Page:Encyclopædia Britannica, Ninth Edition, v. 21.djvu/427

Rh SCHIZOMYCETES 405 FIG. 15. Beggiatoa alba. (After Zopf.) Curved and spiral forms. C, D, separ- ated spirally-wound pieces, which are breaking up still further in H. E, motile tpirilluin form with a cilium at each end. (x 540.) swarm. But, in addition to these straight and more or less rigid forms (which, it will be noticed, simulate Ehren- berg and Cohn's "genera " Micrococcus, Bacterium, Bacillus, and Leptothrix so closely that any of them observed alone would undoubtedly have been formerly placed apart in one of those " gene- ra "), it is interesting to find that some of the filaments become spirally twisted and simulate Spirillum, Spiro- cheete, and Vibrio, the dis- tinctions depending on the relative length and thick- ness of the filament, and the closeness or steepness of the coils. Moreover these twisted filaments also break up into shorter gliding or ciliated portions, which at length fall into rodlets and " cocci" as before. A branched zoogloea form also occurs, and this con- tains cocci, bacterium-like or bacillar rods, or filaments resembling Leptothrix or Vibrio according to circum- stances. In Lankester's Bacterium rubescens we have an- other species which is variable in a high degree. Many other Schizomycetes have now been shown to be more or less pleomorphic, and the researches of Lankester, Nageli, Zopf, Miller, Kurth, De Bary, and others have laid the foun- dation for a know- ledge of the cir- cumstances which induce the changes in form referred to; it is at least certain that alterations in the nutritive me- dium, in the quan- tity of oxygen at the disposal of the organism, and in the temperature, &c., play their part in the matter. It by no means follows, however, that because some species are pleomor- phic all must be so, and still less that no species of Schizo- mvcetes or only i, FIG. 16. Qladothrix dichotoma. A, branched plant, One exist at all; the branches in part spiral and of the form known tTinoo whr^ rlpnv tViP s Vibrio (a) or Spirillum (6) (slightly magnified), tnose WHO deny t, B) a j ong v c 'oiled branch more highly magnified, existence of Species 0, portion of branch resembling Spirillum at one .1 Q t, ' end and Vibrio at the other. D, coiled branches. among the bcniZO- Oj not segmen ted; 6, c, segmented into rodlets and mvcetes On the evi- cocci. E, -SpirocAajte-Uke portions breaking up into , J rodlets and cocci. dence to hand must, to be logically consistent, deny the existence of species altogether. But even if that be allowed, some name of similar intention must be employed to denote any group of organisms which within our experience exhibit periodi- cal repetitions of a process of development, i.e., all the individuals of successive generations go through the same phases periodically. It matters not that variations ill- defined deviations from an average or "type" occur on the part of individuals or generations; the periodically repeated life-history or development marks what we term a species. The difficulties presented by such minute and simple organisms as the Schizomycetes are due partly to the few "characters" which they possess, and partly to the dangers of error in manipulating them; it is anything but an easy matter either to trace the whole development of a single form or to recognize with certainty any one stage in the development unless the others are known. This being the case, and having regard to the minuteness and ubiquity of these organisms, we should be very careful in accepting evidence as to the continuity or otherwise of any two forms which falls short of direct and uninterrupted observation. The outcome of all these considerations is that, while recognizing that the "genera" and "species" as defined by Cohn must be recast, we are not warranted in uniting any forms the continuity of which has not been directly observed; or, at any rate, the strictest rules should be followed in accepting the evidence adduced to render the union of any forms probable. 1 CLASSIFICATION. The limits of this article prevent our ex- Classifi- amining in detail the system of classification proposed by Cohn, cation, or the modifications of it followed by other authorities. Zopf, in the third edition of his work (1885), proposes a scheme based on the modern views as to the pleomorphism : we must refer to the original for the details, simply remarking that, apart from the ex- treme views accepted by the author, his system is impracticable to a degree and recognized by him as provisional only. Indeed any such classification must be provisional, for we are at the threshold only of a knowledge of the Schizomycetes. The best starting-point for a modern classification of these organisms is that suggested by De Bary the two modes of forma- tion of the spores, and as a provisional scheme, and simply to facilitate comparison of the groups, we might perhaps employ De Bary's two groups, and a third one to include those simple forms which show no trace of spore-formation. Many gaps exist, and many changes will probably have to be made. Meanwhile it might be advisable to classify the Schizomycetes provisionally as follows : GROUP A. Asporese. There are no spores distinct from the vegetative cells. I. COCCACE.E (figs. 6 and 7). Genera : 1, Micrococcus (and Streptococcus); 2, Sarcina (and Zopf s Merismopedia); 3, Ascococcus. GROUP B. ArthrosporesB (De Bary). The vegetative cells differ in shape, size, growth, or other characters from the spores : the latter are produced by segmenta- tion. II. ARTHUOBACTERIACE.E. Genera : 4, Bacterium (fig. 8); 5, Lcuconostoc; 6, Spirochsete (?). III. LEPTOTRICHE2B. Genera: 7, Crenothrix (fig. 13); 8, Beggiatoa, (6gs. 14 aud 15); 9, Phragmidothrix (?); 10, Leptothrix. IV. CLADOTRICHE.E. Genus : 11, Cladothrix (fig. 16). GROUP C. Endosporess (De Bary). Genera: 12 (figs. 9-12), Bacillus (and Clostridium); 13, Vibrio (?); 14, Spirillum (at least in part). 2 1 Eay Lankester, Quart. Jour. Micr. Sc., 1873 and 1876; Nageli and Buchner, Niedere Pilze, 1882; Billroth, Untersuchungen uber die Vcgetationsformen der Coccobacteria septica, Berlin, 1874; Klebs, numerous papers in Archiv f. exp. Pathol. und Pharmacol.; Kurth, Bot. Zeitung, 1883; Prazmowski, Biol. Centralblatt, 1884; Zopf, Zur Morph. der Spaltpflanzen, Leipsic, 1882; Cienkowski, Zur Morpho- logie d. Bacterien, 1876. 2 For the definitions of the genera (and species) the reader is re- ferred to the special works, especially those of Zopf and De Bary; also Winter-Rabenhorst, Kryptogamen Flora Pilze, i., 1881; and Grove, Synopsis of the Bacteria and Yeast-Fungi, 1884.