Biology Today - May 2018

Structure of Ovule

• A typical angiosperm ovule is a small structure attached to the placenta by means of stalk called funicle. The body of the ovule
  fuses with funicle and the point of attachment is called hilum.


• The body of the ovule consists of a mass of parenchymatous cells named nucellus.


• Nucellus is surrounded by one (unitegmic ovule, e.g., higher dicots) or two (bitegmic ovule, e.g., monocots and primitive dicots)
  or multicellular integuments.


• The integuments leave a narrow passage known as micropyle at one end of the ovule.


• The place of origin of the integuments usually lies at the opposite end of micropyle, termed as chalaza.


• Embryo sac or female gametophyte is present in the micropylar half of nucellus.


• Depending upon the configuration and orientation of the body of ovule in relation to funiculus there are six types of ovules in
  angiosperms - orthotropous (erect), anatropous (inverted), hemitropous (half-inverted), campylotropous (body curved),
  amphiotropous (both body and embryo sac curved), circinotropous (funiculus coiled around the ovule).

Megasporogenesis

• The process of formation of haploid megaspores from the diploid megaspore mother cell is called megasporogenesis.


• Generally, a single megaspore mother cell (MMC) differentiates in the micropylar region of the nucellus.


• The megaspore mother cell undergoes meiotic division which results in the production of four haploid megaspores.


• In majority of angiosperms, only one of the megaspores is functional while the other three degenerate.


• The functional megaspore develops into the female gametophyte (embryo sac).


• The formation of embryo sac from a single megaspore is called monosporic development.


• The female gametophyte or embryo sac contains 8 nuclei but 7 cells-3
  micropylar, 3 chalazal and one central.


• The three micropylar cells are known as egg apparatus. The middle
  cell of the egg apparatus is called egg which is larger with a central
  vacuole and a nucleus towards the chalazal end while the remaining
  two cells are called synergids.


• Each of the synergids bears a filiform apparatus in the micropylar region
  which is a mass of finger like projections of the wall into the cytoplasm.


• The three chalazal cells of the embryo sac are called antipodal cells.


• The central cell is the largest cell of the embryo sac.


• The central cell contains two polar nuclei which often fuse to form a
  single diploid secondary or fusion or definitive nuclei.

1. What are the functions of tapetum?


2. In which type of ovule funiculus is coiled around the ovule? Give an example.

POLLINATION

• The transfer of pollen grains from the anther to the stigma is called pollination.

Types of Pollination

Autogamy

• Transfer of the pollen grains from the anther to
  the stigma of the same flower.

Self pollination Cross pollination or Xenogamy

• Transfer of pollen grains from anther to
  the stigma of a genetically different plant.
  Geitonogamy


• Transfer of pollen grains from the anther to the
  stigma of another flower of the same plant.


• Some plants such as Oxalis, Viola and Commelina produce two types of flower - chasmogamous flowers with exposed anthers
  and stigma and cleistogamous flowers which do not open of all.


• Cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma.

Chalaza

Two polar

nuclei

Vascular strand

Micropyle Funiculus

Fig.: Structure of a typical ovule (anatropous ovule)

prior to fertilisation.

Egg (oosphere)

Embryo sac

Central cell

Antipodal cells

Integuments

Nucellus

Synergids

Hilum

Raphe