In the race to become the most dominant marine multicellular flora, many lineages of poralp have staked each their own claim; for the Even Greater Poralp Trees to the northern reaches of the Warm Ocean, that claim would be to become among the tallest non-piss flora in the world of 2s2t, becoming the Arboralps - tall, tree-like seaweed that form vast groves in coastlines where it can grow.
The Arboralp has diverged considerably from its ancestry, attaining a hardened stipe analogous to a woody trunk, providing it much support for its impressive height. To distribute sugars and nutrients throughout the flora, a network reminiscent of phloem has developed, as diffusion alone cannot sustain the entirety of an organism of its proportions. Its holdfasts are likewise firm and provide reliable anchorage to whatever surface lies underneath, though preferably coarse, rocky substrate. Arboralp blades branch often to maximize the organism’s total photosynthetic area; older, lower blades grow more distantly than younger blades closer to the water’s surface as to absorb whatever light trickles down there.
But what enabled the Arboralp to dominate the coastlines between the North Cold Ocean and the northern Warm Ocean in veritable forests is its innovation of sexual reproduction. Instead of merely shedding spores under its blades, the Arboralp instead sends gametes into the water column. Transported by the currents, compatible gametes will eventually meet, conjugate, and form spores which will then settle onto a suitable substrate, eventually growing into a new Arboralp. Arbroalps can still propagate from severed blades however, provided there is enough sunlight and nutrients.