On December 2, 1874, the remarkable Mary Treat wrote to Charles Darwin:
I have been studying the bladder-bearing species of Utricularia off and on the last year, and am now fully satisfied that they are the most wonderful carnivorous plants that I have yet seen. The so-called little bladders seem to be receptacles for digesting animal food. Not only small animalcules are lured into these receptacles, but animals large enough to be distinctly seen with the naked eye; and by holding the little bladders up to the light the movements of the animals can be seen with the unassisted eye.
We’ll get to Darwin’s response in a bit, but for now let’s ask: just what are these wonderful bladder-bearing botanical beasts to which Treat was referring?
Utricularia is a genus comprising several species of carnivorous plants called bladderworts. Most (but not all) of these are aquatic and capture their prey by the use of so-called bladder traps or “utricles.”
Utricularia traps, courtesy of Michal Rubeš via wikimedia commons.
According to a review article by Elzbieta Krol and colleagues, Utricularia‘s “[s]uction-traps (bladders) are ranked among the most complex leaf structures ever to have been examined in plants.” The traps work like this: A plant’s bladder forces water out, creating a sort of vacuum within the bladder and stored energy potential in the bladder walls. At one end of the bladder is a tightly sealed valve rimmed by several hairs. These hairs act as triggers for the bladder trap. According to Krol, “each trigger hair acts as a lever, breaking the seal and releasing the energy whenever something (living creature or strong current) disturbs it.” In other words, a light tap on one of these hairs causes the bladder to suck whatever happens to be outside the valve into the bladder. Inside, the captured prey becomes plant food.
These bladders are quite impressive when you consider what they can trap, as listed by Krol (referencing Darwin): “aquatic species [of Utricularia] can hold … substantial prey, such as crustacean zooplankton (e.g. water fleas), nematodes, mosquito larvae, insects, tadpoles and even small fish.” Apparently, Krol notes, they also end up eating a lot of algae.
An aquatic plant with complex bladder traps that allow it to eat fish? Mary Treat called it wonderful, Elzbieta Krol calls it “truly outstanding,” and I say it’s tremendously impressive.
And speaking again of Mary Treat, what of her correspondence with Charles Darwin?
In the months following Treat’s letter to Darwin, he published his book Insectivorous Plants and she published an article in Harpers Magazine titled “Is the valve of Utricularia sensitive?” Darwin had apparently suggested in his book that Utricularia were passive feeders, while Treat very respectfully disagreed:
Mr. Darwin says the valve does not appear to be in the least irritable, and continues (Insectivorous Plants, page 408): “We may therefore conclude that the animals enter merely by forcing their way through the slit-like orifice, their heads serving as a wedge.” But we have seen in the instances of the mosquito and chironomus larvae [each of which was trapped tail-first] that this is not the case; the head does not serve as a wedge. But what is the force that impels them into the utricle? It seems too bad to try to overthrow a plausible theory and offer nothing better in its stead. But what can I do? The play is enacted before me, and I have tried in vain to get behind the scenes to learn what the power is that impels the larva into the utricle. No doubt if Mr. Darwin had had the excellent material that I had to work with, with his keener insight he would have ferreted out the cause.
If within the utricle was a partial vacuum, the sudden opening of the valve would create sufficient force to carry whatever happened to be in close proximity into the utricle; and this illustrates the movement we see executed. But how could a vacuum be formed?
Treat attempted to send Darwin some Utricularia specimens and her article; Darwin at first received the former but not the latter. He responded colorfully:
The specimens which you have sent of Utricularia are most beautiful & excellently preserved. I shall feel great interest in reading your account of them when published.
I am sorry to say that I never received the article in Harpers on the sensitivity of the valve in Utricularia,— a subject which drives me half mad.— If you have been able to prove either side of the case, I beg you tell me exact title & date of the number, which I can then easily pursue.
About a month later, after another letter from Treat, Darwin read the article and wrote to her:
My dear Madam
I have received your kind letter & the article which I have read with the greatest interest. It certainly appears from your excellent observations that the valve was sensitive … but I cannot understand why I could never with all my pains excite any movement. It is pretty clear I am quite wrong about the head acting like a wedge.— The indraught of the living larvæ is astonishing. …
I am not well & am staying away from my home for rest, so pray excuse brevity. Wishing you success of every kind in your admirable work
I remain | Dear Madam | Yours very faithfully | Ch. Darwin
So there you have it.
P.S. In the course of researching this post, I learned that there are native Utricularia — as well as Drosera (sundews) and Sarracenia (pitcher plants) — residing in Rhode Island’s wetlands. I am excited to have new things to look for when tromping around outside this summer!
Krol, E.; Plancho, B.J.; Adamec, L.; Stolarz, M.; Dziubinska, H.; Trebacz, K. 2011. Quite a few reasons for calling carnivores ‘the most wonderful plants in the world. Annals of Botany 109 (1): 47–64.doi:10.1093/aob/mcr249