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:Electron Microscopy and Immunolabeling
Arabidopsis embryos and root tips and tobacco BY2 cells were highpressure frozen/freeze-substituted for electron microscopy analysis as described previously (Otegui et al., 2002).
Category: