I’m so glad Josh was the one who brought up the subject of what Ivy “eats”. Saved me from having to ask the question.
For the record, I don’t REALLY need an explanation for where Ivy gets her energy. In fact, I think it’s good if they don’t try to explain it. It’s not a problem here because it doesn’t come up. If there was a plot point where the warden took away her food, or if she got the power to escape because someone left her too close to a grow lamp, then we’d be obliged to think about this stuff. The game doesn’t explain it, and it doesn’t matter, because fighting her never comes down to directly controlling the energy intake of her physical body.
But I don’t mind discussing the idea of how she sustains herself. It’s a bit of a fan-wank, like asking “which superhero would win in a fight?”, but it’s harmless and fun and sometimes leads to interesting or unexpected conclusions.
I’m sure other people have done this over the years, but let me have a go at it:
Let’s say Ivy does photosynthesize, but she’s also carnivorous via the plants that eat peopleThere are an awful lot of calories stored up in the average person!. (The Harley Quinn DLC in Arkham Knight explicitly shows Ivy’s plants eating cops.) She taps into these huge networks of building-sized plants and soaks up the solar power and calories while she’s on one of her rampagesUndercut somewhat by the fact that these rampages usually happen at night, but shut up.. That gives her lots of power to be ambulatory, grow more plants, produce spores, and whatever else she’s trying to do. Then Batman comes along, foils her plans, and they stick her in a tank with no access to plants. From there she’s living off whatever she’s got stored in her body.
 There are an awful lot of calories stored up in the average person!
 Undercut somewhat by the fact that these rampages usually happen at night, but shut up.
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