The stigma is the receptive surface of a flowering plant’s female reproductive organ, known as the pistil or carpel. Positioned centrally within the flower, the stigma is designed to capture pollen, which contains the male genetic material necessary for reproduction. This capture marks the beginning of the fertilization process, ensuring the successful continuation of the plant species.
Anatomy and Placement
The stigma is consistently found at the apex of the pistil, often sitting directly above a stalk-like structure called the style. Its physical appearance is highly varied across different plant species, reflecting specialized adaptations for different pollinators or environments. Wind-pollinated plants, for example, feature large, feathery stigmas to maximize the surface area for catching airborne pollen.
Insect-pollinated flowers typically have compact and sticky stigmas, sometimes possessing tiny hairs or papillae that aid in adhesion. The surface is frequently coated with a sugary, waxy, or oily exudate. This coating not only traps pollen but also begins the rehydration process for the dry pollen grain. This unique morphology ensures that the pollen is securely held once delivered.
The Critical Role in Pollination
The stigma acts as an intelligent gatekeeper, regulating whether captured pollen is allowed to proceed to fertilization. When pollen lands, the stigma’s surface chemicals initiate a rapid recognition process. This determines if the pollen is from the correct species and if it is genetically compatible. The initial step for a viable pollen grain is to absorb water from the moist surface, which triggers germination.
This recognition relies on complex molecular signaling, often involving a ligand-receptor interaction on the stigma’s surface. For example, in plants like those in the cabbage family (Brassicaceae), a protein on the pollen surface must bind to a specific receptor on the stigma to signal compatibility. If the pollen is recognized as incompatible, the stigma actively prevents the pollen grain from hydrating or inhibits the growth of the resulting pollen tube. This chemical checkpoint prevents inbreeding or fertilization by foreign species, ensuring genetic diversity and reproductive fitness.
The Complete Female Reproductive Unit
The stigma is the topmost part of the complete female reproductive unit, which is collectively referred to as the pistil or carpel. Immediately beneath the stigma is the style, a slender stalk that serves as the pathway for the successful pollen grain. Once the stigma accepts the pollen, the pollen grain sprouts a tube that grows down through the tissues of the style.
The style’s length varies greatly, but it functions to ensure only robust, compatible pollen tubes reach the final destination. At the base of the pistil is the ovary, a bulbous structure that houses the ovules, which contain the plant’s female gametes. The pollen tube travels through the style and ultimately delivers the male gametes to the ovules within the ovary for fertilization, initiating seed development. The ovary itself will mature into the fruit, which protects the developing seeds and aids in their eventual dispersal.