Does Pollen Germinate Indiscriminately?

Proteins

• After pollen lands on the stigma of a flower, a chemical reaction occurs. The proteins in the pollen combine with the proteins in the stigma, creating a firm connection. This connection allows water to pass between the stigma and pollen grain so that the pollen swells to break the protective pollen coat. This process opens the pollen to prepare it for germination. When the proteins from the pollen are incompatible or unrecognizable, the stigma rejects the pollen. Without this protein connection, the pollen cannot absorb the water necessary for germination.

DNA

• The sexual reproduction of plants increases their genetic diversity because it combines the genetic information of two plants to produce offspring. The pollen grains contain half of the DNA required to create a new plant, and the ovule contains the other half. To create a new plant, these two halves must have compatible DNA. In most instances, separate species are too different in their DNA sequences to successfully interbreed.

Chemicals

• When the pollen swells with water, a chemical reaction occurs, and a pollen tube begins to grow. As the pressure from the water increases, this tube erupts from the pollen coat and drills into the stigma. When the chemical signatures are the same between the stigma and the pollen tube, the stigma allows the tube to continue through the style to reach the ovule in the center of the flower. If the chemicals are incompatible, the pollen tube fails to reach the ovule, and fertilization cannot take place.

Exceptions

• When two plant species are similar enough in proteins, chemicals and DNA, fertilization can occur between species. When two species create an offspring, this new plant is called a hybrid. The hybrid plant is often sterile so it cannot produce viable seeds. Plant breeders and scientists attempt to create hybrid plants with positive or beneficial characteristics, but these plants cannot reproduce. A true hybrid propagates through cuttings or root shoots instead of seeds.