A mature flower with its many florets next to a bract covered with milky sap. Photo by Robert Dryja
By ROBERT DRYJA
Plants can have a very mixed relationship with the insects that help pollinate them. On the one hand insects carry pollen from flower to flower while sipping nectar from flowers. This helps assure that more seeds then will develop. On the other hand, insects eat developing flowers and birds eat the resulting seeds.
How can a plant obtain the benefits but avoid the costs?
Different species of plants have different strategies to gain the pollinating benefits of insects. Gumweed has one of the more remarkable strategies. A flower of a gumweed initially grows inside a protective ball of green “bracts”. There is nothing colorful about this green, spiky ball to attract an insect. In addition a white sticky sap is extruded onto the bracts. Any insect that happens to land on the bracts is caught in the sap.
The developing flower that opens from inside the bracts has a circular, plate-like shape. This emerging flower is covered with the white sap. The flower therefore is protected while it is developing. The mature flower stops producing the sap and a bright yellow flower emerges from the white sap. This transition occurs in about a day. Insects now are attracted and pollination begins.
The bracts close around the flower after just a few days and again produce the white sap. The developing seeds again are protected from being eaten by insects or birds. Individual, dry brown seeds eventually emerge from the top of the bract. Gumweed therefore makes its flowers easily available to insects only when the flowers are ready to be pollinated.
Gumweed is one species of asters. Many aster species have their flowers open for days and do not produce milky sap. They are following a different strategy for assuring the survival of at least some of their seeds. Individual plants or groups of plants all bloom at about the same time. The sheer number of resulting flowers and seeds means that some will survive from being eaten by birds.
Flowers can serve other functions. Many insects have color patterns that match the color of flowers.
One of the pictures shows a gumweed flower that has gone to seed. The seeds are brown and stick-like in shape. There is an insect standing among the seeds whose shape and color match the seeds. Another picture shows an insect with an orange and black color pattern that camouflages it at the center of a wild sunflower. The insects may be using the flowers as a place to avoid detection. The camouflage helps protect them from being eaten themselves. Alternatively they can wait in ambush for other unsuspecting insects to be eaten by them.
The bracts have opened, but the milky sap has not yet disappeared entirely. What looks like a single flower actually is composed of many small flowers. Photo by Robert Dryja
The florets are now ready to be pollinated. Photo by Robert Dryja
A wild sunflower plant has its many flowers blooming at the same time. Courtesy photo
Gumweed seeds have matured. Look closely to see an insect camouflaged among them. Photo by Robert Dryja
The orange and black color pattern of an insect’s wings matches those of the flower it is standing upon. Photo by Robert Dryja