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I have a rocky world where land-based vegetation like grass or trees are either very rare and restricted to small and rare areas or non-existent. Rather, vegetation has stayed in the oceans and developed there.

So the question is that in the title, what natural cause could there be for the landmasses of a planet having few or no plants while the seas have plenty?

Edit: Note that animals still spread onto land, maybe feeding off photosynthetic animals rather than plants, or by feeding in lakes with plants

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  • $\begingroup$ You might look into the history of life on Earth. Why was it confined to the sea for most of its history and did something change to enable land life? $\endgroup$ – JDługosz Jul 2 '16 at 1:58
  • $\begingroup$ Could you provide a link to some paper or article on this? $\endgroup$ – ArborianSerpent Jul 2 '16 at 2:09
  • $\begingroup$ Life was in the oceans 200m years before oxygen made air fit to breathe dailymail.co.uk/sciencetech/article-1213287/… $\endgroup$ – Keltari Jul 2 '16 at 2:37
  • $\begingroup$ Life was confined to the oceans for billions of years, so perhaps the real question isn't what keeps plants in the oceans, but why do they leave the ocean in the first place? $\endgroup$ – Thucydides Jul 2 '16 at 4:31
  • $\begingroup$ To escape predation presumably $\endgroup$ – ArborianSerpent Jul 2 '16 at 4:33
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Two ideas:

A periodic increase in ultraviolet or other radiation might allow this, the mobile animal life could move back into the water or other protected areas for protection, while any fixed plant species would be killed.

Or on an evolutionary idea: For some reason plants never evolved a hard outer shell protecting them from drying out, the plants would be limited to wet areas where they would remain moist.

https://en.wikipedia.org/wiki/Plant_cuticle

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  • $\begingroup$ And when the plants have died off, any new attempts to colonize the land are stopped by animals who have filled out the land niche and won't allow the plants to get up on land, until the next mass extinction at least, I should think that works, thanks! $\endgroup$ – ArborianSerpent Jul 2 '16 at 4:32
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A different geology would help. When there are no stable continents, but only short-lived islands that form the land mass of your planet, the time to evolve land based plants may be too short before the land vanishes again while new islands are emerging at other places of the ocean. On such a planet, only very primitive forms of plants (like algae) may be found on land.

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  • $\begingroup$ I like this idea, it seems to neatly toe all the loose evolutionary ends together. It might make building civilization difficult however. I'll look over it $\endgroup$ – ArborianSerpent Jul 4 '16 at 15:52
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Your scenario is one that actually existed, so you may wish to concentrate on (#1) what may have caused it in the past, and (#2) what could constrain it in an arbitrarily specified world, such as yours, where you determine the rules.

There are many causes of transition to land among plants that appear fortuitous, in the sense that the specific adaptations (functions that allow conquest of land) may have appeared in different forms, but for luck and historical contingency. As it turned out, the principal limit was probably time. One could refer to this as a non-equilibrium theory. Evolution is clearly a stochastic, non-equilibrium process at long time scales. Specifically, (#1) some time had to elapse between the "invention" of photosynthesis (plants) and functions that allow plants not to be submerged in water (land plants). During this period, all living plant lineages had a shot at coming up with dry adaptations, e.g. cuticle, adjusted gas exchange, aquaporin expression, etc.; and later vasculature and seeds (prevalent in "vegetation like grass or trees" from your scenario).

Otherwise (#2), you can come up with any number of constraints that could plausibly prevent the evolution of drying out and forming tall structures. This is what Josh King and ArborianSerpent have in mind. Some of those could be arbitrary limits on what plants can do in your physical/abiotic setting, and some could be biotic limits (e.g., animals in your world are pre-adapted to eating plants; microbes are pre-adapted to parasitizing them).

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  • $\begingroup$ I'm honestly not sure what you mean. What I got out of it was that animals and bacteria limit plants from coming onto land by consuming them, which seems plausible, but only if the animals evolved far before plants to conquer the land. $\endgroup$ – ArborianSerpent Jul 2 '16 at 19:12
  • $\begingroup$ What you got out of it is #2--there are other living things that limit land conquest. What you seemed to have missed is #1--there are also inherent limits to adaptation, or at least its pace. It takes a while to develop an adaptation. It is clear from the fact that there was a time when plants existed, but did not yet take to land, that they are limited by time. Anyway, it's your world, so make it as you'd like. $\endgroup$ – Plantaloons Jul 2 '16 at 22:22
  • $\begingroup$ I think I'm gonna do a mix of the two, maybe have the ground be too hard for plant roots $\endgroup$ – ArborianSerpent Jul 2 '16 at 23:02
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A couple of ideas, although I cannot entirely convince myself that they would defeat evolution. They might be "short term" in evolutionary time, persisting for mere tens of millions of years.

A flare star. The planet's sun occasionally undergoes catastrophic brightening. Such stars are known to exist. Anything on the surface of dry land would be incinerated at such a time. The very surface of an ocean might boil off but life that can retreat to deeper water or grow back from roots in deeper water would survive if the flare was short enough.

Too much oxygen. Ocean plants are just too good at pumping out oxygen. Anything carbonaceous trying to live on land will spontaneously ignite or be consumed by fire sparked by lightning strikes. The only safe place is completely immersed in water.

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  • $\begingroup$ The only safe place for plants, right? Animals still have to live on land $\endgroup$ – ArborianSerpent Jul 4 '16 at 9:57
  • $\begingroup$ Fish are animals! If they can avoid spontaneous ignition you might get fish evolving out of the water to avoid predators, but without land plants there could be no food chain away from the coasts. $\endgroup$ – nigel222 Jul 4 '16 at 10:04
  • $\begingroup$ Sure, but land animals is what I meant. And I don't neccessarily think that would matter too much, maybe there are plants which send airborne spores or whatnot which could hit lakes somewhere inland. $\endgroup$ – ArborianSerpent Jul 4 '16 at 15:54
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As others have explained above, it used to be that way for a very long time. The basic idea is that land plants evolved some ways to stop dehydration from not being in the water all the time (all that talk of cuticle, aquaporins etc) and thus could live in dry land. You could either assume:

1 they didn't have time to evolve those yet, as it did happen during a period on Earth, where we had animals living in dry land, but not plant life (mostly arthropods: insects, spiders, scorpions and such)

2 something else is out to get all the plants that did, for any reason, get to survive the dryness of the land. Keep in mind though that these plants would probably get some defenses of their own pretty quickly (in evolutionary terms of course, a couple million years should do) to avoid being eaten, attacked by parasites, etc.

(not trying to plagiarize your answer, Plantaloons, just make it more accessible)

also, photosynthetic animals exist, mostly symbiontic organisms such as corals. I do remember in college a teacher talking about a sloth species that had algae living in their fur, but they couldn't access that food source. Maybe you can think of some animal that could! Fun times ahead!

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