Rusts (order Uredinales) are minute plants, parasitic in the tissues of higher plants. (Fig. 21). They consist of branching filaments which grow through and live upon the host tissues. Certain branches cluster together and form rows of spores by terminal abstriction, each row being initiated by a sexual union of the terminal spores germinate each produces a short fila ment (promycelium) on which four minute spores (sporidia) develop. There are thus four kinds of spores in a typical rust plant, and these have been taken to characterize as many stages in the plant's life history, namely: (1) Cluster-cup stage (aPciospores); (2) Red Rust (urediniospores); (3) Black Rust (telio spores); (4) Promycelium (sporidia). In many rusts these stages occur on the same host in the order given, but in some the first stage occurs on one host, and the remaining stages on an other. The latter is the case with the stem rust of wheat, in which the cluster-cups ,occur on barberry leaves, and the mciospores then ger minate upon and infect the leaves of the wheat, on which the red rust, black rust and promy celium then follow in succession.
Smuts (order Ustilaginales) are still more parasitic than the rusts, and as a consequence have suffered still greater degeneration (Fig. 23). They grow wholly within their hosts, and do not even bring their spores to the surface. They consist of branching filaments which pene trate the tissues of their hosts, and at last form spores homologous with the tcliospores of the rusts. The latter in germinating produce sporidia. Comparing the rusts with the smuts we note that while there are four stages in the or subterminal cells of two adjacent filaments. (eciospores) (Fig. 22 A). The fungus arising from the germination of these spores forms at first large numbers of red or orange colored single terminal spores (urediniospores) (Fig. 22 B), and still later, the one- to several-celled thick-walled, usually dark colored teliospores. (Fig. 22 B). These all begin within the host tissues, but they eventually break through the epidermis into the air. Lastly, when the telio former, there are but two in the latter, th' first and second having apparently disappeared Imperfect Fungi.— At this point should be mentioned the so-called imperfect fungi, an im mense aggregation of many thousand species (16,000 or more), of which we know but one stage (apparently the first) and so are unable to assign them to their proper place in the system. They are minute and mostly parasitic plants, occurring in the tissues of higher plants, and sending their spore-bearing branches out into the air. Some plants formerly placed here have been found to be early stages of certain sac fungi (black fungi, or their relatives) and it is suspected that most, if not all, of them will eventually be so disposed. At present they are grouped under three general kinds, as follows: Spot Fungi (Spheropsidacee), which pro duce whitish or discolored spots, and later de velop closed, spheroidal cases, containing free spores. Septoria and Phyllosticta are common
genera Fungi (Melanconiacee) are like the spot fungi, but there are no spore cases, the spores developing in masses beneath the epidermis which they eventually rupture. Giceosporium is a common genus.
Molds (Moniliacece and related families) produce their spores on branches which grow out through the stomata of the host. Here we find the parasitic species of Ramularia Cer cospora, efc, and the mostly saprophytic species of Monifici. Botrytis, etc.
Basidium Fungi (class Basidiomycetece). — The distinguishing mark of this class is that the spores are produced externally upon club shaped or rounded terminal cells. (Fig. 24). These club-shaped, spore-bearing cells are tech nically known as basidia (singular, basidium), whence the scientific name of the class. The basidia of this class are regarded in this dis cussion as the homologues of the spore-sacs (asci) of the preceding class.
About 14,000 species of fungi of this class are known. Many attain to considerable di mensions, especially their fruits. They are typically saprophytic, but it is now known that many of them are more or less parasitic, also, when the opportunity offers.
About 210 families are commonly recognized, four of which, only, will be noticed here.
(Lycoperdacece) are saprophytes whose branching filaments penetrate decaying wood or earth rich in organic matter, and finally produce globular fruits which rise above the surface. (Fig. 25), These fruits are filled with tortuous canals whose walls are studded with basidia on which the spores are produced. At maturity the interior tissues of the fruits del iquesce, setting free the spores, which escape into the air a little later as a dusty cloud, by the rupture of the fruit wall. From these spores new plants are produced, but we do not know the whole life history of these common fungi. Although the sexual organs should pre cede the formation of the fruits, they have not yet been observed.
(Phallacece) are closely related to the puff-balls, which they closely resemble in all stages excepting the last. Here the spore bearing portion of the globular fruit is confined to a vertical, circular layer of tissue about mid way between the centre and the circumference. At maturity the spore-bearing tissues deliquesce and at the same time the tissues below rapidly elongate, bursting the fruit-wall and carrying up the spores into the air. (Fig. 26). These fruits have very bad odors, which attract insects, and it is thought that these help to distribute the spores. Stink-horns are from an inch or two to six or more inches in height, and grow com monly in lawns and pastures, where their presence is indicated by their intolerable odor.