The most striking difference between the fungi and algae is the absence of chlorophyll in the former, and the consequent inability of fungi to manufacture their organic compounds from inorganic material. Fungi live therefore either as parasites or saprophytes, and as the same species may pass part of its life in a living host to occur at another stage of its development as a saprophyte, it is impossible to distinguish definitely between parasitic and saprophytic forms. The vegetative body of a fungus, that is the portion which is concerned with providing nourishment and preparing the plastic food-substance for the reproductive organs, is known as the _mycelium_. It consists either of a single and branched tubular cell known as a _hypha_, or of several hyphae or thread-like elements (filamentous fungi). The hyphal filaments may be closely packed together and form a felted mass of compact tissue, which in cross section closely simulates the parenchyma of the higher plants. This pseudoparenchymatous form of thallus is particularly well illustrated by the so-called _sclerotia_; these are sharply defined and often tuberous masses of hyphal tissue covered by a firm rind and containing supplies of food in the inner hyphae. They are able to remain in a quiescent state for some time, and to resist unfavourable conditions until germination and the formation of a new individual take place. The reproductive structures assume various forms; in some of the simpler fungi (Phycomycetes) sexual organs occur, as in the parallel group of Siphoneae among the algae, but in the higher fungi the reproduction is usually entirely asexual. An interesting case has recently been recorded among the more highly differentiated fungi in which distinct sexuality has been established[404]. In addition to the reproductive organs, such as oogonia and antheridia, the asexual cells or spores are borne either in special sporangia, or they occur as exposed _conidia_ on supporting hyphae or _conidiophores_. Thick-walled and resistant resting-spores of various forms are also met with. Without going into further details we may very briefly refer to the larger subdivisions of this group of Thallophytes. PHYCOMYCETES. Mycelium usually consisting of a single cell. ZYGOMYCETES, Reproduction by means of conidia, and in many OOMYCETES, cases also by the conjugation of two similar including hyphae or by the fertilisation of an egg-cell =Chytridiaceae=, contained in an oogonium. &c. MESOMYCETES, including the Intermediate between the Phycomycetes and Sub-classes the higher fungi. Multicellular hyphae. No HEMIASCI and sexual organs. HEMIBASIDII.