The stigma is the specialized, receptive tip of the female reproductive organ, known as the pistil or carpel, in a flowering plant. Its purpose is to act as the landing platform for pollen grains, initiating the entire process of sexual reproduction. The stigma must successfully capture, recognize, and facilitate the germination of compatible pollen to ensure the continuation of the species. It determines which pollen is allowed to proceed to fertilization.
Defining the Stigma and its Structure
The stigma is positioned at the apex of the pistil, often sitting atop the style, a stalk-like structure connecting it to the ovary below. Its morphology varies widely across plant species, reflecting adaptations to specific pollination methods. For instance, wind-pollinated flowers, like grasses, often have large, feathery, or elongated stigmas to maximize the surface area for catching airborne pollen.
Conversely, insect-pollinated flowers may have more compact, knob-like, or lobed stigmas designed to brush against a pollinator’s body. Regardless of the shape, the surface is composed of specialized cells called stigmatic papillae. These cells are often covered in fine hairs or have a roughened texture, which increases the surface area for pollen adhesion.
The Mechanism of Pollen Capture
The stigma’s surface is engineered for two distinct functions: physical adhesion and biochemical acceptance.
Physical Adhesion: Wet vs. Dry Stigmas
Physical adhesion classifies stigmas as either “wet” or “dry.” Wet stigmas, such as those found on apples, secrete a thick, sticky fluid called exudate (a mix of lipids, carbohydrates, and proteins). This liquid physically traps the pollen grain, anchoring it in place until germination can begin.
Dry stigmas, common in the cabbage family (Brassicaceae), lack this liquid exudate. They rely instead on surface waxes, proteins, and a thin cuticle for adhesion. The pollen grain’s outer coat interacts directly with the specialized papillae cells to form a stable point of contact, which is necessary for the pollen to hydrate.
Biochemical Acceptance: Self-Incompatibility (SI)
Beyond physical capture, the stigma acts as a sophisticated biochemical checkpoint through self-incompatibility (SI). This recognition system allows the stigma to differentiate between compatible pollen from a different plant and incompatible “self” pollen. In many species, if the pollen grain shares specific genetic markers (S-alleles) with the pistil, the stigma actively rejects it.
For example, in the Brassicaceae, the stigma surface cells contain a receptor protein that interacts with a protein from the pollen coat. If the two proteins match, a signal cascade prevents the self-pollen from hydrating or germinating.
Guiding the Path to Fertilization
Once the pollen grain is captured and recognized as compatible, the stigma facilitates the journey toward the ovule. It provides the necessary moisture and nutrients, enabling the pollen grain to rapidly hydrate and germinate. Germination involves the emergence of the pollen tube, a long, thin cellular extension that must grow down the style toward the ovary.
To grow, the pollen tube first penetrates the cell wall of the stigmatic papillae. The stigma and the transmitting tissue within the style then guide the pollen tube’s growth by providing a nutrient-rich pathway. Chemical signals, such as the STIG1 peptide, further promote the pollen tube’s growth and correct directionality. The stigma’s function is complete once the pollen tube has successfully entered the conducting tissue of the style, setting the path to deliver the sperm cells for fertilization.