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:Embryogenic callus has been obtained in different Citrus species and explants such as nucellus (Kayim and Koc 2006); ovule (Froelicher et al. 2007); style and stigma (Carimi et al. 1999); anther (Germanà and Chiancone 2003); leaf, cotyledon and root segments (Gill et al. 1995).
Category: