The stigma is a specialized part of a flower’s reproductive system, forming the receptive tip of the carpel or pistil. It serves as the primary landing and receiving site for pollen grains during plant reproduction. This floral component is positioned to effectively capture pollen, initiating the events necessary for fertilization. Its function is to ensure that pollen, whether carried by wind or pollinators, successfully adheres to the female reproductive organ.
The Stigma’s Structure and Its Purpose
The stigma exhibits diverse physical characteristics that enhance its ability to capture and retain pollen. Many stigmas feature a sticky or hairy surface, which provides the necessary adhesion for pollen grains. This stickiness often comes from a secreted fluid, known as exudate, composed of water, sugars, lipids, and proteins, which coats the stigmatic surface. Some stigmas have specialized structures like papillae, small, finger-like projections of epidermal cells, further increasing the surface area for pollen capture. These adaptations ensure efficient collection of pollen from various sources, whether from insects or the wind.
The shape of the stigma can vary significantly, adapting to different pollination methods. Wind-pollinated plants, such as grasses, often possess large, feathery, or branched stigmas to maximize their chances of catching airborne pollen grains. Insect-pollinated flowers may have more compact, sticky, or globe-shaped stigmas, sometimes with a velvety texture, designed to effectively pick up pollen from the bodies of pollinators. These structural diversities, including variations from long and slender to lobed forms, link the stigma’s morphology to its efficiency in pollen capture.
The Stigma’s Role in Plant Reproduction
Once a pollen grain lands on a compatible stigma, the stigma plays a role in pollen hydration. The often desiccated pollen grain absorbs water from the stigmatic surface. This rehydration is a prerequisite for pollen germination, enabling the pollen grain to become metabolically active and initiating the growth of a pollen tube.
A function of the stigma is pollen recognition and compatibility, acting as a selective barrier. The stigma determines if the pollen is from the correct species and is genetically compatible for fertilization. This process involves chemical signaling between the pollen grain and the stigmatic surface, where specific proteins and lipids on both sides interact. If the pollen is recognized as compatible, the stigma facilitates the continued growth of the pollen tube. Incompatible pollen, either from a different species or from the same plant in self-incompatible species, is often rejected, preventing its hydration or pollen tube penetration.
Upon successful recognition, the stigma guides the growing pollen tube through the style, which is a narrow extension connecting the stigma to the ovary. The pollen tube navigates through the transmitting tissue of the style, a path often rich in nutrients, towards the ovules located within the ovary. This directional growth, influenced by various cues including water gradients and chemical signals from the pistil, ensures the male gametes are delivered to the ovules for fertilization.