Glossary: Sexual Reproduction in Flowering Plants

This chapter's terms in context

Chapter 21 covers four terms describing how a flowering plant reproduces sexually and how a new plant begins its life: anther, stigma, pollination and germination. The anther is the male part of a flower's stamen, and it is the specific structure where pollen grains, containing the male gametes of the plant, are produced and from which they are eventually released; a typical anther consists of four pollen sacs where pollen mother cells undergo meiosis to produce haploid pollen grains. The stigma, in contrast, is the female receptive surface at the top of the carpel, adapted to capture pollen grains, often through a sticky surface or fine hair-like projections depending on whether the plant is insect-pollinated or wind-pollinated, and it is where a captured pollen grain must land before fertilisation can eventually take place further down inside the ovary.

Pollination is the transfer of pollen grains from the anther to the stigma, and it is important to state precisely that pollination itself is not the same event as fertilisation: pollination is only the physical transfer of pollen, while fertilisation is the later fusion of a male gamete nucleus from the pollen with a female gamete nucleus inside the ovule, which requires the pollen grain to first germinate a pollen tube that grows down through the style to reach the ovary. Self-pollination occurs when pollen from the anther is transferred to the stigma of the same flower or another flower on the same plant, while cross-pollination occurs when pollen is transferred to the stigma of a flower on a genetically different plant of the same species, usually carried by an agent such as an insect, wind, water or another animal; a frequent exam error is treating pollination and fertilisation as interchangeable terms, when the syllabus specifically separates them into distinct content standards with distinct structures and distinct outcomes.

Germination is a separate later event in the plant life cycle: it is the process by which a seed resumes active growth and develops into a seedling, once conditions such as adequate water, oxygen and a suitable temperature are available. During germination, the seed absorbs water, its stored food reserves (in the cotyledon or endosperm, depending on the species) are broken down by enzymes and used to fuel the growth of the embryonic root (radicle) and embryonic shoot (plumule), and the seed coat eventually ruptures as the seedling emerges. A common structural confusion is treating germination as though it happens immediately after fertilisation, when in fact a considerable period of seed development and often seed dormancy usually separates fertilisation from germination, and germination itself requires specific environmental conditions to actually begin.

Sequencing these four terms correctly across the whole reproductive cycle, pollination transfers pollen to the stigma, fertilisation (a separate later event) produces a zygote and eventually a seed, and germination (an even later event, sometimes after a period of dormancy) begins the growth of a new independent plant, gives a clear timeline that most extended-response questions on this chapter are actually built around.

Paper 3 investigations on this chapter frequently test the conditions required for germination, such as comparing seeds given water, oxygen and warmth against seeds deprived of one of these factors, and full marks typically require identifying the correctly controlled variables and explaining a lack of germination in terms of the specific missing condition rather than a vague statement that 'the seed did not grow'.

A related exam angle contrasts self-pollination with cross-pollination in terms of the genetic variation each produces in the resulting offspring: self-pollination combines gametes from the same parent plant and tends to produce offspring genetically similar to that parent, while cross-pollination combines gametes from two genetically different parent plants and produces offspring with greater genetic variation, an advantage that helps a population adapt to changing conditions over generations. A candidate is often asked to explain why many flowering plants have evolved structural features, such as anthers and stigmas maturing at different times within the same flower, that specifically favour cross-pollination over self-pollination.