Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:This is in sharp contrast to the observation that mycoheterotrophic gametophytes show quite specific associations with particular Glomus group A lineages; fungal taxa from the underground massive gametophytes of two Botrychium species (Winther and Friedman 2007) and seven Lycopodiaceae species (Winther and Friedman 2008) form a monophyletic clade, and Psilotum nudum (Winther and Friedman 2009) forms four distinct clades.
Category: