The swellwood plant is widely distributed across the caves of the Shivering Isles, and has distinct sub-species that are each adapted to a different subterranean environment.
Food.
Swellwood bulbs of all variants have potent magical properties, but can be difficult to properly gouge free from the body. The bulbs of screaming maws can be used to contribute to potions. Meanwhile, thorn hook bulbs can be used to contribute to potions.
Screaming Maw

Example.
Thorn Hook

Thorn hooks are a type of swellwood plant that grow from rootcave systems in Dementia. This variant of swellwood is particularly plentiful in Swampgas Hole and Blackroot Ruins, with a modest amount also found growing in the Caverns of Sussuration, in the Gardens of Flesh and Bone.
Some of the plants have found their way into the sewers of Solitude in Skyrim, following the Summer of Madness, and they remain there to this day.