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Monarch Butterflies, Milkweeds and Heart Disease
The fall season is upon us. It's the harvest season when the summer's green, growing plants have gone to seed. There are ears on corn plants, pumpkins on vines, apples on trees and pods on milkweeds. Plants propagate themselves through seeds. Ears, fruits and pods are seed packages, which are devices used by plants to help distribute seeds.
Plant seeds and seed packages provide food for animals, including humans, and that, in a way, helps in seed distribution. For instance, an animal might carry an apple away before eating it and then drop seeds at a location where, if conditions are right, the seed can germinate and grow. In some instances, animals—such as squirrels intent on saving the seeds for future meals—actually plant seeds.
The fruits and seeds of fall are the end result of the spring and summer seasons when plants grow and develop. But all of this is a complicated process. First, the plant must germinate if starting from a seed or leaf out if a perennial plant. Then, it must grow and produce flowers. The flowers need to be pollinated for seeds and fruit to develop. The fruit has to grow and mature. All of this is a difficult and complicated process.
Take the milkweed plant, for example. Poet Robert Frost in "Pod of the Milkweed" sums up parts of that process nicely. In this poem, Frost describes the milkweed as one that flows "in milk and honey." That is because the milkweed plant, like many plants, has showy flowers that attract insect pollinators such as butterflies. The butterflies come to the flowers because of the sweet nectar available for consumption.
But Frost stated, "it is a bitter milk/As anyone who ever broke its stem/And dared to taste the wound a little knows." Here, Frost is referring to the sap of the milkweed plant, which is a white, sticky substance with a bitter taste.
Many plants, like the milkweed, possess anti-feeding chemicals to discourage animals from consuming the plant. For instance, coffee and tobacco plants produce the well-known chemicals caffeine and nicotine, respectively. The chemical in the milkweed is known as cardiac glycoside.
Cardiac glycoside is found in all of some 90 species of plants known as milkweeds. Milkweeds are classified under the plant genus Asklepias, a name based on the mythical Greek god of medicine. An appropriate name because the bitter chemical in the milkweeds has been used to treat heart conditions by strengthening heartbeat. That is why cardiac is in the common name of the chemical.
Milkweed plants benefit from having cardiac glycosides in their tissues because many animals are repelled by the taste and that protects the plant from becoming a meal for an herbivore. High doses of the chemical can even result in death of some animals. But many insects have managed to overcome the toxic effect of this chemical. Milkweed plants are used as food resources by more than 400 species of insects, including the well-known monarch butterfly.
Insects feeding on the milkweed generally store the toxic chemical in their body and that protects them from predators. The monarch butterfly is a good example of such an insect. Both the caterpillars and the butterflies have the chemical in their tissues and that provides protection when a predator tries to make a meal of them.
Birds that try to consume insects that feed on the milkweed give up after one taste of the bitter substance. In fact, it has been known since the mid 1800s that birds trying to consume an insect that had fed on milkweed would vomit.
So this fall when you witness seeds floating from the burst-open pod of a milkweed, remember those seeds are toting genes with the blueprint to produce a toxic chemical, which will protect a plant against some leaf feeders, allow monarch butterflies to cause a bird to vomit and some doctors to treat human heart problems.