Pollination is the process where pollen, containing male genetic material, is transferred to the female reproductive part of a plant to enable fertilization. Corn possesses both male and female reproductive structures on the same plant, giving it the capacity to self-pollinate. However, corn is fundamentally a cross-pollinating species, relying on the transfer of pollen between different individual plants. This preference is due to its unique floral anatomy and a precise timing mechanism in its reproductive cycle.
The Separate Reproductive Organs
Corn is classified as a monoecious plant, meaning it bears separate male and female flowers on a single organism. The male flower, known as the tassel, develops at the very top of the stalk. This tassel is responsible for producing the massive quantity of pollen grains necessary for fertilization.
The female flowers are located lower down the stalk on the developing ear. Each potential kernel on the ear is connected to a single strand of silk, which collectively emerge from the husk. These silks function as the female stigma, serving as the receptive surface for airborne pollen.
Each individual silk must catch a pollen grain and be successfully fertilized for a kernel to develop. The physical distance between the elevated tassel and the lower ear silks promotes outcrossing. This spatial separation of the male and female organs on the same plant is a foundational factor influencing the plant’s reproductive outcome.
The Mechanism Favoring Cross-Pollination
The primary method corn uses to move pollen is wind dispersal, a process known as anemophily. The tassel can produce millions of pollen grains, which are easily carried by the slightest breeze to neighboring plants. This reliance on wind, rather than insects, makes the transfer of pollen to a different plant highly probable, thus favoring cross-pollination.
A precise biological mechanism, called dichogamy, further ensures that pollen from a different plant is used. Dichogamy is the temporal separation of male and female maturity in a plant. In corn, the male tassel typically begins shedding pollen several days before the silks on the same plant emerge and become fully receptive, a condition specifically termed protandry.
The tassel’s pollen shed may begin two to three days before the silks on the same ear appear, meaning the plant’s own pollen is often gone before its female parts are ready. Consequently, the newly emerged silks must rely on pollen blown in from an adjacent plant whose tassel is actively shedding pollen. This biological timing strongly favors fertilization from an unrelated source, resulting in a very low rate of self-pollination, often less than five percent in a field setting.
Genetic Consequences of Self-Pollination
The preference for cross-pollination is a survival strategy, directly related to the genetic concept of outbreeding. Corn has evolved mechanisms like protandry to maintain high genetic diversity, which is advantageous for long-term species health. When self-pollination occurs repeatedly, it leads to a condition called inbreeding depression.
Inbreeding depression results from the accumulation of harmful recessive genes that become expressed when the plant’s genetic makeup becomes too uniform. This reduced genetic diversity often manifests as decreased plant vigor, lower overall yields, and a greater susceptibility to environmental stressors or diseases.
For this reason, most commercially grown corn is a hybrid, created by crossing two distinct, highly inbred parent lines. Their cross-pollinated offspring exhibit a phenomenon known as hybrid vigor, resulting in stronger, more predictable, and higher-yielding plants. The plant’s natural tendency to cross-pollinate avoids the negative consequences of inbreeding.