3 88 SCIENCE PROGRESS 



organisms, but we are well repaid by the fact that we have 

 approached appreciably nearer to an understanding of their 

 peculiar habits. It is no longer true to say that they derive 

 their energy by the oxidation of ferrous to ferric compounds. 

 What their physiological relations with iron are will be dis- 

 cussed in the next section ; it will suffice here to state that 

 the absorption of iron compounds is not necessary for their 

 growth. Another curious fact in connection with these 

 organisms is the great uncertainty which attends all attempts 

 to cultivate them artificially. The virility of the growth of 

 Crenothrix at certain periods has already been mentioned. 

 Garrett mentions most unlikely media on and in which he 

 during such a period secured growth. But at ordinary times 

 the artificial growth of Crenothrix has not been found possible. 

 The same applies to Cladothrix. Whilst Hoflich secured 

 growth with the greatest of ease, others have succeeded only 

 after much trouble, and when successful the growth has never 

 been a prolific one. There is no difficulty in the matter of the 

 preparation of the nutrient medium, for it consists merely in 

 the addition to ordinary drinking water of small quantities of 

 such substances as flesh-extract or peptone or ferrous-am- 

 monium citrate. The sole difference between the ingredients 

 of the successful and the partially successful or unsuccessful 

 media has lain only in a difference of the water employed, 

 this naturally varying with the place of abode of the investi- 

 gator. Whether the determining factor is the presence in 

 certain waters of minute quantities of substances which play 

 the same role as the hormones in the animal kingdom, it is 

 impossible as yet to say. Bottomley claims that they exist, 

 but his evidence as yet is unconvincing. Molisch obtained his 

 best artificial cultures of Leptothrix ochracea by the addition to 

 water of 0^25 per cent of mangan-peptone, and obtained pure 

 cultures by the use of this medium solidified with 10-per-cent. 

 gelatine. He describes its activities in a number of liquid and 

 solid media. It is aerobic, liquifies gelatine, grows in light 

 and in darkness, and shows a tendency to form short rods which 

 exhibit a decided motility. 



Hoflich gives a very complete account of artificial cultures 

 of Cladothrix dichotoma. The addition of half a gramme of 

 flesh extract to a litre of water was sufficient to ensure an 

 abundant growth. Cladothrix liquefies gelatine very slowly ; 



