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:To certain plant species, an explant origin of a species is not specific for the successful regeneration of transformed cells via Agrobacterium-mediated genetic vector (Sangwan et al., 1991a, b, 1992;Ducrocq et al., 1994).
Category: