In _Gymnosporangium_, two kinds of teleutospores are found (distinguished by their size and thickness of exospore). In many species of _Puccinia_, the form of the teleutospores varies very much, so that in the same layer spores have been observed with the characteristic form of other, allied genera.--The teleutospores of _Endophyllum_ resemble æcidiospores, since they are united in chains, whose cells are easily separated, and are produced in the interior of a “peridium.” The multicellular teleutospores of _Coleosporium_ function as basidia, and from each cell immediately produce basidiospores.--The teleutospores of _Coleosporium_ and _Chrysomyxa_, differ from other teleutospores in the absence of exospore and germ-pore. The ÆCIDOSPORES (_Spring-spores_) are produced in chains which are generally enclosed in an _envelope_ of hyphæ, the _peridium_; the _peridium_ enclosing the spores being termed the _æcidium_. The æcidiospores are unicellular, and generally of an orange colour; they are often separated by intermediate cells which wither and so assist in the distribution of the spores. The exospore is made up of minute, radially arranged rods. _Generally germination_ proceeds _immediately_, the æcidiospore producing a germ-tube, which developes into a mycelium bearing either uredo- or teleutospores. The æcidia of many Rust-Fungi were formerly considered as distinct genera. The æcidia of _Phragmidium_, _Triphragmium_, and _Melampsora_, in which the _peridium is wanting_, were in part considered as _Cæoma_. The æcidia with fimbriate edge, or those of _Gymnosporangium_ with longitudinal lattice-like splits, were considered as “_Rœstelia_” (Lattice-Rust); large, sac-shaped æcidia on the Coniferæ were known as _Peridermium_. The ~UREDOSPORES~ (_Summer-spores_) are unicellular and arise singly, seldom in chains (_Coleosporium_). Their colourless, warty exospore bears, _in the equatorial plane_, 2–8 _germ-pores_. In the majority, _germination_ proceeds _immediately_, and a mycelium is produced which at first gives rise to uredospores and afterwards to teleutospores. The uredospore-formations of _Melampsorella_ and _Cronartium_ are enclosed in an _envelope_, and hence resemble æcidia.--Between the uredospores sterile, unicellular hyphæ (paraphyses) may be found. The _spermogonia_ are spherical or pear-shaped _conidiocarps_, generally embedded in the substratum, and are produced before the æcidia, before or simultaneously with the uredospores, or before the teleutospores. The conidia, as far as observations go, do not generally germinate under ordinary conditions.