To this series belong the lowest of the Basidiomycetes. The _basidia_ appear in two principal forms (1 and 2 on page 144) and are _divided_ into four cells, either transversely or longitudinally, each division forming a sterigma which abstricts a basidiospore. The first three orders, Uredinaceæ, Auriculariaceæ, and Tremellaceæ have _gymnocarpic_ fruit-bodies, while those of the Pilacraceæ, on the contrary, are _angiocarpic_. Order 1. =Uredinaceæ (Rusts).= All the Rust-Fungi are parasites, their mycelium living in the interior of the stems and leaves of their hosts, causing red, brown, or black spots--hence their name--and malformations, sometimes of considerable size. The Rust-Fungi are gymnocarpic and destitute of a hymenium; for these reasons they are regarded as the simplest order of the Basidiomycetes. They are entirely parasitic, and their filamentous, branched mycelium ramifies in the intercellular spaces of its host, and often protrudes haustoria into the cells. The mycelium is perennial should it enter a woody tissue; it may also hibernate in the rhizomes of perennial herbs and permeate the shoots springing from them, but in the majority of the Rust-Fungi the mycelium has a very limited growth. The chief means of reproduction of the Rust-Fungi are the _chlamydospores_, which in the more highly developed species occur in three forms, namely, the teleuto-, æcidio-, and uredo-spores. The spores, in the host, are formed immediately beneath its epidermis, which is ruptured on the ripening of the spores, with the production of “rust,” brown, red, or black spots. Those chlamydospores which produce basidia are termed _teleutospores_. The spore on germination produces a _transversely divided basidium_, “promycelium,” on which basidiospores, “sporidia,” generally four in number, are produced on lateral sterigmata. This basidio-fructification is _gymnocarpic_; the basidia neither form a hymenium nor a fruit-body (only _Cronartium_ and _Gymnosporangium_ have a slight indication of a basidio-fructification). Many Rust-Fungi, in addition to basidiospores, have small, _unicellular conidia_, “spermatia,” which are borne in conidiocarps, “_spermogonia_.” The ~TELEUTOSPORES~ (_Winter-spores_) may be either unicellular or multicellular; in the majority of cases they are enclosed in a hard outer cell-wall, the exospore, which in some cases is very strongly developed; they have also a long or short stalk, the remains of the spore-bearing hypha. Each cell of the teleutospore has _one germ-pore_ (a thin portion of the wall, for the protrusion of the germ-tube; in _Phragmidium_ and _Gymnosporangium_ there are, however, several germ-pores). The colour of the teleutospores is generally much darker than that of the uredospores, and it is by these that the majority of the Rust-Fungi _hibernate_.