Stigma

In the syllabus and the exam

Stigma is examined as the receiving structure of a flower's female reproductive part, the carpel, positioned at its top and connected to the ovary below by the style, and candidates should be able to explain how its surface texture, whether sticky or feathery, is adapted to capture pollen grains during pollination. A commonly tested contrast is between insect-pollinated flowers, which usually have a smaller, sticky stigma positioned to brush against a visiting insect's body, and wind-pollinated flowers, which typically have a larger, feathery stigma that hangs outside the flower to catch airborne pollen more effectively. Structured questions often trace the sequence of events after pollen lands on the stigma: the pollen grain germinates, a pollen tube grows down through the style carrying the male nucleus towards the ovule inside the ovary, where fertilisation finally occurs. A common error is stating that fertilisation happens directly at the stigma; the stigma is only the landing site for pollen, and the male nucleus must still travel the length of the style through a pollen tube before it can fuse with the female nucleus inside the ovule. Candidates sometimes also confuse the stigma with the anther, particularly when a flower diagram is presented unlabelled. Candidates should also be able to explain why a stigma that is not receptive, meaning not yet ready to accept pollen, will not lead to successful fertilisation even if pollen lands on it, a timing detail occasionally used to test deeper understanding beyond simple structure recall. This is also why some flowers are structured so that their stigma matures at a different time from their own anthers within the same flower, a timing mechanism that reduces the chance of self-pollination even when both structures are present together.

Example

Comparing flower structures, a candidate writes: 'The stigma, at the top of the carpel, is often sticky or feathery so that pollen grains landing on it during pollination are trapped, after which a pollen tube grows down through the style to the ovary.'

Related terms

PollinationAntherFertilisation

Source:SRC-DSKP-EN