Tulsi (an herb from the basil family) was a beautiful maiden who, when she saw the resplendent Ganesha, fell in love with him and wished to marry him.
Ganesha though was a confirmed bachelor (at least according to some legends) and refused to oblige her. The enraged Tulsi cursed him that one day, he would indeed break his vow of celibacy.
Ganesha cursed her in return, that she would marry a demon and later, turn into a plant.
Tulsi realized her folly and pleaded for mercy until Ganesha relented.
“You will be the most fragrant plant of all”, he said, “one that will please all the gods. Your leaves will be used especially in the worship of Vishnu.
But you will never be present where I am worshiped.”