What's new
Aug 24, 2014
Carnivorous means that something is a "meat eater". It doesn't mean that something kills the animal, or else pure scavengers would not be considered carnivorous. A vulture is a carnivore simply because it eats meat, not because it may on some occasions kill things.

Does that make everything a carnivore?
If an animal decomposes on the skin of a creature and it absorbs nutrients qualifies it as a carnivore, then perhaps everything can be a carnivore, as it's normally the work of skin to absorb at least some nutrients from outside itself.

What you need, for practical purposes, is a more workable definition than that. To eat something means to consume it or break it down or destroy it physically. The plant therefore must perform the action of breaking down the animal's meat. Enzymes are hormones whose sole purpose is to directly perform that task. A plant that makes enzymes to break down the meat is carnivorous.

What if there are no enzymes? It is still carnivorous if it is acting to destroy the meat for the nutrients. That is what one must show to show that it is carnivorous. To show that a man destroys a building, one need not show that he used explosives, directly made to blow up things. One could also show that he designed it for that purpose.

One difficulty is that "design" means intent. You can ask a person why he designed explosives, but you can't ask a plant why it designed its surfaces.

Nonetheless, if there are no enzymes, one could still show that a plant designed itself in order to eat meat.
Darlingtonia is said to not produce enzymes. Yet its trap is so clearly designed for trapping and digesting insects that it is said to be carnivorous. Its resemblance to other carnivorous plants like Sarracenia Rubra that digest meat is so similar that its traps are considered designed for the purpose. So a matching resemblance to known carnivorous plants is one way to show it.

Further, there are plants like Stylidium with surfaces that sometimes catch animals, but it is debated whether they are carnivorous. If nutrient absorption from the captured meat is so low that it is practically the same as plants that don't catch or eat animals, then it isn't carnivorous.

Is it enough to show that a plant captures insects and absorbs efficiently absorbs its nutrients? No, because it leaves the question open of whether it was designed for that. Since plants are more passive then animals, it can be left open whether the two processes are related and a coincidence. It can be that the plant, like Stylidium, uses substances to protect from animals, and then, separately finding meat on itself, absorbs it well. It wouldn't mean that the plant catches things for digestion.

For example, a team of golfers may consistently hit to the right of their practice field, where the team's ball collector cart sits. It may end up being simpler for the team, because their collector spends less time carrying balls. However, this may not be the reason they always hit to the right.

In any case, one way to show that a plant does catch things for digestion, is to show that the plant's only purpose for capturing the insects is consumption. Thorns have the purpose of wards off animals, and are for protection. But what purpose does nectar have on Sarracenia leaves, except to attract them. So if the plant is trying to attract insects with its traps, then it is not for protection but for consumption.

A final way to show carnivory would be to show that the plant's surface has a quality that could only be to encourage digestion. If the plant produced a chemical that encouraged bacterial growth that would only perform decomposition and digestion for the plant, then it would be carnivorous.

In conclusion, it must be shown that the plant was designed or acted in order to consume or break down the meat. This can be shown by enzymes, by attractive methods, by an inarguable match with carnivorous plants, or by a quality that can only serve a digestive purpose.

Let's look at two examples:
Roridula and Teasel.

Roridula has sticky hairs that trap insects so that the Assassin Bug can come and eat it, after which the plant eats its waste. Since the plant scavenges the waste, rather than the insects, it's not carnivorous. Still, it's close enough to carnivory with its traps that carnivorous plants enthusiasts should put it in their category of interests.
By the way, since the Roridula eats the waste, how do we know that it is not designed to digest the other animals on it? Or is it simply unproven that it tries to encourage bacterial digestion directly?

Teasel collects water and animals drown in it at a higher rate than in normal water, and the plants' seeds are heavier after the plant benefits from the nutrients. Would teasel be killing insects for protection, with nutrients being absorbed as a coincidence? Unless there are enzymes, a means of attracting the animals, or it can be shown that the plant encourages digestive bacterial growth more than normal, then it remains in the uncertain quasi-carnivorous plant category.
Aug 24, 2014
I think the short, practical answer is that you have to show that the plant makes enzymes like Venus Fly Traps do or that it attracts the animals to its traps for bacteria to break it down like Darlingtonia. Otherwise, if it kills insects and may absorb nutrients, it's a quasi-carnivorous plant.

Some Bromeliads make enzymes, so people should categorize them with carnivorous plants. If Roridula's enzymes break down meat, it's carnivorous.

Heliamphora and Darlingtonia have no enzymes, but attract insects for no other purpose.

There are many plants that trap insects with sticky sides, but it could be for protection and there are neither enzymes nor attraction to the surfaces, so those are quasi or non-carnivorous.

I haven't read of teasel attracting or using enzymes to digest the trapped insects. Could it be trapping and innoculating them so that they don't eat the stalks, with the beneficial nutrient absorption just a coincidence? If it can't be shown otherwise, then it's quasi-carnivorous.