OTHER FORMS OF SYMBIOSIS 137 



men tons bacterium forming " zooglea." The second is a lactic-acid- 

 producing bacillus, and the third is a yeast. By these agents a 

 fermentation is set up in the milk of cows, goats, or sheep. " The 

 yeast and the bacteria, either jointly or separately, split up the 

 lactose or milk-sugar into two other sugars, galactose and glucose. 

 The yeast then forms alcohol from the latter, and the bacterium 

 lactic acid from the former" (Green). This filamentous bacillus 

 probably affects the casein. The outline is, it is true, only the 

 probable course of action, as full details as to the whole function of 

 the separate factors are not yet known. 



The gingerbeer plant is the agent of fermentation in the so-called 

 "stone gingerbeer," and is composed essentially of two organisms, 

 one a yeast, Saccharomyces pyriformis, the other a bacillus, Bacterium 

 vcrmiforme. It rarely happens that these two forms are found pure, 

 there being as a rule an admixture of other organisms with them. 

 Professor Green describes B. vermiforme as growing in two different 

 ways, namely, as long rods or convoluted threads, invested by a 

 translucent wrinkled sheath, and as constituent microbes contained 

 within the sheath, yet able to escape from it. The sheathing form 

 of the organism can only be produced when oxygen is replaced by 

 carbon dioxide. In the symbiotic association the yeast absorbs the 

 oxygen, and during its fermentative activity produces carbon dioxide, 

 thus providing the necessary conditions for the formation of the 

 sheath. The bacterium benefits by substances excreted by the yeast, 

 and the latter profits in its turn by the removal of these matters 

 through the agency of the former. The yeast sets up the usual 

 fermentation of cane-sugar. 



A third organism manifesting symbiosis occurs in Madagascar 

 as a curious gelatinous substance found attacking the sugar-cane, 

 and consisting again of a yeast and a bacterium associated together 

 in very much the same way as are the organisms in the gingerbeer 

 ferment. 



Before leaving this subject of symbiosis as illustrated in the 

 lichens, in Winogradsky's Clostridium, in the nodule-bacteria, in the 

 gingerbeer plant, and in Kephir, we may suitably inquire whether 

 anything is at present known as to how the symbionts are related to 

 each other. Obviously the matter presents many difficulties, and 

 the problem is by no means solved. There are, however, three chief 

 hypotheses. First, the provision of definite food materials by the 

 one symbiont for the other may be an important factor ; e.g. an alga 

 supplies a fungus with carbo-hydrates, or a fungus converts starch 

 into the fermentable sugars which the associated yeast needs. In 

 other cases the advantage derived is one of protection from some 

 injurious agent; e.g., the aerobic bacterium prevents the access of 

 oxygen to the anaerobic one. Thirdly, there is some evidence, 



