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FITC is fluorescein isothiocyanate, a reactive derivative of fluorescein used to attach a fluorescent label to suitable targets. Its reactive group couples to accessible amino groups, so labeling a protein can make that protein observable by fluorescence microscopy or flow cytometry.

The label's presence is distinct from the specificity of the experiment. Unbound dye must be separated from the intended conjugate, and an unlabeled control estimates autofluorescence. Labeling can alter the target's behavior, while bleaching and environment-dependent emission affect measured intensity. ^[ref]

The new bud contains newly deposited wall rather than simply inheriting a stretched patch of the mother's old labelled wall. concanavalin A conjugated to FITC was applied before growth resumed, so it marks the pre-existing accessible biological cell surface. The bright mother outline persists while the emerging bud is weakly labelled or unlabelled. That spatial contrast places substantial new, unlabelled wall material at the growth site. Drug-treated biological cells that eventually bud show that at least some localized new wall deposition still occurs under treatment.

The pulse label does not mark all wall made subsequently, and it does not prove that absolutely no new material is inserted elsewhere in the mother. Signal dilution, shedding and accessibility can affect brightness. A second distinguishable wall label applied after refeeding would more directly map new deposition. If the original lectin remained free in the medium, it would also bind newly exposed wall, so the interpretation assumes the stated prelabelling step functions as a pulse rather than continuous staining.