Slimy Trees are generated on Slime Islands in quantities of two to three. They can also be grown on your own using the Slimy Saplings that drop from their leaves.
Slimy Trees grow six to nine blocks tall and nine blocks wide. They are composed of two to five Congealed Green Slime blocks and a canopy of Slimy Leaf blocks.
Magma Slime Variant
Slimy Trees also occur in the Nether as Magma Slime Trees. They are composed of Congealed Magma Slimes and orange Slimy Leaf blocks.