The stigma is the pollen-receiving tip of a flower’s female reproductive system. Sitting at the top of the pistil, it captures pollen grains, determines whether they’re genetically compatible, and initiates the process that leads to fertilization. While it looks like a simple sticky surface, the stigma is a sophisticated gatekeeper that controls which pollen gets to fertilize the plant’s egg cells.
Where the Stigma Sits in a Flower
The female part of a flower, the pistil, has three connected structures. The stigma is the knob at the very top. Below it runs the style, a tube-like stalk that connects down to the ovary, which holds the egg cells (called ovules). Think of it like a funnel: pollen lands on the stigma, grows a tube down through the style, and eventually reaches the ovary to fertilize an egg. Without the stigma as a landing platform and starting point, that chain of events can’t begin.
How the Stigma Captures Pollen
The stigma’s most visible job is physically catching pollen grains, whether they arrive on a bee’s body or drifting in the wind. It does this through a combination of surface texture and liquid secretions. The stigma surface is covered in tiny cone-shaped projections called papillae, which are flexible enough to bend around and cradle incoming pollen grains.
These papillae are often coated in a thin layer of liquid that forms tiny bridges between the pollen grain and the surface, anchoring the grain in place through capillary forces, the same physics that makes water climb up a narrow straw. The main forces holding pollen to the stigma are these liquid bridges plus molecular attraction at very close range. Together, they ensure a pollen grain stays put once it arrives rather than bouncing or blowing off.
Wet Stigmas vs. Dry Stigmas
Not all stigmas work the same way. Botanists classify them into two broad types based on how much fluid they produce.
- Wet stigmas secrete a visible layer of exudate on their surface. Tobacco is a classic example. These stigmas are less selective at the front door: the fluid allows pollen from many species to hydrate and begin germinating, even pollen from unrelated plants.
- Dry stigmas produce little to no visible secretion. Arabidopsis, rice, and crocus all have dry stigmas. In these species, water flow to the pollen grain is tightly controlled, and the stigma runs a compatibility check before releasing moisture. This makes dry stigmas pickier about which pollen they accept.
The exudate on wet stigmas is far from plain water. It contains sugars, lipids, proteins, amino acids, and calcium ions. This fluid serves as both a welcoming environment and a nutrient source. Enzymes in the exudate break down large molecules into smaller units that a growing pollen tube can absorb, essentially providing fuel for the tube’s rapid journey down toward the ovary.
Hydrating Pollen to Trigger Germination
Pollen grains arrive at the stigma in a desiccated, dormant state. Before a grain can sprout a pollen tube and begin its journey toward the egg cell, it needs water. The stigma controls this hydration step, and it’s one of the most critical checkpoints in the entire fertilization process.
In dry-stigma species like those in the mustard family, a compatible pollen grain begins absorbing water from the stigma’s papilla cells within minutes of landing. Research on Arabidopsis shows that hydration happens in two phases: a rapid uptake during the first ten minutes, followed by a plateau where the grain stops swelling and starts germinating instead. The pollen grain needs to reach a specific hydration threshold before germination kicks in. Meanwhile, the stigma cell reorganizes its internal scaffolding and directs tiny transport packages (vesicles) toward the contact point, delivering water and other materials precisely where the pollen grain sits.
Screening for Genetic Compatibility
Perhaps the stigma’s most remarkable function is its ability to distinguish “self” pollen from “non-self” pollen. Many flowering plants have a built-in system to prevent inbreeding, called self-incompatibility. The stigma is where this screening happens.
The system works through a single genetic region called the S-locus, which comes in many different versions across a population. Each version produces unique proteins on the stigma surface that act as receptors. When pollen lands, these receptors check for a matching molecular signature from the pollen grain. If the pollen carries the same version of the gene as the stigma, the receptors trigger a chemical cascade inside the stigma cell that shuts down the hydration and germination process. The self-pollen simply never gets the water it needs to grow.
If the pollen carries a different version, no alarm is raised, and the stigma proceeds with hydration and supports pollen tube growth. This elegant system ensures that plants outcross with genetically different individuals, maintaining genetic diversity in the population without relying on any external mechanism.
Stigma Shape Varies With Pollination Strategy
The physical form of a stigma reflects how a plant gets pollinated. Wind-pollinated plants like grasses face a numbers problem: airborne pollen drifts randomly, so the odds of any single grain reaching a stigma are low. To compensate, these plants have large, feathery stigmas with many branching surfaces that sweep through the air like a net, maximizing the chance of intercepting a passing grain.
Animal-pollinated plants take a different approach. Their stigmas tend to be compact and sticky, positioned where a visiting bee, butterfly, or hummingbird will brush against them while reaching for nectar. The shape and placement are tuned to the specific pollinator. Since animal visitors deliver pollen directly and in relatively large quantities, the stigma doesn’t need to cast a wide net.
Timing of Stigma Receptivity
A stigma isn’t always ready to accept pollen. It goes through a window of receptivity that depends on the flower’s developmental stage and the surrounding environment. In almond trees, for example, the stigma doesn’t reach peak receptivity until the flower is past the fully open stage, when petals have flattened or even begun to fall. Younger, freshly opened flowers show lower rates of pollen germination and tube growth.
Temperature plays a significant role in how quickly this window opens and closes. Warm conditions can push an almond flower from early bloom to petal fall in a single day, compressing the entire receptive window. Cooler weather extends it. For fruit growers, this matters enormously: the timing of bee-assisted pollination needs to align with the narrow period when stigmas are most receptive, and that period shifts with the weather rather than following a fixed calendar. The same principle applies broadly across flowering plants, with each species having its own receptivity timeline shaped by genetics and climate.