I think its something in Norfair maybe?
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There's an impassable plant barrier right in Norfair's doorstep (one of those that you need to get the bugs to eat) that goes away only when you pick up Power Grip. Theoretically, if that wasn't there skipping Power Grip would be possible. But I figure in a game as open-ended as Zero Mission, you don't just put in a barrier like that for no reason.