Yogy's biology

01 Reproduction in Lower and Higher Plants - part 11 - Apomixis ,Parthenocarpy and Polyembryony Apomixis :  Phenomenon of  formation of embryo(s ) through  asexual method  of reproduction without formation of gametes and the act of fertilization.  Unusual sexual reproduction where there is  no meiosis  and  syngamy.  Embryo develops in the  ovule  and ovule developes to form  seed .  In apomixis, when a gametophyte organ or cell  produces embryo like structure without fertilization , it is termed as apogamy.  Similarly when diploid sporophyte cell produces a diploid gametophyte without undergoing meiosis is called apospory,  e.g.  Orange, Mango . Main categories of apomixis are:  a. Recurrent apomixis :  In this type, the embryo sac generally rise either from an archesporial cell  or from some other part of the nucellus.  In diplospory, the unreduced embryo sac is derived from t...

01 Reproduction in Lower and Higher Plants - part 10 - Seed and Fruit Development Seed and Fruit Development :  The goal of reproduction, in every living organisms including plants, is to  create offsprings for the next generation.  One of the ways that plants can produce offpsrings is by forming (making)  seeds .  The flowers must be pollinated in order to produce seeds and fruit. Seed development is initiated by  fertilization .  The integuments of the  fertilized ovule persist and get transformed into the  seed coat  of mature seed. Seed sometimes consists of two distinct coverings, a typical outer seed coat, the  testa  and the inner thin, membranous  tegmen.   The  nucellus  in the ovule may persist in some genera like  black pepper and beet  as a thin, papery layer, the  perisperm .  In some seeds, the food reserves in the endosperm are partially used up in the development of an emb...

01 Reproduction in Lower and Higher Plants - part 06 - Outbreeding devices Outbreeding devices (contrivances):  Many plants have mechanisms that discourage or prevent  self pollination.  To promote cross pollination and increase genetic diversity, plants have evolved a wide variety of sexual strategies.  Genetic diversity  is an essential factor for evolution by natural selection. Continued self pollination results in the inbreeding depression.  Thus plants have developed many devices to  encourage cross pollination.  The examples of outbreeding devices are as follows:  Unisexuality :  In this case, the plant bears either male or female flowers.  It is also called as  dioecism .  As flowers are unisexual, self pollination is not possible.  Plants may be monoecious, e.g.  Maize or dioecious,   Mulberry, Papaya. Dichogamy  :  In this device,  anthers and stigmas mature at different times...

01 Reproduction in Lower and Higher Plants - part 05 - pollination Pollination :  Pollen grains being non motile, angiosperms have evolved the strategy to use - Abiotic agents (wind, water) and biotic agents (birds, insects, snails) to their flowers,  Feeding the visitors and  Exploiting their mobility for pollination and also seed dispersal.  Pollen grains are non-motile and they are usually carried from flower to flower by means of external agents.  Pollination is the  transfer of pollen grains from anther to the stigma  of the flower.  It is the prerequisite for fertilization because both the male and female gametes are non-motile.  Moreover gametes are produced at two different sites. Self pollination : It is a type of pollination which occurs  in a single flower or two flowers on a single plant.  It results in inbreeding or selfing.  In contrast cross pollination is the transfer of pollen grains from the anther of one flow...

01 Reproduction in Lower and Higher Plants - part 09 - Development of Embryo Development of Embryo :  The process of development of zygote into an embryo is called  embryogenesis .  The embryo is developed  at the micropylar end  of embryo sac.  The growth of embryo triggers only  after certain amount of endosperm is formed.  After fertilization the embryonic development begins. The zygote divides to form  two- celled proembryo.  The larger cell towards the micropyle is called  basal or suspensor initial cell  and smaller cell towards chalaza is called  terminal or embryonal initial cell.  The  suspensor cell divides  transversely in one plane to produce  filamentous suspensor of 6-10 cells. The  first cell of the suspensor  towards the micropylar end becomes swollen and function as a  haustorium .  The  lowermost cell of suspensor  is known as  hypophysis . ...

01 Reproduction in Lower and Higher Plants - part 08 - Development of Endosperm Development of Endosperm :  The triploid primary endosperm nucleus repeatedly divides, mitotically to form nutritive tissue, called endosperm.  In post-fertilization changes within the ovule, the  embryo and endosperm are seen to develop simultaneously.  The other cells of embryo sac disorganized sooner or later.  The formation of triploid endosperm nucleus triggers cell division which leads to the formation of endosperm. Types of endosperms [on the basis of mode of development] : Nuclear type Cellular type Helobial type  1. Nuclear Type :  It is the most common type found in  161 angiospermic families.  Here, the primary endosperm nucleus repeatedly divides mitotically  without wall formation to produce large number of free nuclei.  A  big central vacuole  appears in the centre of cell pushing the nuclei towards the periphery. Later, walls dev...

01 Reproduction in Lower and Higher Plants - part 07 - Pollen - Pistil Interaction and Double Fertilization Pollen - Pistil Interaction :  It is the interaction of pollen grains with sporophytic tissue (stigma).  It  begins with pollination  and  ends with fertilization.  All the events from the deposition of pollen grain on stigma to the  entry of pollen tube in the ovule (synergid)  are referred as  pollen - pistil interaction.  Pollination does not guarantee the transfer of right type of pollen, often wrong type also land on stigma.  The pistil has the ability to recognise and accept the  right or compatible pollen  of the same species. Thus wrong type of pollen is discarded by pistil.  Compatibility and incompatibility of the pollen-pistil is determined by  special proteins.  This process involves pollen recognition followed by promotion or inhibition of pollen.  The stigmatic surface of flower refu...