Sunday, September 24, 2017

Pop! goes the seed pod

Earlier this week, I posted about the profusely blooming Impatiens capensis (jewel-weed), particularly a few visitors I managed to film visiting the flowers.

Today, I want to share something rather different. It's not as much in my area, but it is a rather awesome feature of this and some other plants: explosive seed dispersal. Yes, that's a thing.

The biological purpose of a flower, of course, is reproduction. Remember from the last post that I. capensis individual plants make male flowers, which donate pollen to fertilise ovules, and they also make female flowers, which produce ovules that, when fertilised with pollen, will develop into seeds.

Impatiens capensis male flower (middle) and female flower (right).

Impatiens capensis is an annual, so any given individual won't grow back in the spring; consequently, it won't compete directly with its offspring for suitable growing space. The seeds are all at least half-siblings, however, so it's in the interest of the genes not to have (half-)siblings too close together.

Basically, there's good reason to suppose that a strategy that disperses seeds within a few meters of the plant (i.e., close to where suitable conditions for growth and reproduction were found, since the parent grew and successfully reproduced there, and because the plant is an annual there's no major incentive to disperse offspring further from the parent plant to avoid parent-offspring competition), but not all too close to one another (since they're at least half-siblings and share genes, so the genes would spread better through a population if they don't spend too much energy directly competing with each other), would be advantageous for this species.

There are all sorts of seed dispersal mechanisms. Explosive seed dispersal (a.k.a. projectile dispersal) is one of my favourite, however, because it's very dramatic. Here's what a seed pod looks like in I. capensis:

Impatiens capensis seed pod

Inside this elongated structure, there are a bunch of seeds (somewhere around 8-10, if I remember correctly). Each plant can produce quite a lot of these. So the explosive seed dispersal may be how I. capensis got one of its common names, "touch-me-not", but I think that's a misnomer because it's so fun to pop them that I highly recommend that anybody who gets a chance should definitely touch them.

They pop quite audibly:

So how does this even work? I tried to slow my videos down so that it's a bit easier to see what's happening, but it's so rapid that I have limited success. Watch for the seeds and pod flesh flying off:

In order for the seed pod to explode like this when touched, it has to be storing energy. Fortunately for me, I don't have to speculate too much in explaining what's happening here, because New Phytologist just published an article by Hugo Hofhuis and Angela Hay on the topic of explosive seed dispersal in Cardamine hirsuta, a species with extremely similarly shaped seed pods to I. capensis (though the two species are not closely related). I'm going to presume that the broad strokes, at least, are pretty similar in I. capensis. The article is really neat and I highly recommend that you read it, but here's my very brief summary of the findings from the article that may apply to I. capensis explosive seed dispersal.

The TL;DR is that these seed pods (likely) have specially shaped cells and extra lignin in places; essentially they're shaped so that they're always just on the very edge of curling up together, and touching them disturbs the delicate balance that is holding the seed pod's shape. Notice what happens to the fleshy parts of the pods afterwards:

Impatiens capensis seed pod after explosion is triggered
Neat, eh?

No comments:

Post a Comment