Cereals, wheats and grasses, suffer from other well-known forms of microscopic fungi termed _rusts_ and _smuts_, which cover the blades or infect the full ear of the fruit. The name given indicates their colour, and these belong, for the most part, to the genus Uredo and the family of the Basidiomycetes. They have no endogenous spores but as many as four forms of exogenous. This is also the case with wheat and barley, whereby they are distinguished as _Uredo_ or _Puccinia graminis_ (see Figs. 273 and 274, and Plate I., Nos. 19 and 22, _Æcidium berberidis_). For a long time it was believed that _Uredo linearis_ and _Puccinia graminis_ were so many distinct species, but it is now known that there are only three successive phases of the developmental stages of a single species--that, as a matter of fact, puccinia presents the phenomenon of alternation of generations, that is, that the complete development of the fungus is only effected by its transference from one plant to another. Other uredines, Ustilago and Tilletia smuts, are more apt to affect the ears of wheat, rye, and other grasses than puccinia. Bread made from wheat affected by smut has an acrid and bitter taste, while that made from rye flour often produces a serious form of disease. The propagation of either, then, should be stopped as quickly as possible by destroying all barberry bushes growing near or within the vicinity of corn fields, and by other means. The ergot of rye is due to distinct species of fungi having endogenous spores enclosed in a sac or _ascus_, hence the name of the family, Ascomycetes or _Tuberaceæ_, which are reproduced by the spores contained in these asci. Truffles belong to this family. But other members of the same family have several forms of spores, and these again present us with the phenomenon of alternation of generations. [Illustration: Fig. 276.--Fungi, Moulds. _p._ Spores of _Tilletia caries_; _q._ Spores of _Tilletia caries_, when germinating, produce a fœtid olive-coloured spore in cereal grains; _r._ Telentospores of _Puccinia graminis_; _s._ _Crystopus candidus_, spores growing in chains; _t._ _Petronospora infestans_, mildew of turnips, &c.; _u._ A transverse section of ergot of rye, showing spores in masses; _v._ _Claviceps purpuræ_, associated with ergoted rye.] Ergot of rye is used in medicine, but if not used with care it will produce a dangerous disease. This parasitic fungi consists of minute microscopic masses of spores, which cover the young flower of the rye with a white flocculent mass, formerly termed _sphacelium_. The mycelium formed spreads over the ear of corn in thick felt-like masses, termed _sclerotis_. The sphacelium changes its form in the following spring. Other changes are brought about, and it seems to pass through a cycle of alternations of generations.