Category : 12th Class
The fusion of two dissimilar sexual reproductive units (gametes) is called fertilization. This process was discovered by Strasburger (1884).
(1) Germination of pollen grain on stigma and growth of pollen tube : Pollen grains reach the receptive stigma of the carpel by the act of pollination. Pollen grains, after getting attached to the stigma, absorb water and swell. Subsequent to mutual recognition and acceptance of pollen grains, the pollen grain germinates (in vivo) to produce a pollen tube which grows into stigma towards the ovarian cavity.
G.B. Amici (1824) discovered the pollen tube in Portulaca oleracea. Generally, only one pollen tube is produced by a pollen grain (monosiphonous). But some plants like members of Cucurbitaceae produce many pollen tubes (polysiphonous). The pollen tube contains a vegetative nucleus or tube nucleus and two male gametes. Later, the vegetative cell degenerates. The pollen tube now reaches the ovule after passing through the style.
(2) Entry of pollen tube into ovule : After reaching ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by any one of the following routes :
(i) Porogamy : When the pollen tube enters the ovule through micropyle, it is called porogamy. It is the most common type. e.g., Lily.
(ii) Chalazogamy : The entry of pollen tube into the ovule from chalazal region is known as chalazogamy. Chalazogamy is less common. e.g., Casuarina, Juglans, Betula, etc. It was first observed by Treub (1981) in Casuarina.
(iii) Mesogamy : The pollen tube enters the ovule through its middle part i.e., through integument (e.g., Cucurbita, Populus) or through funicle (e.g., Pistacia).
(3) Entry of pollen tube into embryo sac : The pollen tube enters the embryo sac only from the micropylar end irrespective of its mode of entry into the ovule. The pollen tube either passes between a synergid and the egg cell or enters into one of the synergids through filiform apparatus. The synergids direct the growth of pollen tube by secreting some chemical substances (chemotropic secretion). The tip of pollen tube enters into one synergid. The penetrated synergid starts degenerating. After penetration, the tip of pollen tube enlarge and ruptures releasing most of its contents including the two male gametes and the vegetative nucleus into the synergid.
(4) Double fertilization : The nuclei of both the male gametes are released in the embryo sac. One male gamete fuses with the egg to form the diploid zygote. The process is called syngamy or generative fertilization. This syngamy was discovered by Strasburger (1884).
The diploid zygote finally develops into embryo. The other male gamete fuses with the two polar nuclei (or secondary nucleus) to form the triploid primary endosperm nucleus. The process is called triple fusion or vegetative fertilization. These two acts of fertilizations constitute the process of double fertilization. The process was discovered by S.G. Nawaschin (1898) and Guignard in Lilium and Frittillaria. Double fertilization occurs in angiosperms only.
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