Fungi and algae retain their individual genetic make-up even in symbiosis, but the composite body they form is known as a thallus. The shape and appearance of lichens is dependent on the species of fungi present, not the algae. Lichens grow relatively slowly and their propagation is unclear. Scientists generally agree, however, that the most frequent kind of lichen reproduction is vegetative, a portion of an existing lichen breaking away and starting a new growth in an adjacent area.