Saturday, July 28, 2012

Is the population of pickerelweed in a pickle?

Species name: Pontederia cordata

Common name: pickerelweed

Location: Ontario

This species of aquatic plant is native from southeastern Canada (Ontario, Quebec, New Brunswick, Nova Scotia) all the way south to southern Argentina in South America. There have been arguments by plant taxonomists in the past about whether the South American populations should be considered the same species as the North American populations, and so many Latin synonyms have been given to this plant in the past. DNA sequencing has shown that this plant is the same species throughout its entire range, but with small divergent sub-populations. These sub-populations may one day become their own species with enough time to genetically diverge from the parent population, but this has not occurred yet (since all sub-populations can still breed with their parent population). Due to land use change throughout much of the native North/South American range of this species, the chances of these sub-populations having enough time to genetically diverge from the parent population to produce distinct species is pretty slim. In fact, back yard garden ponds are some of the richest sources of this plant, with it escaping back into boggy and swampy areas if left unattended (it is a very vigorous seed producer!). Again unfortunately, it is not a very good competitor with invasive species (which we have many, many invasive species in North American wetlands) so it rarely establishes a new population for long.

Since this species of plant can grow submerged in water like rice can, it, like rice, requires some sort of mechanism in its tissues to be able to transport gas from where it is absorbed in the leaves to where it is needed for growth in the roots. To do this, the plant forms a special type of tissue called "aerenchyma," which looks a bit like empty honeycomb. The large, empty cells are used to store oxygen and carbon dioxide so the plant doesn't suffocate under water. A neat adaptation! The flowers also have a special adaptation to attract bees that you can see in the third picture, that yellow dot on the top petal. Once the flowers are pollinated by bees, the plant undergoes a process called "geotaxis," which refers to the entire inflorescence stalk bending to submerge the developing seeds under water (peanut plants also do this with their developing seeds; this is why peanut shells always have a bit of dirt left on the outside!). The seeds then finish maturing and are released into the muck at the bottom of the pond. This is also a pretty incredible adaptation; without a prolonged exposure to water, the seeds cannot germinate.

With the exception of ornamental value, there is no other major use of this plant by humans.