Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/866

 830 more than tropical, and that those of a woody texture, like the Polyporei, inhabit tropical in preference to temperate countries. In the Arctic regions fungi are represented almost solely by the Lichenes. Of the distribution of micro scopic fungi&quot; nothing of any value is known. Classification. Fungi are divided into orders and sub orders as follows : Phycomycctcs. 1. Saprolcgniese. 2. IVronosporesB. 3. Mucorini. Hypodcrmice. 1. Uredinese. 1. 11, IV. Ascomycctcs. 1. Discomycetes. 2. Erysiphese. 3. Tuberacete. 4. Pyrenomycetes. 5. Lichenes. 2. Ustilagineae. III. Easidiomycetes. V. Myxomycclcs. 1. Tremellini. 2. Hymenomycetes. VI. Schizomycetet. 3. Gasteromycetes. The above classification is broadly the same as that pro posed by Professor De Bary. The principal characters of these orders and suborders are described below. Order I. PHYCOMYCETES. The Phycomycetes possess both sexual and asexual organs of reproduction. The sexually produced reproductive body gives rise to a genera tion bearing first asexual and then sexual organs, and the asexually produced reproductive body gives rise to a similar generation. The mycelium in this order consists of long densely-ramifying tubes or hyphae, on which the repro ductive organs are borne. Suborder I. Saprolegniece. In this order the male organs are called antheridia and the female organs oogonia. In the monoecious forms these arise beside each other on the same plant. The oogonia are usually to be found at the end of short branches of the mycelium very rarely sessile or intercalary. They are globular cells with one enclosing membrane containing dense protoplasm. In some cases (as in Saprolegnia monoica) the cell-wall soon becomes resorbed at various places, and the protoplasm divides into several globular bodies, which float in a watery fluid, the whole enclosed by the wall of the oogonium. In Aphanomyces, Pythium, and several species of Saprolegnia, only one globe is formed in the oogonium. The fluid in which these globes float is part of the watery contents of the globes ex pelled when the contraction took place. While these altera tions are proceeding in the oogonia, the antheridia grow out from the same or some adjoining branch of the mycelium. These are thin, crooked hyphae, which frequently entwine the stalks of the oogonia in their growth ; the tips of the tubes, however, on reaching the wall of the oogonium ad here, and the upper part becomes separated from the rest by a septum. This process is usually contemporary with the contraction or separation into globules of the contents of the oogonia. The antheridia then push through the walls one or more tubes, which discharge from the points minute motile spermatozoids (about -g-^j- millim. in size), and these fertilize the globules. The globules then assume a cellulose membrane and are called oospores. The oogonia are formed in the dioecious as in the monoe cious species, but the antheridia in a totally different way. They arise within bladder-like protuberances of the mycelium, which are divided by transverse septa into a series of cells, each of which is an antheridium. In Sapro legnia- dioica, the antheridium breaks up into numerous spermatozoids ; but in Achlya dioica a preliminary division of the antheridium into several portions precedes the forma tion of the spermatozoids. When free these spermatozoids move actively by means of one long cilium. Fertilization, and the consequent formation of oosporos, are effected in a similar way to that described as occurring in the monoecious forms. ^ Doubt has been expressed as to the described monoecious fertilization, but on the whole it may be re garded as generally true. The oospores of the Saprolegniece are double-coated. After a long period of rest they omit germ-tubes, but it has been recorded that they sometimes produce zoospores, when the resting state is shorter. The zoosporangia or asexually-produced organs of repro duction are terminal cells (very rarely intercalary), some times irregularly cylindrical, sometimes ovoid, of a constant form in some genera (Saprolegnia, Achlya) but variable in others (Pythium, Monoblepharis). The reproduction by means of the zoosporangia is much more extensive than that by means of the oospores. Each zoosporangium contains a great number of zoospores. The mycelium is composed of long ramifying tubes, of consider able diameter, and nearly always aseptate. The Saprolegniece grosv for the most part in water, chiefly on the dead bodies of insects. One species, Saprolegnia ferax, is notable on account of its extensive ravages on the bodies of fishes (especially salmon) and other water animals. Suborder II. Peronosjwrece. The Peronosporece are very closely allied to the Saprolegniece the modes of repro duction both sexual and asexual being similar to those of the monoecious forms of the latter suborder. The oogonia are terminal as a rule, and the protoplasmic contents contract always into one central globule. Two forms of germination have been observed in the oospores. In Cystopus they burst FIG. 1. C ystopus Candidas. A. a, conidia; 6, conidiophorcs; r, conidium emitting zoospores; (/, free zoospore. B. og, oogonium; os, oospherc; an, antheridium. C. Formation of zoospores by oospores. z, free zoospores. (After De liary.) (x400.) and produce zoopores, and in Peronospora the usual mode of germination is by the emission of a hypha. As regards the asexually-produced spores the same rule holds. In Cystopus and Phytophthora they are zoosporangia, producing in the former case each the same number of zoospores as the oospores; and in Peronospora they are usually conidia, emitting hypha3 on germination. Both zoospores and hyphse produce mycelia directly. The zoosporaugia of Cystopus are borne in vertical series on short, club-shaped filaments ; the conidia of Peronospora in most cases, and the zoosporangia of Phytophthora, on branching, tree-like sporophores. The mycelium, which is usually septate and ramifies densely, possesses frequently haustoria of various shapes. The Peronosporece are parasitic on living phanerogams, the tissues of which they penetrate and destroy. The most notable example is the well-known potato-disease, PliytQ- phthora infestam, the ravages of which are too well known to need description. Suborder III. Mucorini. Sexual reproduction in this