Submerged Growth System Supporting Oaks Etc.
Puzzle 15 of Under the Ground Easy Pack
Question is: Submerged Growth System Supporting Oaks Etc.
and the answer is: TREEROOTS
Word Meaning:
TREE — A perennial woody plant taller and larger than a shrub with a wooden trunk and, at some distance from the ground, having leaves and branches.
Definitions from Wiktionary.