Tulsi is known to be an incarnation of Mahalaxmi who was born as Vrinda. Tulsi was married to demon king Jalandhar. She prayed to Shri Vishnu that her demon husband should be protected, with the result no God was able to harm him. However on the request of the other Gods, Shri Vishnu took the form of Jalandhar and stayed with the unsuspecting Tulsi.
When the truth emerged after Jalandhar’s death, Vrinda cursed Shri Vishnu and turned him to stone (Saaligram) and collapsed. From her body emerged the Tulsi plant. That is why Vishnu puja is considered incomplete without Tulsi leaves.