Here is your short essay on Fertilization

Fertilization involves the fusion of male gamete with female gamete. It was first discovered by Strasburger (1884). In angiosperms the female gametophyte is seated deep in the ovarian cavity quite away from stigma. The partially developed male gametophyte (pollen grains) are normally held at stigma.

To effect fertilization, the pollen grains germinate on the stigma by putting forth pollen tube which grows through the style and reach the ovules where they discharge the male gametes in the vicinity of egg. The fertilization in angiosperms can be studied under the following headings.

(A) Germination of Pollen Grains and Growth of Pollen Tubes:

The pollen grains are usually monosiphonous (i.e., develop one pollen tube) but some plants are polysiphonous e.g. develop more than one pollen tubes. 10 pollen tubes in Althea rosa (Malvaceae) and 14 pollen tubes in Malva neglecta (Malvaceae) have been reported. Similarly, polysiphonous conditions have been reported in members of cucurbitaceae. On the basis of movement of pollen tube through style, styles are of three types:

(1) Open Style:

The style is hollow and pollen tubes creep on the canal e.g. Papavaraceae and most monocots. Here the canal cells are lined with secretary cells and transmitting tissue is developed.

(2) Half Closed Style:

Here the canal is surrounded by a rudimentary transmitting tissue e.g. Cactaceae.

(3) Closed Type:

No open channel is present but instead a solid core of elongated cells with rich protoplasm, through which pollen tube moves is presented, e.g. Datura, Cotton.

(B) Entry of Pollen Tube into Ovule:

The generative cell divides to form two male gametes.

After arriving at the top of ovary, the pollen tube may enter the ovule either through the micropyle or by some other routes. The former is usual condition and is known as Porogamy. In some plants, the pollen tube enters the ovule through Chalaza. This condition is known as chalazogamy. e.g. Casuarina, Rhus. In Alchemilla and Cucurbita the pollen tube enters the ovule through funiculus or the integument. This is known as mesogamy.

(C) Entry of Pollen Tube into Embryo sac:

After entering the ovules the pollen tube passes through the nucellus and reaches the embryo sac. The apex of the pollen tube may either pass between the egg and one synergid or between the embryo sac wall and a synergid or directly into the synergid. As a rule one synergid is destroyed by the impact of the pollen tube and the other remains intact until sometimes afterwards.

(D) Discharge of Male Gametes from Pollen Tube:

Male gametes are discharged into embryo sac by one of the following methods:

(i) Two subterminal openings are produced, one gamete is discharged through each opening.

(ii) The apex of the pollen tube bursts at the apex releasing both male garnets.

(iii) The tip of the pollen tube divides into two short branches one of which is directed towards the egg and other towards the secondary nucleus: Later the apical end of each pollen tube bursts releasing the male gametes.

(E) Fusion of Male and Female Gametes (Syngamy) and Fusion of Male Gamete with Secondary Nucleus (Triple Fusion):

After discharge from pollen tube one of the male gametes fuses with the egg and forms the oospore (zygote). This is called true fertilization (also called syngamy = Amphimixis). The oospore is diploid (2n). The second male gamete fuses with the secondary nucleus (2n) and forms a triploid (3n) nucleus called endosperm mother nucleus. This is called triple fusion.

Thus both the male gamete participate in fertilization, i.e., fertilization takes place twice, this is called Double fertilization (double fertilization includes syngamy as well as triple fusion). Zygote produced as a result of syngamy develops into embryo and the triploid nucleus (= Endosperm mother nucleus) develops into the endosperm which is used for nutrition of embryo.

Antipodals and synergids degenerate after fertilization.

Significance of Double Fertilization :

Double fertilization was discovered by Nawaschin in 1898. As a result of fertilization, the second male gamete fuses with the secondary nucleus to produce endosperm. The endosperm produced as a result of double fertilization, supplies food materials to the developing embryo, due to which embryo is differentiated properly leading to the formation of healthy seeds. According to Brick and Cooper (1940)” the endosperm is a tissue developed to compensate for the extreme reduction of female gametophyte in angisoperms. Embryo and its Development (Embryogeny)

After syngamy the egg is called oospore (zygote). After some rest period it develops into embryo. Although in the earlier stages of development there is no difference in monocotyledonous and dicotyledonous plants, but in later stages their development is different.