Cotton blue or resourcin blue for callose

A cross-section of pinewood showing xylem (below) and phloem (above). In the phloem, blue-stained deposits (arrows) may be observed which are callose, a β-1, 3-glucose polymer.

The callose has a strong and specific reaction with resourcin blue (also designated as lacmoid) or cotton blue, which results in dark blue deposits.