Tuesday, June 10, 2008

Plant of the week - Milkweeds (Asclepias)

I thought I'd start a feature where I could post some cool stuff about plants. Let's start with the milkweeds, in the genus Asclepias, and the namesake of the family Asclepiadaceae. There are 75 species of milkweed in the US, and the genus is distributed throughout. The common name comes from the milky sap, which contains several chemicals that are toxic to many herbivorous insects and can cause contact dermatitis if it gets on your skin.

Members of this genus are famous in that they are the sole food of the Monarch caterpillar (Danaus plexippus), which probably merits a post by itself, given its hefty migration. The caterpillars feed exclusively on the leaves of milkweed, and this confers toxicity to the individual through subsequent stages of its life cycle. I can remember learning about this in school and seeing a video clip of naive blue jays being offered monarchs, eating, vomiting, and then being offered a Viceroy butterfly, which is a mimic. Those savvy jays declined anything that looked like a monarch, thus showing us how mimicry works in nature.

The flowers are attractive to many pollinators, but this genus has an unusual pollination mechanism that favor certain sized pollinators. The flowers are unusual in appearance because they have an extra whorl of parts in addition to the usual four (sepals, petals, stamen, pistil) called the corona. Here's a page that I got these flower pictures from. Tucked inside each flower is a small black structure that is attached to something called a pollinarium (plural pollinaria). The pollinarium is the jackpot of pollen. It is a large amount of pollen in one neat package.

This is so cool. The pollinating insect, while moving around the flower looking for nectar, hooks a foot around the pollinarium and, if conditions are right, pulls the entire structure out of the flower. Upon removal, the pollinarium starts to dry, and the arms holding the packets of pollen, called translator arms, reorient themselves such that, you guessed it, when the insect lands on another (hopefully) milkweed flower, the pollinarium is inserted into the proper place on the new flower.

The fluff on attached to the seeds is an effective dispersal mechanism that helps spread offspring away from their parent plants, presumably to reduce competition. It is as good or better than down as an insulator.

A nice gallery of eastern US species can be found here.
Blue jay pic from here. Pollinarium from here. Pink flower pic from here. Orange ones from here.

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Hi, sorry to make the humans do an extra step.