Among the Rust-Fungi some species are found which only form basidiospores and teleutospores (_Puccinia malvacearum_, _Chrysomyxa abietis_). Other species have in addition uredospores; others spermogonia and uredospores; others spermogonia and æcidia; others spermogonia, uredospores and æcidia. Those species in which all the methods of reproduction are not developed must not be considered as incomplete forms. As a rule the mycelium, which is produced from the basidiospores, developes æcidia; in the species, however, without æcidia, it developes the uredo-form, and when the uredospores are also absent, the teleutospore-form. It has been established in some species of _Puccinia_ and _Uromyces_ that the formation of æcidia can be suppressed, and it is not a necessary part of the cycle of development of the species. The majority of Rust-Fungi hibernate in the teleutospore-form. Many species are able to hibernate in the uredospore-form (_Coleosporium senecionis_). Others pass the winter in the æcidio-form, and develope æcidia on new hosts (_Uromyces pisi_, on _Euphorbia cyparissias_; _Phragmidium subcorticium_, on _Rosa_; _Æcidium elatinum_, on _Abies alba_). In _Chrysomyxa abietis_, the mycelium, developed from the basidiospores, survives the winter. Among the Rust-Fungi, with several forms of reproduction, there are about sixty whose development can only be completed by an _alternation of hosts_, that is, on one host only uredo-and teleutospores are produced, while the further development of the germinating basidiospores, and the formation of the æcidia and spermogonia from its mycelium, can only take place on a second quite distinct and definite host (_heterœcious_ or _metoxenous_ Fungi). Those Fungi which have all their forms of reproduction on the same host are termed _autœcious_ or _autoxenous_. It is not, however, always necessary that the heterœcious Rust-Fungi should regularly change their hosts; for example, _Puccinia graminis_ can hibernate in the uredo-form on the wild Grasses, and in the spring can distribute itself again in the same form. As a consequence of the alternation of hosts the various forms of development were considered as independent genera (_Uredo_, _Æcidium_, _Rœstelia_, _Cæoma_, _Peridermium_), until De Bary and Oersted established, about the same time (1865), the mutual connection of some forms, and paved the way for the right conception of these Fungi. [Illustration: FIG. 146.--_Puccinia graminis_.]