I'm calling bs on this one, riot
http://i.imgur.com/4awiEv3.jpg
How do you explain it working on them: 
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Rito, how could you forget one of the most basic bits of knowledge?
Grass is super effective vs Rock.
If anything, 
should heal DOUBLE from towers!
Tree magic can't sap stone beams?
Especially Galio, who's build out of magic repelling stone.
It wouldn't work on Galio anyway.
Magic damage doesn't exist to him.
Yes you can just forgot about me! I dont care... 
Technically Galio isn't a stone monument anymore. if i remember right he's made from an ancient magic absorbing wood with the strength of stone.
Because they're both magic rocks, so he's sapping the magic, not the stone...
I think...
kappa