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Sims who are teens or older can become PlantSims by planting a Forbidden Fruit and eating it. Sims at university who take part in a science experiment have a 20% chance of becoming PlantSims. In either case, the affected Sim will receive a "Botanitis Minorous" moodlet, which will last 4 hours; if the moodlet is not cured or removed in that time, the Sim will become a PlantSim.
i have a plant sim