Jewelweed (wikipedia link) is a common plant in the northeastern US, where its late summer flowers are very popular with bees and hummingbirds. Its most notable feature, however, is its seed pods. When the tiny pods are ripe the slightest touch causes them to snap open, flinging the seeds in all directions (scroll down to see a video). Hence, the alternate name: touch-me-not.
The common explanation for this explosive seed-flinging is that it is a way for the plant to disperse its seeds more widely. I’ve heard and repeated that many times myself, but it has always felt a little unsatisfactory. The seeds don’t go very far, and it seems like an overly simplistic explanation for a very complex adaptation.
This summer we have a large stand of jewelweed right outside our back door, and Rose-breasted Grosbeaks have been spending a lot of time in there. A few weeks ago I was able to watch a grosbeak as it foraged in the jewelweed, and I was a little surprised to see it feeding on the seed pods. Watching the grosbeak attempting to eat jewelweed seeds led to a Eureka! moment, an idea that immediately made sense and seemed obvious and logical…
Maybe jewelweed seed pods explode to prevent predators like grosbeaks from eating the seeds!
The grosbeak moved deliberately among the stems, looking for ripe seed pods. When it spotted one it leaned over and, with a quick lunge, took hold of the pod. Instantly the pod exploded, and if the grosbeak had been quick enough, and managed to get a good grip on the middle of the pod, it might still have a seed or two in its bill. Other times the pod exploded as soon as the grosbeak made contact, seeds flew, and the grosbeak was left with nothing.
I don’t know if this is a new idea. I couldn’t find any reference to it, and would be very interested to know if it has been proposed and studied before.