Pollen grains are minute structures produced by seed plants, serving as the male reproductive cells. These microscopic particles, often appearing as a fine dust, are released into the environment. Varying in size from approximately 3 to 200 micrometers, they represent a microgametophyte, carrying the male gametes necessary for plant reproduction.
Pollen Grain Structure
Each pollen grain is encased by two layers. The outer layer, the exine, is tough and resistant, composed of sporopollenin. This material allows the exine to withstand harsh environmental conditions. The exine often displays intricate, species-specific patterns, allowing identification of the plant species.
Beneath the exine lies the intine, a softer inner layer. Within these protective layers, a mature pollen grain contains two specialized cells. A larger vegetative cell, also known as the tube cell, forms the pollen tube during germination. The smaller generative cell is contained within the tube cell and later divides to produce the male gametes, the plant’s equivalent of sperm cells.
Pollen’s Role in Plant Reproduction
Pollen’s primary role is to facilitate plant reproduction through a process called pollination. This process begins with the transfer of pollen grains from the anther, the male reproductive part of a flower, to the receptive surface of the stigma, which is part of the female reproductive organ. Pollen can be transported by various agents, including wind, insects, birds, and sometimes water. Wind-pollinated plants, for example, often produce abundant, lightweight pollen designed for atmospheric dispersal.
Once a compatible pollen grain lands on the stigma, it begins to germinate. The tube cell within the pollen grain initiates the growth of a pollen tube, which emerges through a specific pore in the exine. This slender tube then grows downward through the style, a stalk-like structure connecting the stigma to the ovary. The pollen tube serves as a conduit, guiding the male gametes from the generative cell to the ovule, located within the ovary. The fusion of the male gametes with the egg cell inside the ovule completes fertilization, leading to the development of seeds.
Pollen and Human Health
Pollen is a common trigger for allergic reactions in humans, often manifesting as hay fever, medically termed seasonal allergic rhinitis. This occurs when a sensitive individual’s immune system mistakenly identifies the harmless proteins in pollen as a threat. The immune system then mounts a defense, producing specific antibodies, such as IgE, which attach to mast cells.
Upon subsequent exposure, these sensitized mast cells release inflammatory chemicals, most notably histamine. Histamine is responsible for familiar allergy symptoms, including sneezing, a runny or congested nose, itchy and watery eyes, and an itchy throat or ears. Not all pollen types cause allergies; the fine, dry pollen from wind-pollinated plants like trees, grasses, and weeds is the primary culprit because it is easily inhaled. In contrast, the larger, sticky pollen carried by insects is less likely to become airborne and cause widespread allergic responses.