In the other section of _Coniomycetes_ the species are parasitic upon, and destructive to, living plants, very seldom being found on really dead substances, and even in such rare cases undoubtedly developed during the life of the tissues. Mostly the ultimate stage of these parasites is exhibited in the ruptured cuticle, and the dispersion of the dust-like spores; but in _Tilletia caries_, _Thecaphora hyalina_, and _Puccinia incarcerata_, they remain enclosed within the fruit of the foster-plant. The different genera exhibit in some instances a liking for plants of certain orders on which to develop themselves. _Peridermium_ attacks the _Coniferæ_; _Gymnosporangium_ and _Podisoma_ the different species of Juniper; _Melampsora_ chiefly the leaves of deciduous trees; _Roestelia_ attaches itself to pomaceous trees, whilst _Graphiola_ affects the _Palmaceæ_, and _Endophyllum_ the succulent leaves of houseleek. In _Æcidium_ a few orders seem to be more liable to attack than others, as the _Compositæ_, _Ranunculaceæ_, _Leguminosæ_, _Labiatæ_, &c., whilst others, as the _Graminaceæ_, _Ericaceæ_, _Malvaceæ_, _Cruciferæ_, are exempt. There are, nevertheless, very few natural orders of phanerogamous plants in which some one or more species, belonging to this section of the _Coniomycetes_, may not be found; and the same foster-plant will occasionally nurture several forms. Recent investigations tend to confirm the distinct specific characters of the species found on different plants, and to prove that the parasite of one host will not vegetate upon another, however closely allied. This admission must not, however, be accepted as universally applicable, and therefore it should not be assumed, because a certain parasite is found developed on a special host, that it is distinct, unless distinctive characters, apart from habitat, can be detected. _Æcidium compositarum_ and _Æcidium ranunculacearum_, for instance, are found on various composite and ranunculaceous plants, and as yet no sufficient evidence has been adduced to prove that the different forms are other than varieties of one of the two species. On the other hand, it is not improbable that two species of _Æcidium_ are developed on the common berberry, as De Bary has indicated that two species of mildew, _Puccinia graminis_, and _Puccinia straminis_, are found on wheat.