Short essay on Pollen Germination and Fertilization

Pollen tube enters the ovule through micropyle (porogamy), sometimes it enters through chalaza (chalazogamy) or through the base of ovule or through funicle or integuments (mesogamy).

Pollen tubes produce enzymes which digest the tissues of the stigma and the style. After entering into the embryo sac, pollen tube liberates the male gametes and disintegrates.

Syngamy:

One of the male gamete fuses with the egg resulting in the production of zygote (diploid).

Triple fusion:

Second male gamete fuses with the secondary diploid nucleus, producing a triploid primary endosperm nucleus.

The process of fertilization was discovered by Strasburger (1884).

Double fertilisation:

Takes place in angiosperms only; discovered by Nawaschin (1898) in Frittilaria and Lilium and later by Guignard (1899). It is the fusion of:

(i) One male gamete with the egg cell (real fertilization or syngamy) and

(ii) Another male gamete with the secondary or definitive nucleus or polar nuclei.

In double fertilization, above all five nuclei are involved.

Endosperm and Embryo Development :

i. Endosperm (triploid, nutritive tissue) develops after fertilization from secondary nucleus.

ii. 3 types of endosperm, depending on the development.

(A) Free Nuclear:

Most common; the first division and many subsequent divisions of primary endosperm nucleus are not followed by wall formation e.g., cotton, maize, wheat, rice, sunflower etc.

(B) Cellular:

Every nuclear division is followed by cytokinesis, making it cellular from the beginning.

(C) Helobial (Intermediate type)-First division of primary endosperm nucleus results in the formation of a large micropylar chamber and a small chalazal chamber. Rest of the divisions are free nuclear.

i. Perisperm: Remnant of nucellus or persistent nucellus surrounding the embryo.

ii. Chalazosperm: Perisperm like tissue in chalazal region ; subtitute for endosperm.

iii. In dicots like pea, bean etc. reservesjn the endosperm are consumed by the developing embryo and visible only as few cell layers in the mature seed.

iv. In many other plants e.g., cereals, coconut etc. endosperm enlarges considerably by cell division and is much bigger than the embryo on seed maturation.

v. The first division of the zygote is transverse and a basal (hypobasal) cell towards the micropyle and a terminal (epibasal) cell is formed towards the chalaza.

vi. The epibasal cell divides repeatedly to produce a row of 4-8 cells.

vii.The terminal cell divides in various planes to produce a cluster of cells, called proembryo.

viii. The hypobasal and the remaining cells constitute the su&pensor which pushes the proembryo into the endosperm to enable the developing embryo to receive nutrition.

ix. A few cells of the proembryo nearest to suspensor develop into hypocotyl and radicle. The other cells give rise to epicotyl, plumule and cotyledons (two in dicots and one in monocots).

x. As the embryo and endosperm develop and mature, the integument of the ovule becomes hard (seed coat) and leads to the formation of seed.

xi. Post-fertilization changes : Ovary (carpel): fruit; Ovule (megasporangium):seed ; Nucellus : perisperm ; Egg cell : embryo ; Secondary nucleus : endosperm ; Outer integument: outer seed coat (testa); Inner integument: inner seed coat (tegmen) and Micropyle : an opening in seeds (micropyle).