Jakers, you were probably too slow and should have returned another day.
Triffid Heart is the QUEEN tag for triffids. It can not only spawn more triffids, biollantes, and creeper hubs to protect itself, but also close up its roots to block out evil Heart Attackers–such as you. Best practice is to clear out the Triffid Roots and leave, then repair & reload and return for the Heart. Wall-demo, flamethrower/extinguisher, and tear gas/mask are all recommended.
Brain blobs are not a guaranteed spawn because there’s currently no way to ensure that any given level of a Slime Pit is the bottom. Should probably steal the code from Labs, come to think of it…