Bilva Tree associated with Lord Shiva is also known as Sirphala (head fruit). The tree is also called Sirphala because the fruit resembles a skull with a white bone like outer shell with soft pulp inside.
The botanical of Bilva Tree is name was Aegelemarmelos. It is called the Indian bael or Bengal quince and belonged to the lemon and orange family.
In the Atharva Veda, the Bilva tree is described as being so sacred that its timber should not be burnt.
According to the Puranas, Goddess Lakshmi was born as a divine cow and from her appeared the Bilva tree.
Lakshmi Ashtottara Nama Stotram refers to Goddess Lakshmi having her abode in the Bilva tree - bilvanilaya. So, the tree is also called Sri Vriksha, the tree of prosperity and good fortune. The fruits are also referred to as Sriphala - the fruit of plenty.
Sri Suktam too associates the tree with Goddess Lakshmi.
Practically every part of the tree has some medicinal use: root, leaf, trunk, fruit and seed, are used for curing one human ailment or another.