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I imagined that in the far future, humanity has become space-faring and is running experiments with seeded worlds. One of these planets would be an earth-sized moon with almost identical atmosphere to Earth (I know it's not likely). Unlike the other seeded world however, the only multicelular lifeforms on this world are plants and fungi. As millions of years pass, would any 'predatory' plants evolve?

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Not predatory, parasitic.

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The picture above is from a plant called Cuscuta racemosa. The thing that makes this plant interesting is that, despite being a plant, this thing is an obligatory parasite, aka it can't survive on its own, and must feed on the sap of other plants in order to survive.

Truth is that when it comes to plants, predator prey relationships aren't like we'd see on animals. An attacking plant can take its sweet time approaching another one, because plants aren't nearly as mobile as an animal, nor can they unroot and re-root themselves whenever, so unless we're talking about an extreme exception, the plant being attacked isn't going anywhere. Because of this, the closest you can get from a "predator" plant is a parasitic plant. Cuscuta racemosa is a more extreme example because, unlike some other plants who also engage in parasitism in order to save their own resources, this particular plant has completely lost the ability to survive on its own, and is probably the closest you'll get from a plant that hunts other plants in terms of "diet".

As for the closest you can get in terms of behavior, look no further than the dodder vine, also known as strangle weed. This leafless parasitic plant has an almost animal-like behavior, in the sense that it finds its preferred "prey" through smell. Not only that, it has preferences in the sense that they will choose some plants over others through the smell each emits. It's theorized this plant "hunts" by literally growing towards the source of smell, which they identify through special chemo receptors, in a likely example of chemotropism. Once it reaches the host plant, it starts to wrap around it as it grows and begins its pararitism.

Regarding how we found out about that, we tested if it found the plant through smell, and found out it did.

When the researchers isolated tomato plant odor chemicals and smeared them on a piece of rubber, dodder tried to attack that. Seventy-three percent of the seedlings headed toward the piece of rubber with tomato chemicals compared to a plain piece of rubber.

So summing up, if by predatory plants you mean a plant that actively moves around like an animal made of leaves a and vines and goes eating its relatives left and right, then likely no, you won't get predatory plants without some serious handwaving. If however you're fine with a less active, but more realistic equivalent of a predator-prey relationship between 2 plants, then absolutely you can. It will however be ultimately considered as a type of parasitism, simply because plants aren't nearly as dynamic as even the slowest of animals in terms of needing move around from place to place.

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Vegetarian sundew.

https://www.cpukforum.com/forum/index.php?/topic/61472-vegetarian-sundew/

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Vegetarian sundews: Few people know, that carnivorous plants like also vegetarian diet, as long as it is rich in protein like flower pollen. To show this, we fed fresh stamens of a Sarracenia leucophylla with adhering pollen to four different Drosera species (photos). D. capensis and D. ultramafica rolled in the leaf as intensive as with animal prey.

I was thinking about the predatory plant the sundew. Sundews live in nitrogen poor soil and supplement their nitrogen by catching insects. But what if they caught nitrogen rich plant matter? I was thinking seeds but these folks tested it with pollen and the sundews liked it.

On your world there could be plants which consume mobile parts from plants and fungi like seeds, pollen and spores. These parts are nutrient rich because they are supposed to give rise to a new organism and so perfect for a predator to eat.

I am thinking now about a riff on sundews - if a piece sticks to a seed but the seed "gets away" - maybe it is blown or washed away - the piece of sundew goes with it. It can regenerate the sundew parent plant but now in the context of this great big tasty thing that blundered into it, which will jump start the plant.

Maybe human explorers will brush into one of these seed-eating sundews and have them stick. They think they have pulled them off. But some is left behind.

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Plants and fungi are more closely intertwined than most people realize. Most plants have strands of fungi, called mycorrhizae, growing on their roots in a mutualistic relationship. The fungi increase the plant's access to water & minerals, and in turn take nutrition from the plants. (The associations can be much more complex than this, too: the mycorrhizae can pass nutrients between species, increase resistance to insects & disease, and more: https://en.wikipedia.org/wiki/Mycorrhiza )

An interesting example of a plant that takes advantage of this relationship is the Snow Plant (Sarcodes sanguinea). It has no chlorophyll itself, so is a parasite/predator on the mycorrhizae of pine trees, and indirectly on the pine trees themselves. https://www.fs.fed.us/wildflowers/plant-of-the-week/sarcodes_sanguinea.shtml

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the only multicellular lifeforms on this world are plants and fungi

neither of them is mobile, and carnivorous plants on Earth evolved to integrate their element supply by feeding on mobile targets.

The only "predatory" style that plants might evolve in your case is to prey on other plants, in the sense that their seeds, carried by wind or water, would use other plants to grow. Sort of what the mistletoe or the straggling ficus do on Earth today.

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