So, when a person is in love, a vine with flowers grows on their arm (It starts on the wrist and grows upward until the elbow), the flowers are the signature colour of the person they're in love with and when they hold hands for the first time, the vines connect and basically "bind" them together, so they can't fall in love with a different person. More flowers grow when the person falls deeper in love.

When the person doesn't love them back, the flowers die and the vines grow all over their body until they can't move.

The vines can also grow inside the person (but only when the love is unrequited) and suffocate the person.