If your professor desires you to make a phylogenetic tree of orchids and she provides you tissue from seven orchid species and one lily, then the most likely reason why she gave you the lily is for it to serve as an outgroup.
To add, in cladistics or phylogenetics, an outgroup is a group of organisms that serve as a reference group when determining the evolutionary relationship among three or more monophyletic groups of organisms. The set of organisms under study that precisely allows the phylogeny to be rooted is called the outgroup and it is used a comparison point for the ingroup.