The Mythological story

According to the Puranas once a fierce battle broke out between Lord Shiva and the demon Jallundhara. They both appeared equally matched with no one ready to lose, and hence the battle continued for a long time. It was not an ordinary battle between the gods and the demons, but a match between the chastity of two chaste women Parvati and Vrinda. Lord Shiva knew it well that unless the chastity of Vrinda, the wife of Jallundhara, was spoiled he could not be killed. But it was against the nature of gods to spoil the chastity of a chaste woman. Hence despite knowing everything the Gods were helpless.

But for Jallundhara nothing was a taboo. So, thinking that by spoiling the chastity of Parvati he could defeat Lord Shiva Jallundhara reached Parvati in the guise of Lord Shiva. But Parvati, who is herself is the supreme ruler of illusion Maya, knew the craftiness of Jallundhara and ended his guise and signaled Lord Krishna (Vishnu) that it was now just to spoil the chastity of Jallundhara's wife, Vrinda, and it will not be a sin. So, Lord Krishna approached Vrinda in the guise of Jallundhana. Vrinda could not understand the craftiness and lost her chastity. Later when Vrinda came to know the reality, she was infuriated and gave four seeming curses to Lord Vishnu to become stone, grass, tree and plant. Hence Lord Vishnu took four avatars (incarnations). He became Saligram-stone, Kusha-grass, Pipal-tree and Tulsi-plant. Since then these became four are considered holy and are worshiped.