280 THE SULPHUR BACTERIA 



200. The Species of the Genus Thiothrix, 



which has been newly established by Winogradsky, differ from Beggiatoa by the 

 absence of free motility, they being sessile, i.e. attaching themselves at one ' 

 extremity by means of a mucinous sucker to the walls of 

 the culture vessel, the cover-glass of the microscopical 

 preparation, to stones, remains of plants, and similar 

 quiescent substrata in the situations where they occur 

 naturally; whilst the other end extends into and grows 

 in the liquid. Such a one is shown in Fig. 73. In this 

 genus, also, the articulation of the threads is ordinarily 

 concealed by the abundant content of sulphur, but if the 

 latter be washed out with absolute alcohol and the cells 

 stained, e.g. with fuchsine, the transverse walls are plainly 

 revealed. The length of the joints gradually increases 

 towards the free end, as will be seen from the subjoined 

 measurements given by Winogradsky : Length of joint 

 near the point of attachment, 4-8.5 p; at the apex, 

 FIG. 73 .-Thiotrix nivea. S ~ I 5 /* However, there is no scarcity of considerably 

 . shorter cells. So far as the breadth of the threads is 

 Group of young threads , ,, . ... , ,. ,. . 



with one end firmly at- concerned the above -conditions are reversed, the threads 

 tached to the substratum tapering off towards the free end, where, for example, 



Z^wZ. their diameter is on] y '-5 * compared with 2.0 M at the 

 900. (After Winogradsky.) base. Consequently the cells are more slender towards 

 the tip. 



A second characteristic point of difference from the genus previously described 

 is the appearance of a (merely slight) sheath, whereby the moribund members 

 are partly held together, whereas the Beggiatoa threads at this stage break into 

 short fragments and finally into separate cells. 



A third characteristic of the genus Thiothrix is the dislocation (termed 

 conidia-formation by Winogradsky) of the uppermost joint of the thread. The 

 rod-shaped cell, thus loosened from the chain, crawls a short distance along the 

 solid substratum, then develops a mucinous sucker and grows into a new thread, 

 from which in turn conidia subsequently wander and settle in the vicinity, the 

 result being the formation of the whitish, tufted, thread colonies characteristic 

 of Thiothrix. 



Here also the thickness of the threads constitutes a criterion for the classi- 

 fication of species. One of them, named by Winogradsky Thiothrix nivea, has a 

 diameter of 2-2.5 ** near the base, 1.7 p. in the middle, and 1.4-1.5 p, at the tip. 

 In a second species the diameter is almost uniformly i.o-i.i p throughout the 

 whole extent of the thread. It is known as Thiothrix tennis, and is probably 

 identical with a fission fungus discovered by ENGLER (I.), in the so-called " dead 

 ground " of the Bay of Kiel, and which he held to be a Beggiatoa and called by 

 the specific name B. alba var. universalis. The threads of a third species (Thio- 

 thrix tenuissima), from a sulphur spring at Adelboden (Switzerland), measure 

 only 0.4-0.5 p. in breadth. W. ZOPF (VII.) regarded the sessile sulphur bacteria 

 as belonging to the morphological cycle of the Beggiatoa, and named them 

 " sessile Beggiatoa," until Winogradsky proved that two distinct genera are here 

 in question. 



As will be shown later on, the life of the sulphur bacteria is indissolubly 

 connected with the presence and availability of free oxygen. In the mode of 

 satisfying their needs in this respect the two genera differ. The Beggiatoa, 

 being endowed with the power of locomotion, can more readily accomplish this 



