 The International Committee on Taxonomy of Viruses now recognises over 900 plant viruses.By the end of the 19th century, viruses were defined in terms of their infectivity, their ability to be filtered, and their requirement for living hosts.Up until this time, viruses had only been grown in plants and animals, but in 1906, Ross Granville Harrison (1870–1959) invented a method for growing tissue in lymph, and, in 1913, E Steinhardt, C Israeli, and RA Lambert used this method to grow vaccinia virus in fragments of guinea pig corneal tissue.