Bred and nurtured as a food source by the Elf Thelon of Havenwood long, long ago, Thallids have become a race unto themselves. Their spore-based reproduction is so efficient that they bud off sporelike organisms known as Saprolings almost constantly. Thallids thrive in the presence of these Saproling byproducts, sometimes consuming them for fuel.
Thallids are, with a few notable exceptions, small. Almost all of them have the capability of producing Saprolings. Saprolings can blossom from a larger plant one at a time or burst from a pod in a huge wave. They can bud off of the body of a Thallid, such as happened all the time around Sarpadia, or occur on a plane where Thallids don't exist, such as on Ravnica.
Different strains of thallids may have different physical characteristics, yet still function identically. One common thallid, for instance, grows long, hair-like fronds that drape over its trunk, shielding its saproling buds from view and potential harm.