The style is a specialized part of a flowering plant’s female reproductive organ, the pistil or carpel. It is a slender, stalk-like structure that connects the receptive tip (stigma) to the chamber that houses the ovules (ovary). The style plays an active role in reproduction, engaging in a complex chemical conversation with the pollen grain. This ensures that only compatible pollen can successfully complete the journey necessary for seed production.
The Style’s Position in the Flower
The style is the middle segment of the pistil, which is the collective term for a flower’s female reproductive unit. The pistil is generally located in the center of the flower, surrounded by the pollen-producing stamens. This central position offers protection to the reproductive structures.
The style originates from the top of the ovary, the swollen base of the pistil where the ovules are housed. It extends upward to terminate at the stigma, the specialized surface designed to capture and recognize pollen grains. Functionally, the style acts as a physical bridge, positioning the stigma to efficiently receive pollen while providing a direct, protected route to the ovules.
The Style’s Role in Reproduction
The style’s most significant function is to act as the pathway for the pollen tube, which carries the male genetic material to the ovule for fertilization. After a pollen grain lands on the stigma, it germinates and begins growing a tube that must burrow down the entire length of the style to reach the ovary. This growth relies on the stylar tissue for both guidance and sustenance.
The style operates as a biological filter, selectively enabling or blocking the growth of pollen tubes. This regulation is maintained through intricate chemical signaling between the pollen tube and the specialized stylar tissue, often called the transmitting tract. For instance, in plants exhibiting gametophytic self-incompatibility (GSI), the style produces a specialized ribonuclease (S-RNase). If the pollen tube is genetically incompatible, the S-RNase destroys the tube’s RNA molecules, arresting its growth and preventing self-fertilization.
In compatible pollinations, the transmitting tract tissue secretes an extracellular matrix composed of proteins and carbohydrates, such as arabinogalactan proteins (AGPs). These provide a nutrient-rich and adhesive environment for the pollen tube. Signaling molecules stimulate the pollen tube’s growth and guide its trajectory toward the ovules. The style ensures that only genetically suitable pollen tubes are allowed to proceed, promoting outcrossing and maintaining genetic diversity within the plant species.
Diversity in Style Structure
The structure of the style is highly varied across different plant species, reflecting diverse pollination strategies. Internally, styles are generally classified as either open (hollow) or closed (solid). Open styles, common in monocots like lilies, feature a central canal lined with secretory epidermal cells, and the pollen tube grows along the surface of this canal.
Closed styles, found in many eudicots, are packed with a solid core of specialized transmitting tissue cells. In this case, the pollen tube grows through the extracellular matrix that is secreted between these cells.
Morphological variation also exists in the style’s placement on the ovary, which can influence how pollen is delivered by pollinators. While the most common arrangement is the terminal style, emerging from the apex of the ovary, some species have a lateral style that projects from the side, or a gynobasic style that appears to emerge from the base, often seen in the mint family. Variations in style length, known as heterostyly, also exist in some species to promote cross-pollination.