IGCSE Biology Paper-3: Specimen Questions with Answers 294 - 295 of 358

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Fig. is a diagram showing the events from pollination to fertilization in a species of flowering plant.

Question 294 (4 of 6 Based on Passage)

Short Answer▾

Seed formation occurs after fertilization. Seeds are formed inside the fruits and then dispersed.

State an advantage of seed dispersal.

Short Answer▾

Fertilization occurs at R as shown in Fig.

Describe what happens at fertilization in flowering plants.

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 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 one synergid. The penetrated synergid starts degenerating.
After penetration, the tip of pollen tube enlarges and ruptures releasing most of its contents including the two male gametes and the vegetative nucleus into the synergid. The nuclei of both the male gametes are released in the embryo sac. One male gamete fuse with the egg to form the diploid zygote (2n) . The process is called syngamy or generative fertilization.
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 (3n) . The process is called triple fusion or vegetative fertilization. These two acts of fertilizations constitute the process of double fertilization.