thallus (noun, pl. thalli) – in general: the physical structure of any plant-like organism that is not differentiated like a cormus of vascular plants and therefore lacks the three main organs root, stem and leaf. The term is thus defined by reference to and exclusion from another group (i. e., "thallophytes" are all non-vascular plants). In lichens: the vegetative and assimilative body of both myco- and photobiont. In non-lichenized fungi: the entire mycelium including all reproductive structures; some definitions emphasize a more or less organized and differentiated structure and therefore exclude undifferentiated mycelia from a more clearly differentiated and organized thallus.