants.
• Anemophilous plants bear small and inconspicuous flowers without any bright colours, fragrance and nectar.
• Flowers are produced in large numbers.
• Stamens are long with versatile anthers.
• Stigma is feathery, exposed to receive the pollen grains coming along with the wind.
• e.g. Grasses, maize, Jowar and Palms.

Hydrophily : The transfer of pollen grains with the help of water is called hydrophily.

• Plants that are pollinated by water are called hydrophilous plants.
• Hydrophilous plants possess small, inconspicuous unisexual flowers.
• Flowers lack fragrance, nectar and bright colour.
• Pollen grains and other floral parts are protected from getting wet.
• Stigma is long and sticky. e.g. Zostera, Vallisneria, etc.

 (2) Biotic (Living) :

• Insects-Entomophily
• Birds-Ornithophily
• Bats-Chiropterophily
• Animals-Zoophfly
• Human beings-plant breeding-Artificial

Entomophily : Pollination with the help of insects is called entomophily.

The insect pollinated flowers are called entomophilous flowers.

Entomophilous flowers show the following adaptations :

• 1) Flowers are large and attractive.
• 2) Flowers are brightly coloured with pleasant smell.
• 3) Flowers produce nectar which is food for the insects.
• 4) Pollen grains are spiny and sticky for easy adherance to the rough and sticky stigma.

Entomophily is seen in plants like rose, Jasmine, Cestrum, Salvia, etc- I

 Ornithophily : The transfer of pollen grains through birds is called ornithophily.

• Bird pollinated plants are called ornithophilous plants.
• Ornithophilous plants bear large and showy flowers.
• Flowers brightly coloured to attract birds for pollination.
• Ornithophilous flowers lack fragrance as birds have poor sense of smell.
• Pollen grains are sticky and spiny e.g. Callistemon, Bignonia, Bombax, Butea, etc.

Chiropterophily : The pollination that occurs with the help of bats is called chiropterophily.

• In chiropterous plants the flowers are large and stout enough in such a way that bats can hold onto the flowers.
• Chiropterous flowers are nocturnal, i.e. they open during the night time only.
• Flowers emit rotten fruits like fermenting fruity odours which attract bats.
• Flowers produce copious nectar.
• Flowers possess large number of stamens which produce large amount of edible pollen grains.
• Anthocephalus, Kigellia and Adansonia are chiropterous flowers.

Devices observed in plants : Unisexuality, dichogamy, prepotency, heteromorphy and herkogamy are the outbreeding devices.

(1) Unisexuality :

• The plants bear either male or female flowers.
• Due to unisexual nature, self-pollination is avoided.
• Plants are either dioecious, or monoecious,
• e.g. Maize or Monoecism e.g. Papaya, Mulberry

(2) Dichogamy : When stamens and carpels mature at different times in a bisexual flower, the condition is known as dichogamy.

• Maturity at different times for anthers and stigma
• Owing to dichogamy self-pollination is avoided and cross pollination is favoured.
• Dichogamy is of two types, protandry and protogyny.
• Protandry is seen in sunflower in which pollen grains are released much before stigma becomes receptive.
• In protogyny, stigma becomes ready to receive the pollen grains before the anthers mature. It is seen in plants like Gloriosa.

(3) Prepotency

• Rapid pollen germination on other stigma of same type e.g. Apple.

(4) Heteromorphy :

• In same plants different types of flowers are produced.
• In these flowers, stigmas and anthers are situated at different levels.
• Divided into two types - (1) Heterostyly, (2) Heteroanthy.
• This prevents self-pollination e.g. Primrose.

(5) Herkogarny :

• In bisexual flowers we may come across mechanical device to prevent self-pollination.
• Natural physical barrier between sex organs avoids contact of pollens with stigma.
• E.g. Calotropis where pollinia are situated below the stigma.

(6) Self-sterility: Self-incompatibility or self-sterility is a genetic mechanism that prevents germination of pollen on stigma of same flower.

• E.g. Tobacco, Thea

Artificial hybridization : It is one of the major approaches used in the crop improvement and plant breeding programmes.

• Only the desired pollen grains are hand pollinated and used for fertilization.
• This is accomplished through emasculation (Removal of anthers before opening of flowers) and bagging procedure.

Importance of the double fertilization :

• It ensures seed formation with food storage for embryo developed from fertilized egg.
• Diploid zygote develops into embryo which further forms a new plant.
• Triploid PEN forms endosperm which is nutritive tissue for embryo.
• Restoration of diploid condition by syngamy.

Characteristic feature of angiosperms

Porogamy : Pollen tube entering the ovule through micropyle.

(Most common type - enters synergid and releases its content — 2 non-motile male gametes)

Chalazogamy : Entry of pollen tube through chalaza.

Mesogamy : Entry of pollen tube through integuments.

