Thereon is defined as on that or on it.
(adverb)An example of thereon is things growing on the earth such as in this quote by Kate Chopin: “It is greater than the stars - that moving procession of human energy; greater than the palpitating earth and the things growing thereon. “
See thereon in Webster's New World College Dictionary
adverb
See thereon in American Heritage Dictionary 4
adverb