Tuesday, May 26, 2015

Sometimes Flowers are Jerks Part 2: Jack-in-the-Pulpit - Arisaema triphyllum - Petit prêcheur

Another plant currently in bloom is Arisaema triphyllum (jack-in-the-pulpit), a rather intriguing-looking plant with large three-lobed leaves and unusually shaped flowers.

Arisaema triphyllum
This species is native to eastern North America (range maps: North America, Canada). This plant has no protected status listings in the US [1] and is listed as secure in its Canadian range, except in Manitoba where it may be at risk [2].

Arisaema triphyllum

The flower of this plant tends to attract a lot of interest and attention because of its unusual shape. These two structures are called the spathe (the striped, green-purple hood section) and the spadix (the cylinder in the centre). The spadix is actually the inflorescence (structure to which the flowers are attached); the flowers are hidden inside the spathe.

Arisaema triphyllum spathe & spadix
Why would a plant be shaped this way, carefully shielding and reducing access to its flowers? Wouldn't that reduce pollination?

Well, certainly that would be the case if the plant were wind-pollinated. Or if it were pollinated by larger animals eg bats.

But A. triphyllum is pollinated by fungus gnats [3]*. And this plant is a giant jerk about it.

The plant is visited by fungus gnats, which crawl on in to get access to the flowers. But then once they're inside, the size and shape of the hood make it seem totally closed [4]; the flies have a hard time getting out. The male flowers, which have some interest in the pollinator escaping, have a small hole at the bottom of the spathe through which pollinators can escape (after brushing past all the male flowers on the way down) [4]. The females have no such hole, so pollinators are more likely to die in there [4]. But while they're stuck in there they fly around and thrash and generally get themselves coated in pollen, or in the case of the female flowers, coat the pistils with the pollen they've already collected.

So this flower traps its pollinators to increase its odds of successful pollen transfer, and with the female flowers has a high chance of killing the pollinator outright. Harsh.

Other plants do this, of course, with varying degrees of harm. For example, in yesterday's post I talked about Cypripedium acaule, which also traps its pollinators (but it always lets them out!). C. acaule has a one-way hole in the front of the flower through which pollinators enter, and then a sort of pollination-tunnel through which they can exit up around the top of the flower [5]. First, the pollinator is forced to rub past the pistil, which ensures that if they're carrying pollen it's transferred over [5]. Next, they get brushed with a ton of little pollen-covered hairs, ensuring that the pollinator leaves with plenty of pollen for the next flower [5].

Cypripedium parviflorum and Cypripedium arietinum are both also pollinator trappers (sometimes referred to as kidnapping).

Trapping helps to increase the odds of successful pollination when a pollinator does come by, by increasing the odds that pollen will be deposited on the pistil.

A. triphyllum is often considered dioecious [6]; bisexual flowers have been noted but whether these flowers reproduce bisexually is debated [7]. An interesting aspect of this species' gender segregation is that it is not fixed by generation; a given individual can vary its sex from year to year; this sex variation is generally thought to be linked to the size/age (available stored resources in the roots) of the plant [3,4,6,8]. The general idea is that female function (production of fruit and seeds) takes more energy than male function (production of pollen), so a plant with more available resources in its roots, eg one which is larger or one which was able to store more energy the previous year (which would be more possible if it was male the previous year), is more likely to be female for the season.

I did manage to get a photo of a unisexual female, showing the flowers at the bottom of the spathe. I apologize for the poor image quality, I was in a hurry.

Female flowers of Arisaema triphyllum
As you can see, this flower has very plain flowers, just a little fruiting bulb (an ovary) with a pistil on the tip (the white part). Given that their visual characteristics are not related to attracting pollinators, it is no surprise that the flowers are unremarkable to look at; resources are redirected to more valuable functions such as producing fruit.

*there is debate and disagreement about this [4]

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