158 ANIMAL AGGREGATIONS 



by Wildiers (1901) to a mysterious organic substance which he be- 

 Heved to be necessary for the prohferation of yeast cells. After ap- 

 proximately a quarter of a century of work upon the subject, Tanner 

 summarizes the situation regarding bios as follows : 



"One group of investigators denies the existence or need on the 

 part of the yeast plant, of a substance like 'bios.' They feel that 

 yeasts will grow without this accessory substance. 



"Another group believes that 'bios' is necessary for the growth of 

 yeasts. They are unable to secure growth of yeasts in pure solutions 

 without it. Certain of these investigators have reported fractiona- 

 tion of 'bios' into components which are necessary to one another. 



"A third group of investigators believe that yeast will grow in 

 pure nutrient solutions without 'bios' but that the addition of a 

 'bios' containing substance may cause increased growth. Whether 

 this acceleration in growth following the addition of a 'bios' con- 

 taining substance is due to 'bios' or to some other factor in the pre- 

 parate has not been satisfactorily estabhshed. In this connection it 

 is well to point out that even a medium such as beer- wort which is 

 rich in 'bios' may be improved by the addition of other 'bios' con- 

 taining substances. 



"A fourth group may also be recognized including those who have 

 isolated 'bios' or substances having 'bios' properties." 



Throughout his review Tanner's attitude is satisfactorily critical; 

 and from a study of it, supplemented with certain of the original 

 research reports, it seems to me that the evidence favoring the view 

 that there is a growth-promoting substance which markedly stimu- 

 lates yeast growth is too strong to be disregarded at the present 

 time. Concerning whether the yeast cells are able to synthesize this 

 substance from nutrient solutions lacking it, as has been claimed, the 

 evidence is not yet so clear. 



The same problem in a somewhat different guise is met with in the 

 studies concerning whether inorganic substances taken alone are ade- 

 quate for the growth of green plants. This question was most re- 

 cently raised by Bottomley (191 5 and subsequent papers) and 

 Mockeridge (1920, 1927), who showed that certain complex organic 

 substances, when partially broken down by bacterial action, stimu- 



