The region of the thallus that produces food in the process of photosynthesis is called the photosynthetic region. This region is made up of many filaments. Filaments are slender, threadlike, long, sessile, multicellular and green in colour. These are called assimilation formulas. Each sutra consists of one large and colorless cell at the tip. The space between two adjacent cells is called an air pocket. It contains air. Each air cell has an air hole at the top. Air moves through these holes. This region is green and produces food through the process of photosynthesis.