94 SULPHUR BACTERIA 



Sheath Formation, 



The existence of a sheath-covering in Beggiaioa was re- 

 corded as early as 1884 by Winter, in Rabenhorst's Krypto- 

 gamen Flora; and in more recent work it is mentioned by 

 Selk, 1907, and by Keil, 191 2. Further, Koppe, 1923, has 

 recently shown that Beggiatoa arachnoidea occasionally be- 

 comes attached to objects in the water by the development 

 of cushions of slime at one end of the thread. The occurrence 

 of slime formation, however, as a normal feature in the life- 

 history of Beggiatoa alba has not hitherto been recognized. 



Sheath formation is characteristic of all conditions of 

 the Beggiatoa cell, but in motile threads its amount is too 

 slight to be perceptible without special treatment. A similar 

 formation is found in the iron bacteria, e.g. Crenothrix poly- 

 spora, but whereas in these the sheath formation is a normal 

 occurrence of healthy cells, in Beggiatoa alba, as in yeast, 

 active slime formation is abnormal. The autolysis of the cells 

 is always accompanied by excessive slime formation (see below). 

 The thread appears to be divided transversely by walls such as 

 are normally formed in the division of cells of the higher plants. 

 In this case, however, they are merely transverse bands of 

 hardened slime. Their mode of formation is described below. 



Growth. 



Under favourable conditions a thread grows to double its 

 length in twenty-four hours, and during this time undergoes 

 one division. According to Winogradsky, this rate of growth 

 continues as long as the thread receives an adequate supply 

 of sulphuretted hydrogen. The amount of this substance 

 daily absorbed by a normal thread is substantially greater 

 than the amount of protoplast in that thread. Growth is 

 intercalary. 



Autolysis.* 



Mass cultures of Beggiatoa alba in nature usually disappear 

 completely with the advent of unfavourable circumstances. 



* Autolysis refers to a process initiated inside the cell which results in 

 the disappearance of the organism by self-liqueiaction. 



