366 THE SULPHUR BACTERIA. 



ing off towards the free end, where, for example, their diameter 

 is only 1.5/0,, compared with 2.0 /z at the 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 Thiotlirix. 



Here also the thickness of the threads constitutes a criterion 

 for the classification of species. One of them, named by Wino- 

 gradsky Thiotlirix nivea, has a diameter of 2-2.5 ^ near ^ ne base, 

 1.7 fi in the middle, and 1.4-1.5 /a at the tip. In a second species 

 the diameter is almost uniformly i.o-i.i //, throughout the whole 

 extent of the thread. It is known as Thiothrix tenuis, and is 

 probably identical with a fission fungus discovered by ENGLER 

 (L), 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 (Thiothrix 

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

 measure only 0.4-0.5 & 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 object by their 

 ability to proceed at will to the surface of the liquid. Conse- 

 quently this species gains the upper hand in stagnant or quietly 

 flowing waters, in which they search about so eagerly that very 

 little of the oxygen diffusing into the water can reach the bottom 

 where the Thiothrix species rest. The latter, however, have the 

 advantage in rapid running water, the loose Beggiatoa species being 

 washed away by the current. In either event, whitish mucinous 

 masses highly characteristic of sulphur springs accumulate in time 

 e.q. those of Bareges in the French Pyrenees and are known 

 in France as baregine or glairine. 



