The thallus of a fungus is usually called a mycelium. Characteristic of fruticose lichen is the shape of the thallus. "Platycerium" gametophytes are a small heart-shaped thallus. It is composed of a thallus and a holdfast. In lichenized species, the thallus of the fungus defines the shape of the symbiotic colony. The nonreproductive tissues, or vegetative body parts, are called the "thallus". Both male and female gametes are formed on the one thallus. Such colonies of bacteria growing inside the thallus give the hornwort a distinctive blue-green color. The asexual reproductive structures soredia and isidia are present on the thallus surface. Each cell of the thallus usually contains just one chloroplast. Many species have a branching thallus that is more than two centimeters wide. The thallus must be saturated with liquid water for cyanobionts to photosynthesize. The erect portion of the thallus bears cryptostomata and caecostomata (sterile surface cavities). There may also be small "slime pores" on the underside of the thallus. It forms calcium carbonate deposits within its cells which serve to strengthen the thallus.