Chapter 8 - 6

06 Botany

How a Plant Attracts Pollinators

All species of plants have basic functions. They produce oxygen and take in carbon dioxide. They use chlorophyll and sunlight to produce food.

A) They also multiply or reproduce through pollination. While all plants need to be pollinated, this can occur in several ways. For example, pollination most commonly occurs by the wind blowing, by people or insects carrying the pollen from plant to plant, or by self-pollination.

B) Scientists have researched a primitive plant called a cycad and have subsequently developed a theory regarding it.

C) This plant is pollinated by insects called thrips, but the method by which it attracts bugs makes it markedly unique.

D) Cycads are known as “living fossils” partly because of their primitive structure, but additionally, fossils of the plant indicate that it has been around since the era known as the Permian Period, which occurred 290 to 250 million years ago.

A) Along with conifers such as pine and fir, they are considered gymnosperms, meaning the plant’s seeds are exposed on the surface rather than being enclosed.

B) Cycads have a trunk that exists mainly underground and leafy fronds sticking up above ground situated around a single cone. The cone is a greenish color when it is young, and as it matures, it changes to a shade of yellow.

C) The scales of the cone are typically close together except during pollination, when they spread to leave a tiny space in between the scales.

D) Previously, scientists believed that cycads were pollinated by the wind. This can occur when the wind blows pollen from male cones to nearby female cones, but scientists began to question this theory because the space in which the wind needed to penetrate the cycad cone was too small, preventing this means of pollination.

A) Researchers soon began to study cycads and their miniscule pollinators, the insects known as thrips.

B) Closer inspection led them to believe that thrips are the plants only pollinators and that their relationship is twofold: thrips eat the cycad’s pollen and cycads reproduce through the pollination carried out by thrip activity.

C) Individual cycads have a pollination period, or coning season, that occurs once a year and lasts four weeks or less.

D) During this time, male cones have the unique ability to raise their temperature up to twenty-five degrees Fahrenheit warmer than the surrounding air by using the store of sugars, starch, and fats generally used by the plant for normal function.

This heating of the cone occurs between eleven a.m. and three p.m. and takes place each day of the pollination period.

When the temperature rises, a substantial odor emits from the cone.

The odor is described as harsh and overpowering, unlike any other common odor. One of the odors is a chemical known as beta-myrcene, which increases to toxic levels, proving lethal to the thrips and driving them from the cones.

As they leave the male cones, they take pollen along with them on their bodies.

As the plants cool down, the cones emit much lower levels of beta-myrcene, which then attracts the thrips, and the pollen they carried away, back to the plants, specifically the female cones.

Thus, the thrips effectively remove pollen from the male cones and deposit it in the female cones, and the pollination process is complete.

Scientists believe that plants have evolved by producing chemical defenses in order to drive away animals that would eat them.

Another belief is that flowering plants use fragrance as well as vivid color to attract the insects necessary to pollinate them.

The method of pollination characterized by the relationship between cycads and thrips is referred to as “push-pull pollination”.

Researchers view the push-pull pollination process as an intermediate evolutionary stage for plants, using odor to attract pollinators, repel herbivores, and be pollinated by wind-blown pollen.

Compelling evidence supports this theory of evolution as a means of protection and reproduction to further a species.

Over centuries, plants have refined the functions they are capable of to become a sustainable species.

As a result, plants such as cycads have existed over a timeline of more than 250 million years and will be in existence for millions of years to come.