Labelled nitrogen answered the question Darwin left open

The question left unanswered in 1875

In 1875 Charles Darwin suggested that some Saxifraga species might be carnivorous, ran a few experiments, and left the question without demonstrating digestion. The question itself was not hard; testing it was. To count as carnivorous a plant has to do four things at once: attract prey, capture it, digest it and absorb nutrients from it. The first two are visible to the eye. The last two ask for instruments nobody had in the nineteenth century.^[1]

Saxifraga candelabrum, which grows in the stone crevices of Wufeng Mountain near Shangri-La in Yunnan, meets the first two conditions easily. Mature plants carried an average of 71 trapped insects, mostly small gnats of the family Chironomidae. The trap is selective: the larger insects that pollinate the flowers do not stick to the secretion. The plant lets go of the visitor it needs and holds on to the one it does not.^[1]

The trail the nitrogen left

The team tested the last two conditions along three separate paths. Phosphatase staining showed digestive activity in the glandular hairs, and the same stain gave nothing in the close relative Saxifraga filicaulis. Then the plants were fed fruit flies labelled with ¹⁵N. In Saxifraga candelabrum δ¹⁵N rose significantly (p = 0.0001), as it did in the known carnivore Drosera peltata (p = 0.009). In two non-carnivorous comparison species it did not move at all: p = 0.978 in Salvia przewalskii and p = 0.853 in Saxifraga diversifolia. The insect's nitrogen came out in the plant's tissue.^[1]

The third path runs through the genome. Across six independent origins of carnivory the study found convergence in genes for leaf morphogenesis, digestion and nutrition; lineages far apart from one another reached for similar tools to do the same job. Convergence on its own does not count as proof of carnivory; taken together with the isotope result it shows the finding is more than a resemblance at the surface.^[1]

Where does the boundary move?

What this study really opens is a method rather than a species. The genus Saxifraga contains around 450 species, and many are known to carry the same sticky hairs. The same three tests can be applied to others. Whatever comes out, something more interesting than a change in the count is at stake: carnivory is read from the test applied to a plant rather than from how the plant looks. Nature stayed as it was; where we stand to measure it moved.^[1]

That the open questions of a discipline can wait 151 years is a sign of its durability rather than its slackness. Darwin's guess was written down, was not forgotten, and remained testable; when the instruments to test it arrived, the question was still there. Some of the questions left open today ask for the same patience. The answer depends on the question having been written properly.^[1]