222 METABOLISM 



quent renewal of the fluid, we maintain a constant proportion of sulphuretted 

 hydrogen. If we now introduce under the cover-glass some active plants of 

 Beggiatoa, we can see the filaments rapidly wandering towards the edge of the 

 preparation, and forming a thick white border (visible to the naked eye) at a 

 distance of about i mm. from the edge of the cover-glass. Beggiatoa avoids 

 the periphery of the drop, where oxygen is abundant, and also the central 

 region, which contains no oxygen. On the fluid being renewed, however, the 

 filaments retreat more and more to the centre of the preparation as the 

 sulphuretted hydrogen gets gradually used up. When a Beggiatoa filament 

 has found the region of optimum oxygen tension, it is able by small changes of 

 position to reach zones either where the sulphuretted hydrogen is abundantly 

 present, or where oxidation of that gas can take place. 



The behaviour of Beggiatoa in nature conforms to that which it exhibits 

 in a microscopic culture. In a mud swamp, just as in the culture, it always 

 attempts to find a region with an optimum oxygen tension, as it lives on the 

 surface of the culture, inhabiting places subject to overflow and avoiding 

 deeper hollows. Hence both oxygen and sulphuretted hydrogen play a part 

 in determining its distribution, since a definite and not excessive concentra- 

 tion of that substance is essential to its existence. 



But Beggiatoa not only accumulates sulphur in its cells, but also dissolves 

 it out, and both processes may go on simultaneously, though it is not 

 possible to note this fact directly. The removal of sulphur from the cells 

 can only be determined if formation of the substance is prevented by 

 withdrawal of sulphuretted hydrogen. The amount of sulphur removed in 

 this way is enormous. When WINOGRADSKY supplied an active culture 

 every two or three hours during one day with fresh supplies of Langenbriicken 

 water containing sulphuretted hydrogen, the filaments became full of sulphur 

 by evening (Fig. 39, a), and after the supply of sulphuretted hydrogen 

 had ceased, the whole of the sulphur accumulated became dissolved out in 

 twelve to fifteen hours. Fig. 39, b, shows the filaments after remaining for 

 twenty-four hours without any sulphuretted hydrogen, while Fig. 39, c, 

 represents the same culture after an interval of forty-eight hours. According 

 to WINOGRADSKY'S estimates, the protoplasm of one cell uses up daily an 

 amount of sulphur equal to four or more times its own weight. Such quantities 

 as these render it impossible that the sulphur is devoted to the formation of 

 proteid or the synthesis of any other substances, since Beggiatoa grows relatively 

 slowly, only rarely doubling the length of its filaments in twenty-four hours. 

 As a matter of fact, it can be shown that the sulphur subserves an entirely 

 different purpose ; it is oxidized in the cell, and the sulphuric acid so formed 

 in turn attacks the carbonates taken into the cell from the water, and passes 

 back again to the medium in the form of calcium sulphate. Beggiatoa oxidizes 

 the sulphuretted hydrogen into sulphuric acid, and to some extent accumu- 

 lates the intermediate product, sulphur, as a reserve. If the supply of sul- 

 phuretted hydrogen be limited by offering to the plant extremely dilute solu- 

 tions, it is possible to grow these Bacteria without their showing any accumula- 

 tion of sulphur in their cells. Just as the formation of sulphur from sulphuretted 

 hydrogen may be diminished, so also may the formation of sulphuric acid 

 from sulphur, without the aid of the Bacteria ; while, however, the former 

 process appears to go on in the cells of Beggiatoa about as actively as in water, 

 the organism has obviously the means of greatly accelerating the formation 

 of sulphuric acid (possibly by means of an enzyme ?). 



The oxidizing process above described, which, as we have said is absolutely 

 essential to the existence of Beggiatoa, is one which is eminently characteristic of 

 this organism, but which is absent in the vast majority of cases. Yet it is not the 

 only peculiarity of this remarkable plant. It possesses neither chlorophyll nor 

 any other allied colouring matter which would lead us to imagine that it was 



