One of the first and earliest suspected cases of dualism, which long puzzled the older mycologists, was observed amongst the Uredines, and many years ago it was held that there must be some mysterious association between the "red rust" (_Trichobasis ruligo vera_) of wheat and grasses and the "corn mildew" (_Puccinia graminis_) which succeeded it. The simple spored rust first makes its appearance, and later the bilocular "mildew." It is by no means uncommon to find the two forms in the same pustule. Some have held, without good reason, that the simple cells became afterwards divided and converted into _Puccinia_, but this is not the case; the uredo-spores are always simple, and remain so except in _Uredo linearis_, where every intermediate stage has been observed. Both are also perfect in their kind, and capable of germination. What the precise relations between the two forms may be has as yet never been revealed to observers, but that the two forms belong to one species is not now doubted. Very many species of _Puccinia_ have already been found associated with a corresponding _Trichobasis_, and of _Phragmidium_ with a relative _Lecythea_, but it may be open to grave doubt whether some of the very many species associated by authors are not so classed upon suspicion rather than observation. We are ready to admit that the evidence is strong in favour of the dimorphism of a large number of species--it _may_ be in all, but this awaits proof, or substantial presumption on good grounds. Up to the present we know that there are species of _Trichobasis_ which have never been traced to association with a _Puccinia_, and doubtless there will be species of _Puccinia_ for which no corresponding _Uredo_ or _Trichobasis_ can be found. Tulasne remarks, in reference to _Puccinia sonchi_, in one of his memoirs, that this curious species exhibits, in effect, that a _Puccinia_ may unite three sorts of reproductive bodies, which, taking part, constitute for the mycologists of the day three entirely different plants--a _Trichobasis_, a _Uromyces_, and a _Puccinia_. The Uredines are not less rich, he adds, in reproductive bodies of divers sorts than the _Pyrenomycetes_ and the _Discomycetes_; and we should not be surprised at this, since it seems to be a law, almost constant in the general harmony of nature, that the smaller the organized beings are, the more their races are prolific.