When the culture must be grown in Petri plates to observe individual colonies, several methods are available. Laboratories that work with relatively few culture plates at a time can use systems that can incubate the microorganisms in sealed boxes and jars in which the oxygen is chemically removed after the culture plates have been introduced and the container sealed. Some systems require that water be added to an envelope of chemicals before the container is closed, as shown in Figure 6, and require a catalyst. The chemicals produce hydrogen and carbon dioxide(about 4-10%) and remove the oxygen in the container by combining it, in the presence of the catalyst, with hydrogen to from water. In another commercially available system, the envelope of chemicals (the active ingredient is ascorbic acid) is simply opened to expose it to oxygen in the container's atmosphere. No water or catalyst is needed. The atmosphere in such containers usually has less than 5% oxygen, about 18% Co2, and no hydrogen. In a recently introduced system, each individual Petri plate (OxyPlate) becomes an anaerobic chamber. The midium in the plate contains an enzyme, oxyrase, which combines oxygen with hydrogen, removing oxygen as water is formed.