Symbiotic bacteria digest cellulose with the help of enzymes.
"Those that have become specialist leaf-feeders can only extract nourishment from it by enlisting the aid of a very different kind of living organism--bacteria. They, unlike any animal, can digest cellulose." (Attenborough 1995: 58)
Book The Private Life of PlantsBBC BooksFebruary 20, 2017 David Attenborough