AFinn is a human boy who was found by Jake's dad, Joshua, in the forest. His duty is to protect the Land of Ooo from evil such as the Lich. Jake is a magical dog who can stretch to any length, shape, or size. He got his stretching power by rolling in a magic puddle as a puppy. Jake's blood relatives consist of Joshua, his dad, Margaret, his mom, and Jermaine, his brother. All of his blood relatives are dogs as well, but none of them have stretching powers like Jake. Finn is also Jake's adopted brother. Jake's quest is the same as Finn's, being righteous and such, but Jake is more laid back and like's to goof around and mess with Finn.
AIn 'Escape from the Citadel' we see the Sap used when Finn's dad gets the tissue on his leg burnt off so he is only left with the bone in his leg connected to his foot. When the Sap is put on the bone it creates new muscle and skin tissue over existing bone. This also happens when The Lich gets covered with the Sap, he begins rapidly growing muscle and skin over his skeleton being the case with both examples we can assume the Sap only works if there is at least a bone.When Finn's grass sword merges with his right arm which is them torn off his arm was previously, regardless of how briefly, made of grass. This is why the Sap grew a flower because it was growing based on what the arm previously was. As there was no bone after Finn's arm is torn off it was the only thing the Sap could do.