What are Cleistogamous Flowers?

The word comes from Greek: kleistos means 'closed', and gamos means 'marriage'.

Cleistogamous flowers are small, inconspicuous flowers that do not open at all, even at maturity. Because the flower remains completely sealed, the anthers (male parts producing pollen) and the stigma (female part receiving pollen) lie very close to each other tightly inside the closed petals.

Because the flower never opens, pollen from other plants cannot enter, and its own pollen cannot escape. When the anthers split open inside the flower bud, pollen grains automatically fall onto the stigma.

Therefore, cleistogamous flowers are invariably autogamous (strictly self-pollinating). There is absolutely no chance of cross-pollination.

Why would a plant do this? It seems like a bad idea since it reduces genetic diversity.

However, it is a highly effective evolutionary survival trick:

1. Assured Seed Set: Because pollen doesn't have to travel, seed production is 100% guaranteed. The plant does not have to worry about the weather, wind, or lack of bees.
2. Energy Efficient: The plant saves a massive amount of energy because it doesn't need to produce large, colorful petals, sweet nectar, or fragrant scents to attract insect pollinators.

Many plants produce two types of flowers on the exact same plant:

1. Chasmogamous flowers: Normal, open flowers that invite insects for cross-pollination.
2. Cleistogamous flowers: Closed, underground flowers acting as an 'insurance policy'.

Classic examples of plants that produce both are Commelina (dayflower), Viola (common pansy), and Oxalis (wood sorrel).