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I've seen somewhat similar questions to this one (mostly about outright mobile plants or "planimals") but the idea I'm trying to conceive for my worldbuilding is not exactly that so I wanted some guidance to how viable it is, since I have a lot more knowledge about animals than plants.

My idea was for the "plants" of this planet to have a sessile sporophyte (like most Earth plants) and an independent motile gametophyte, vaguely inspired in the fern lifecycle and its motile sperm. I know lots of plants are somewhat motile, often utilizing animals to do the hard work for them. However, in this case I wanted the gametophyte to take it a bit further and be able to move for a limited period of time, using this mobility to reach other plants by itself before releasing the gametes and dying. A bit similar to how some insects have a very short adult life to just reproduce and die. Ideally this would happen out of water for these plants.

How would it be possible for plants in a different planet to develop something like this? Maybe with something alike to a very primitive and simple hydrostatic skeleton that allowed them to have maybe a few days or hours worth of motile lifetime? Or by accumulating energy and using it to move until they "starve out"?

The planet in question is mostly Earth-like, with a higher average temperature of 16ºC, an axial tilt of 36.2º, 80% of Earth's gravity and a denser atmosphere, though it has the same elements as Earth's, just with slightly different amounts (such as more oxygen).

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    $\begingroup$ Mosses are actual plants and they have flagellate spermatozoa, which do actively swim in search of a receptive archegonium. So that the aswer is trivially yes, some plants do indeed have a temporary motile phase. $\endgroup$
    – AlexP
    Commented Jan 10, 2023 at 20:17
  • $\begingroup$ @AngryMuppet I added some more information about the planet. Everything else I have not mentioned is meant to be similar to Earth's conditions, since it's an habitable planet. Distance traveled by the gametophytes themselves I don't imagine more than a couple of km at most, if possible. Maybe more with external help such as wind. $\endgroup$
    – Umbrace
    Commented Jan 10, 2023 at 20:29
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    $\begingroup$ Looks good. That sounds pretty good for buoyancy purposes. Whilst we're on that topic, check out: On the viability of living balloons for a related post that might be of interest to you. $\endgroup$
    – Escaped dental patient.
    Commented Jan 10, 2023 at 20:35
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    $\begingroup$ Is the biochemistry of locomotion necessary for the stories you're going to tell about your world? Could it be sufficient to describe the behavior of the plantlike things? Remember that anything you attempt to explain to your readers will be something that they can find fault in. $\endgroup$
    – sphennings
    Commented Jan 10, 2023 at 20:50
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    $\begingroup$ @sphennings This is not exactly for any stories. I'm currently working on this as a personal worldbuilding exercise by creating a biosphere of my own (I'm an artist) and making cool but feasible designs out of it. $\endgroup$
    – Umbrace
    Commented Jan 10, 2023 at 20:59

3 Answers 3

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There is no reason why it shoudn't. On Earth it didn't evolve that way because it is more efficient to use animals as a vehicle to spread plants' genetic material. But in an environment without animals, plants could easily evolve an alternative way. Especially if using wind is not an option.

Lets look at the numbers. An average apple contain around 100 calories of energy. Depending on age, a single tree can produce between 200 and 800 apples. If we take that as 500, that is 50000 calories of energy. Human on average consume 2000 calories/day - same as beehive with 50000 bees. So a tree definitely produce enough excess energy to "fund" a mobile seeds. And that is in the case when a tree keeps on living. If a plant were to put all its energy into making mobile offsprings, the energy budget would be even higher. And that is all that really matter. Energy. With selective pressure a complex mobile form would be only a matter of time. And it could be drastically more complex than simple hydrostatic skeleton mentioned in question.

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Sure. The comments already mentioned mosses, which have motile gametes.

Consider also upside-down jellyfish and corals. Each type of organism has a motile phase, and they both have a sessile phase where the sit on the seafloor or in reef structures and rely on photosynthesis for energy. On earth, photosynthetic jellyfish and corals rely on symbiotic algae to perform photosynthesis, but...

  1. There's no reason your plant-analog has to be a single organism with no symbiotes, and
  2. It's a pretty short hop--just one endosymbiosis event--from there to a unitary creature which has native chloroplasts of its own, if that's what you want.
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    $\begingroup$ Just wanted to add a honorary mention of Elysia viridis, the photosynthetic nudibranch. i0.wp.com/www.nudibranch.org/Scottish%20Nudibranchs/images/… Right now it engulfs chloroplasts from a specific alga that it eats and then uses them itself (which is crazy to start with) but one could imagine an alternative in which this engulfment somehow became a necessary step of the algal lifecycle. $\endgroup$
    – Laura
    Commented Jan 12, 2023 at 9:39
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Many plants do already move, albeit often at a rate too slow for us to notice. Some clinging plants, for example, move around to wrap around their support.

Your plant can do something similar, shooting out one or more shoots which then plant roots where they find a good place and repeat the process over and over. Nothing too dissimilar from what strawberries do.

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