The cast iron plant, unusually for its kind, is named for its personality.
As a houseplant and groundcover, it is virtually indestructible. It can get by on the barest of illumination, tolerates over- and under-watering, and has few pests. Its one drawback -- that it is a sluggish grower -- is probably an asset when grown outdoors, since that renders it non-invasive [See also: kudzu].
As popular and commonly cultivated as it is, as a botanical specimen, it is somewhat unusual. In the wild, it exists on only a few small islands in the entire world. And for over 100 years, it was thought to be the world’s only flower pollinated by a slug. Yes, you read that right. A mollusk.
It turns out that might not have been 100% accurate.
Some Japanese scientists decided to test this hypothesis by actually staking out some cast iron plants in their homeland in southern Japan. Because, as it turns out, no one had actually done that before throwing around their land-gastropod-pollination ideas. That hypothesis was formulated in 1889 by watching plants imported to Europe. And for another 100 years, no one else performed that most basic of natural history experiments: watch the organism in nature.
It should be said that idea that a slug could pollinate its flowers was not quite as crazy as it seems. The genus to which the plant belongs, Aspidistra, is known for having a variety of oddball, unassuming flowers with a forest floor zip code. This suite of characteristics makes for some unusual pollinators, many of whom remain unknown.
A typical flower has both male and female parts. The male parts are the stamens, which contain sacs of pollen called anthers, while the female part is the carpel, surmounted by the stigma, the surface upon which pollen must land in order to fertilize the flower.
In the flower of a cast iron plant, the stamens are tucked under a fat stigma that nearly fills the floral cup. This configuration renders wind pollination or self-pollination impossible and means that only a very determined or very tiny pollinator can gain entry. It hints at something sinister -- a trap
Like its kin, cast iron flowers grow partially buried in soil and covered by leaf litter (another reason wind pollination is out of the question). In captivity, they often make these flowers right in the pot they’re growing in, so if you own one, take a peek from time to time to see if your plant might be in cryptobloom.
For about 30 hours, the scientists surrounded and patiently watched wild cast iron flowers on Kuroshima Island, using red light at night to disturb pollinators as little as possible. They also tagged 253 of the flowers to identify which later set fruit.
The flowers were visited not by slugs, but by the considerably smaller and less sexy fungus gnat. A fungus gnat is a tiny and, should your houseplants be infested with any, extremely irritating insect. They like to find ways to land in your tea, get accidentally inhaled, or insinuate to your guests you are a poor housekeeper. When they’re not busy doing that, their other hobbies include mating and flying around like they’ve recently spent time in a stale puddle of Natty Light.
I’m being unreasonably harsh here; fungus gnats are mosquito relatives, so in the plus column, they aren’t blood-sucking parasites. However, fungus gnat larvae (a.k.a. maggots) spend their days peristalsing through the soil eating rotting plant parts, fungi, and roots. Although that’s technically called “nutrient cycling” and it’s important, that last menu item is why it’s even more irritating when they’ve turned your ficus into their own personal Club Med, because what they’re actually doing is chewing on your beloved houseplant. [angrily shakes fist at Terminada, the Evil God of Failed Pest Eradication Efforts as fungus gnat lands on monitor]
To return to the cast iron plant. Over the 30 hours the scientists spent scrutinizing their flowers, five fungus gnats landed, crawled through the gap between the stigma and the petals, and emerged looking bewildered but covered in pollen.
None were female, which means they cannot have been there to lay eggs. When they landed on the flowers, they inevitably landed on that stigma helipad first, ensuring that any pollen grains stuck to them from previous visits would immediately find their target.
There was one other clue to what might be going on in the flowers of Aspidistra eliator. Unlike other species of Aspidistra whose flowers are generally odorless, the scientists noticed the cast iron flower emitted a “musty” odor.
Connecting the dots, they surmised the cast iron plant flower has a dirty little secret: it has evolved to look and smell like a fungus, the chosen food of the aptly-named fungus gnat. In my opinion, it strongly resembles an earthstar.
An earthstar would no doubt provide a delicious meal. A cast iron plant flower, on the other hand, though full of pollen a fungus gnat cannot digest, provides nothing. The fungus gnat, on the other hand, has just provided the flower with advanced reproductive services free of charge.
Interestingly, fungus gnats were not the flowers' only visitors. Little soil invertebrates called springtails and sand fleas also ducked inside occasionally. But they were so tiny that they could carry no more than a few pollen grains at a time, the scientists said, making them unlikely to be important pollinators.
There were two more visitors of note: diaprid wasps. These parasitoids attack the larvae of fungus gnats, infesting them with their own parasitic offspring. What that means is that these flowers are such convincing fakes that they attract not only fungus-eating insects but also the insects that prey on them. Unlike the fungus gnats, however, they at least sometimes find a decent meal inside.
As a reproductive strategy, deception’s not perfect. Out of those 253 flowers, only 12 got lucky and ended up setting fruit. A low fertilization rate is probably the price of a deceptive pollination strategy, since smart insects quickly learn to avoid such traps, and smart insects leave more offspring. But of the five flowers visited by fungus gnats, two set fruit. The goods are odd and the odds aren’t good, but they get much better when a fungus gnat gets involved.
Suetsugu, Kenji, and Masahiro Sueyoshi. "Subterranean flowers of Aspidistra elatior are mainly pollinated by not terrestrial amphipods but fungus gnats." Ecology 99, no. 1 (2018): 244-246.