Pollen grains are microscopic structures that carry the male gametes necessary for flowering plant reproduction. Encountering fine, sticky dust in spring or summer is common, especially when brushing against certain flowers. This stickiness is not accidental, but a highly evolved biological feature. The mechanism involves a specialized chemical coating whose purpose is tied directly to efficient plant reproduction.
The Role of Pollenkitt
The physical mechanism behind pollen’s adhesive quality is a substance known as Pollenkitt, or the pollen coat. This viscous, oily material is synthesized by the tapetum, a specialized tissue inside the anther. As the pollen grain matures, the tapetum breaks down, releasing Pollenkitt components that form a coating on the grain’s surface.
Chemically, Pollenkitt is a hydrophobic mixture, meaning it repels water, which helps protect the internal genetic material. Its main constituents are various lipids, or fats, including a complex mix of saturated and unsaturated fatty acids. This oily matrix also contains proteins, carbohydrates, and flavonoids.
Carotenoids are a notable component, often imparting a distinct yellow or orange color to the pollen grain. This coating fills the intricate surface sculpturing and cavities of the pollen grain’s outer wall (the exine). This creates a dense, sticky layer that allows the pollen grains to cohere in clumps and provides the strong adhesive quality observed.
The Function of Pollen Adhesion
The biological reason for the adhesive coating is to facilitate successful and targeted reproduction. This stickiness is an effective mechanism for plants that rely on animals, such as insects or birds, to move their reproductive material. The cohesive nature of the coating ensures that when a pollinator brushes against the anther, pollen grains adhere firmly to the animal’s body.
The viscous layer allows the pollen to stick securely to the fine hairs or exoskeletons of insects, preventing it from falling off. This attachment increases the likelihood that the pollen will survive transport and successfully reach the stigma of another flower. The adhesive quality also helps the pollen grains remain clumped together, which is advantageous for collection and deposit. When the animal visits the next flower, the sticky mass makes contact with the stigma, ensuring transfer and increasing fertilization efficiency.
Differences Between Pollination Types
The sticky characteristic is not universal, as it relates directly to the method of pollen transfer. Plants that utilize insects, birds, or other animals for reproduction are classified as entomophilous, and these plants possess adhesive pollen. In contrast, many plants, like grasses and oaks, rely on wind pollination, a method known as anemophily.
Anemophilous pollen is adapted for aerial movement, making it dry, small, and lightweight. This non-adhesive, smooth structure allows the grains to be carried effortlessly by air currents for long distances. Therefore, the sticky coating only occurs in species where the pollen needs to physically attach to a vector for targeted delivery, rather than being broadcast randomly.