Yew Spirit

Oracle Text

2GG: Yew Spirit gets +X/+X until end of turn, where X is its power.

Card Rulings

5/1/2012 The value of X is determined when the ability resolves. The bonus won’t change later in the turn if the creature’s power changes.
7/14/2017 If this creature’s power is negative as its ability resolves, X is considered to be 0. (This is a change from previous rulings.)