Some of these sponges have green algae living deep inside them. The sponges use these glass fibres as optical cables to channel light to the algae, so keeping them alive. Part of the reason that stinging nettles can sting is that the tips of the stinging hairs are made of glass. But we do know that rock is made from silicon, so plants live in a very dilute slush of silicon. In fact, silicon is the most common element on the Earth.
Plants suck in the silicon through their roots, and then somehow make little fibres of glass.
These glasses made by plants are called phytoliths , from the Greek phytos , a plant , and lith , a stone . These phytoliths are useful to archaeologists. While plants will rapidly break down in soil, a phytolith can last for thousands of years. For example, the glasses of Indian corn are very different from any other plant glasses so far discovered. So, by digging in ancient camp sites and looking for corn glasses, archaeologists can follow the spread of corn across America.