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As landlubbers, we often let ourselves think that if salty seawater is undrinkable for us, it could be even worse for plants. However, certain types of angiosperms have found ways to not only thrive on marine environments but specialize on them. These are the mangroves and four families of alismataleans collectively called "seagrasses".

But what about the other major groups of plants--the conifers, the ginkgoes, the ferns, the cycads, the liverworts, the mosses, the clubmosses? In an alternate Earth, could any of them have what it takes to adapt for saltwater life and grow into marine forests as complex if not more so than the kelp forests of back home?

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There's no rule against it. Salinity is not a barrier for plants, as seen by sea grasses and mangroves. If it evolved once it would evolve again.

Growing underwater has certain mechanical considerations because water currents are a lot harsher than air currents. Your new seaplants would probably start to resemble the current seaplants, because the mechanical considerations of not having stuff break off would lead to similar solutions evolving. (MESA link)

Seagrasses have evolved adaptations to survive in marine environments including salt tolerance and resistance to the energy of waves (rhizomes and roots firmly anchor seagrasses to the sediments and flexible blades offer little resistance to water movement.

If you have a clubmoss underwater, I'm not sure how much resistance those spikes are going to give, but it will likely develop a lower structure similar to current seagrasses, because it'll be trying to collect sunlight from all the way at the bottom of the water.

As far as pulling a mangrove and growing in shallower brackish water, go nuts. There's less water to deal with forces, and the shallower water means that most plants can actually get their leaves above water so they can have the typical shape.

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  • $\begingroup$ What about the other plant groups? How will THEY adjust their anatomy for ocean living? $\endgroup$ – JohnWDailey Oct 15 at 21:57
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    $\begingroup$ "Your new seaplants would probably start to resemble the current seaplants," Because seaplants look a lot more similar to each other than landplants. I did only mention clubmosses in the answer but it really would apply to all of them. The mechanical considerations - and thus the shapes - have more constraints, and those constraints are tighter. Mangroves are one thing, but living underwater is another problem altogether. $\endgroup$ – ltmauve Oct 16 at 2:34

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