(i) Photosynthetic region: 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.
(ii) Storage Zone: The area of the thallus where food is deposited or stored is called the storage zone. It is colorless, multicellular, devoid of intercellular spaces and composed of parenchyma cells. This region does not produce food, but stores carbohydrates. Unicellular rhizoids and multicellular scales are produced from this region.
(iii) Genus: Riccia is a companion plant. Its male and female genitalia are located in the same thallus. Male genitalia are called antheridium and female genitalia are called archegonium.
* Antheridium: On the surface of the thallus, several stamens antheridia grow. It is round and pear shaped. Inside the antheridium are the sperm mother cells. Two biflagellate spermatozoa are produced from each sperm mother cell.
* Archegonium: Within the thallus several cup-shaped or flax-shaped female archegonia grow. It consists of two parts. Neck and Abdomen. The neck is slightly curved and consists of few neck duct cells. Abdomen contains an ovary and an abdominal duct cell.
(iv) Hypodermis: The hypodermis is composed of single cells below the accumulation zone of the thallus. Unicellular smooth or rough rhizoids and multicellular scales are formed from the epidermis.