The small plastic bag contains a special stain, Alizarin, that is used to mark Halimeda individuals. The bag is left on for 12-24 hours, during which the individuals take up the red dye. Alizarin dye is specifically taken up by the algae during calcification. After the bag is removed, any growth that occurs is without stain, which makes it possible to identify the production of new segments because they do not contain stain.