In the Uredines, to which much of this volume is devoted, the genera known as _Lecythea_ and _Trichobasis_ are by some mycologists excluded altogether, as containing only species which are mere forms of more highly-developed uredines, such as _Puccinia_, _Aregma_, and others. On the other hand, they are retained by those who possess a lingering doubt whether both forms may not be distinct, though developed from the same pustule. As the two forms are distinct in appearance, it will better answer our present purpose to treat them separately, notwithstanding the belief that, in a scientific point of view, the evidence is all in favour of their union. In fungi of this kind the mycelium, or delicate root-like threads, consists of thin filaments, which are spread through all parts of the plant occupied by the parasite, traversing the intercellular passages, but rarely perforating and entering the cells. This compacted and interwoven mycelium forms a kind of cushion beneath each pustule, on which the fruits of the parasite rest. By the increase of this cushion and the swelling of the fruit, the epidermis which covers them is distended, and ultimately ruptured, so that, when ripened, the spores escape. It must be remembered that the fruit is of from two to four kinds. Small bodies, called _spermatia_, which are derived from the spermogones, and which have not yet been known to germinate; _Stylospores_, produced either singly, or in bead-like, or moniliform, strings, and which either precede or are associated with the true spores; _Spores_, sometimes simple, but often complex; and _Sporidia_, or secondary sporules, which are produced on the germinating threads of the true spores.