These are a few Initial ideas of the plants when i first read the extract from the book.
"The forest consisted if large treelike plants, and as soon as we entered beneath its vast arcades, I immediately stuck by the strange manner in which the branches grew--something i had never seen before", "All grew straight up toward the surface. Every filament, every strip of vegetation, stood up rigidly, like a stalk of iron. The fucus and the lianas grew in perpendicular lines"
"I observed the padinae spreading out like the tail of a peacock"
For this i wasn't sure what to make of it. I found that a 'Phyllaplysia padinae' is a type of Sea Slug so I ran with it and used my imagination, by no means are these final ideas just rough sketches to get first impressions down.