Pollen is a fine, powdery substance produced by seed plants that serves as the vehicle for their male genetic material. These microscopic grains are fundamental to the sexual reproduction of nearly all plant life. Pollen is an ever-present component of the atmosphere, often becoming noticeable when it forms a yellow dusting on surfaces or triggers an immune response in humans.
The Microscopic Structure of Pollen
Pollen grains are durable biological structures designed to protect the plant’s reproductive cells. They are typically spherical, measuring between 25 and 50 micrometers in diameter, and are encased in a protective, two-layered wall called the sporoderm.
The outer layer, the exine, is composed of sporopollenin, a complex biopolymer. This makes the exine highly resistant to chemical degradation, desiccation, UV radiation, and extreme temperatures, allowing ancient pollen to be preserved in the fossil record. The exine’s surface features species-specific patterns of ridges, spines, or pores that aid in identification.
Beneath the exine lies the intine, a thinner, more flexible layer made primarily of cellulose and pectin. The exine contains specific weak spots, known as apertures (pores or elongated furrows called colpi), through which the pollen tube emerges.
Inside the wall is the male gametophyte, which consists of two main cells at maturity: the vegetative cell and the generative cell. The larger vegetative cell forms the pollen tube upon germination. The generative cell, contained within the vegetative cell, later divides to produce the two sperm cells necessary for fertilization.
The Role of Pollen in Plant Reproduction
The purpose of the pollen grain is to transport male genetic information to the female reproductive structure of a compatible plant. This process, called pollination, precedes fertilization and seed formation. After landing on a receptive surface, the grain absorbs moisture, causing the vegetative cell to germinate.
The vegetative cell grows outward through an aperture in the exine, forming a slender pollen tube that penetrates the female tissue. This tube guides the male gametes toward the ovule, which contains the egg cell. The generative cell migrates down the tube and divides to produce the two sperm cells.
In flowering plants (angiosperms), the process culminates in double fertilization. One sperm cell fuses with the egg cell to create the embryo. The second sperm cell fuses with two other cells in the female gametophyte to form the endosperm, which is the nutritive tissue for the developing seed. Gymnosperms, such as conifers, deposit pollen directly onto the female cone scales using a similar pollen tube mechanism.
How Pollen Moves: Mechanisms of Transfer
Plants move pollen using two primary strategies: non-living agents (abiotic transfer) or living agents (biotic transfer). Abiotic transfer is most commonly achieved through wind, a strategy called anemophily, employed by many grasses, trees, and weeds. Wind-pollinated plants produce vast quantities of small, lightweight grains to compensate for random dispersal.
These plants typically produce inconspicuous flowers that lack bright colors, nectar, or strong scents. Conversely, biotic transfer, or animal pollination (entomophily), utilizes agents like bees, butterflies, birds, and bats. To attract animals, these flowers have evolved elaborate colors, patterns, scents, and a nectar reward.
Pollen from animal-pollinated plants is generally larger, heavier, and often sticky or spiky, making it better suited to adhere to an animal’s body. Water pollination (hydrophily) is a minor form of abiotic transfer limited to aquatic plants, where pollen floats or travels submerged to reach the female flower.
Pollen’s Impact on Human Health
The most familiar interaction with pollen is the immune response known as allergic rhinitis, or hay fever. This condition occurs when the immune system mistakenly identifies certain harmless pollen proteins as a threat. Upon initial exposure, a sensitized person produces the antibody Immunoglobulin E (IgE).
When that person encounters the same pollen again, the IgE triggers the release of inflammatory chemicals, such as histamine, from mast cells in the nasal passages and eyes. This reaction causes seasonal allergy symptoms, including sneezing, nasal congestion, and itchy, watery eyes.
The pollen responsible for these reactions almost exclusively comes from wind-pollinated plants, such as grasses, ragweed, and certain trees, because they release massive amounts of airborne grains. Research suggests that other components of the pollen grain, such as lipid mediators, can also contribute to inflammation and exacerbate respiratory symptoms.
Seasonal variations in pollen counts are tied to plant life cycles. Tree pollen peaks in the spring, grass pollen in late spring and summer, and weed pollen dominates the late summer and fall.