Pollination and Fertilisation in Flowering Plants

Types of pollination

• Self-pollination, pollen is transferred from the anther to the stigma of the same flower, or another flower on the same plant.
• Cross-pollination, pollen is transferred from the anther of one plant to the stigma of a flower on a genetically different plant of the same species, usually carried by insects, wind, water or animals.
• Cross-pollination increases genetic variation in the offspring, while self-pollination is more reliable but produces less variation.

Growth of the pollen tube

When a compatible pollen grain lands on the stigma, it absorbs nutrients from the stigma's surface and germinates. The tube nucleus controls the growth of a pollen tube that grows down through the style towards the ovary, guided partly by chemical signals from the synergid cells in the embryo sac.

The generative nucleus travels down the tube and divides to form two male gametes, which are carried to the ovule at the tip of the pollen tube.

Double fertilisation

When the pollen tube reaches the embryo sac, it releases its two male gametes. One male gamete fuses with the egg cell to form a diploid zygote, which develops into the embryo.

The other male gamete fuses with the two polar nuclei in the central cell to form a triploid endosperm nucleus, which develops into the endosperm, a food store for the developing embryo. Because two separate fusion events happen at once, this process is called double fertilisation, and it is unique to flowering plants.

How it is examined

You may be asked to distinguish pollination from fertilisation, to describe the pathway and role of the pollen tube, or to explain double fertilisation by naming the two fusion events and their products (zygote and endosperm). Frame any exam-question analysis strictly as looking at how the topic has previously been examined, not as predicting future questions.

Worked exam-style question

Question. A hypothetical flowering plant species relies on a specific insect for pollination. In one growing season, the insect population in the area declines sharply, and far fewer flowers on this plant species develop into seeds compared to a normal season.

(a) Explain, in terms of pollination, why fewer seeds would form when this insect declines. (b) State what must still occur, after a pollen grain reaches a compatible stigma, before a zygote can form.

(c) Name the two products formed during double fertilisation, and state which structure in the pollen tube gives rise to the male gametes involved. (d) Suggest one reason a small number of seeds might still form even with very few of these insects present.

Model answer. (a) With fewer insects to carry pollen, less pollen is transferred from anther to stigma, so fewer flowers are pollinated; since pollination must occur before fertilisation can take place, fewer flowers go on to form seeds. (b) A pollen tube must grow from the stigma down through the style to the ovule, carrying the male gametes to the embryo sac, before fertilisation and zygote formation can occur.

(c) Double fertilisation forms a zygote (from a male gamete fusing with the egg cell) and an endosperm nucleus (from the second male gamete fusing with the two polar nuclei); the male gametes are produced when the generative nucleus in the pollen tube divides. (d) A small number of seeds could still form through self-pollination, or because a few of the declining insects still visited some flowers, or through other pollinating agents such as wind occasionally carrying pollen between flowers.

Practice question

Try this. A student states: "Pollination and fertilisation are the same process, because both are needed to produce a seed." Explain why this statement is inaccurate.

Exam tip

Key terms

These glossary terms connect directly to this standard:

• Pollination, the physical transfer of pollen from the anther to the stigma, which must occur before fertilisation.
• Fertilisation, the fusion of a male gamete with the egg cell (and, in double fertilisation, a second male gamete with the polar nuclei).
• Zygote, the diploid cell formed when a male gamete fuses with the egg cell, which develops into the embryo.
• Germination, the process by which the seed produced after fertilisation later begins to grow into a new plant.

Source:SRC-DSKP-EN

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