Planet Swamp. As the name suggest, it is a planet fully covered with swamps.
A new kind of carnivorous plant has been discovered on this planet: when its leaves are touched by an animal or moved by the wind they emit a sweet scent, something like vanilla and sugar. This sweet scent attract unaware animals close to the plant.
Its fern like leaves are covered with tiny and fragile thorns, and as soon as an animal touches one of these thorns, it gets injected a small amount of toxin.
This toxin, when injected in small amounts initially enhances the sensitivity to the smell emitted by the plant, giving pleasure and euphoria to the animal inhaling it. Therefore the animal keeps touching the plant leaves, rolling in them, and it's not scared when the leaves and branches start enveloping and immobilizing its body until the toxin leads the animal to death. The enveloping movement is triggered by multiple local torn ruptures, and forces the branch to bend in the direction of the broken thorns, and can be as fast as in the Mimosa pudica.
The same toxin, once injected in large amounts as consequence of the enveloping, speeds up the body decomposition, resulting in a slurry that will feed the plant, providing it nutrients else not available in the swamp soil.
What can be the optimal size of preys that this plant should target? In other word, does it make sense that such a plant grows until it is able to catch an elephant sized animal, or it would be better for it to target a sheep sized (or even smaller) animal?