Siphonogamy : Non-motile gametes present in pollen tube.

Process of double fertilization in embryo sac

(1) Syngamy (Generative fertilization)

1 male gamete unites with egg cell -> (n) + (n) = 2n -> Zygote (2n) -> Embryo

(Generative fertilization)

(2) Triple fusion (Vegetative fertilization) : Triple fusion is also called second fertilization.

Out of the two male gametes in angiosperms, the first one fuses with the egg to form the zygote, while the second one fuses with the secondary nucleus to form primary endosperm nucleus. This is called triple fusion.

Since each of the polar nuclei is a sister nucleus of the egg, it is called second fertilization.

First fusion involves the fusion of a male gamete with the egg; the second fusion involves the fusion of two polar nuclei to form the secondary nucleus and the third fusion involves the fusion of the other male gamete with the secondary nucleus.

• 1 male gamete unites with Secondary nucleus ->n + (2n) = 3n -> PEN-Primary Endosperm Nucleus (3n) ->Endosperm — Nutritive tissue (Vegetative fertilization)

Significance of double fertilization :

• Unique feature of angiosperms.
• Concerned with production of seed.
• Zygote develops into embryo which in turn forms new plant.
• Triploid PEN forms endosperm — nutritive tissue for developing embryo.
• Restoration of diploid chromosome number.
• Avoids polyembryony.

Types of endosperm :

There are three types of endosperm, viz., nuclear, cellular and helobial.

Nuclear endosperm :

• Nuclear endosperm is the most common type of endosperm.
• During the formation of nuclear endosperm, the primary endosperm nucleus (PEN) undergoes free nuclear division forming a large number of triploid nuclei which remain freely suspended in the common cytoplasm of central cell.
• A central vacuole pushes the nuclei towards periphery.
• Later on wall formation takes place around these nuclei to form a cellular mass.
• It is seen in plants like maize, sunflower, wheat, coconut, etc.

Cellular endosperm :

• In this type of endosperm, the triploid primary endosperm nucleus undergoes nuclear divisions followed by cytokinesis.
• Owing to this, the development of endosperm occurs in cellular form.
• It is less common and seen in dicot plants like Datura, Petunia, Balsam, Adoxa.

Helobial endosperm :

• In helobial type of endosperm, the first division of the primary endosperm nucleus is followed by the formation of cell wall.
• Owing to this, the central cell is divided into a large micropylar cell and a small chalazal cell.
• In both micropylar and chalazal chamber, the further development of the endosperm is of nuclear type.
• Walls develop between nuclei in micropylar chamber.
• This type of embryo development is seen in plants belonging to order Helobiales of Monocots. e.g. Asphodehis.

Oospore divided into 2 celled pro-embryo :

The oospore undergoes a transverse division to form a large basal cell towards the micropyle and a small apical or terminal cell towards the chalaza of the embryo sac. This two celled structure is called proembryo.

• The basal cell or suspensor initial undergoes repeated transverse divisions to form a multicellular structure called suspensor.
• The suspensor pushes the embryo towards the endosperm to draw its nutrition.
• The apical cell or embryonal initial of the proembryo undergoes a transverse division followed by two vertical divisions at right angles to form an 8 cells octant stage.
• From octant, the lower four cells form hypocotyl and radicle while four cells of upper side form plumule with two cotyledons.
• The lowermost cell of suspensor is hypophysis and by its further division forms part of radicle and root cap.
• The cells from upper side of octant divide repeatedly to form heart shaped which elongated further to form two lateral cotyledons.
• Enlargement of hypocotyl and cotyledon results into curved embryo which appears horse shoe shaped.

Significance of Seed and Fruit Formation:

• Nourishment to seed that is present inside fruit.
• Protection of seed.
• Propagating unit.
• Dispersal, spread of species.

Seed dormancy : It is a state of metabolic arrest which helps in survival of organism in unfavourable environmental conditions.

• Structure or physiological adaptive measures of seed that are helpful in adverse conditions is called dormancy.
• Seeds are dispersed during their dormancy.
• When dormancy period of seeds is completed then only the viable seed germinate.

There are three main categories of apomixis :

(a) Recurrent (b) Non-recurrent (c) Adventive embryony.

Recurrent apomixis : In this diploid sporophytic cell, archesporial cell or nucellus form embryos, When diploid megaspore mother cell forms embryo sac it is known as diplospory. It is also called apospory.

Non-recurrent apomixis : Haploid embryo sac is formed but the embryos arise either from egg cell or any other haploid cell. It ii also known as apogamy.

Adventive Embryony : In this in addition to normal zygotic embryo, additional embryos develop from nucellus or integuments. It results in polyembryony.

Advantages :

• Genetically uniform seedlings.
• Emergence of multiple seedlings.
• Beneficial in horticulture — Growing fruits