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Recently, I asked whether or not the other plant groups besides the angiosperms could evolve to adapt to being submerged under salty ocean water 24/7. While marine conifers, ginkgoes, cycads, ferns, horsetails, lycopods, liverworts and mosses are not possible on Earth, in an alternate Earth, they could have the opportunity.

So in an alternate Earth, the world's shallow waters are green with forests as complex as our kelp forests, if not more so. But plants being plants, their diversity raises some concerns. In marine jungles like this, what sorts of changes would the conifers, ginkgoes, cycads, ferns, horestails, lycopods, liverworts and mosses need to adapt to life in salty ocean water?

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    $\begingroup$ They would have to adapt to become algae-like. Algae have been at it for billions of years, they have marine photosynthesising all figured out. $\endgroup$ – Renan Oct 25 at 1:09
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    $\begingroup$ Underwater trees need not bear weight, so expect them to lose the habit of spending resources on wood. $\endgroup$ – Anton Sherwood Oct 25 at 3:57
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    $\begingroup$ You're seeing close votes because you're asking about a vast range of different plants in one question. Trees are a long way from moss. $\endgroup$ – Separatrix Oct 25 at 7:14
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    $\begingroup$ Mangroves may be halophytes, but underwater trees they are not. That just seems to leave you with seagrasses, and they're quite thoroughly un-treelike. $\endgroup$ – Starfish Prime Oct 25 at 7:39
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    $\begingroup$ @AntonSherwood expect them to lose it fairly promptly; waves and currents are a lot stronger than storm winds, and tall, strong, inflexible trunks would be a liability. Kelp forests are filled with things that are well adapted to their environment, and aren't particularly tree-like... $\endgroup$ – Starfish Prime Oct 25 at 7:41