MICRO-ORGANISMS 249 



cultivate these micro-organisms when removed from their hosts, 

 but the results are by no means conclusive. Berlese, in 1906, for 

 example, succeeded in growing the yeast-like organism, found in 

 association with the Coccid Ceroplastes rusci, on artificial media. 

 A fungus, which he termed Oospora saccardiana, resulted, and it 

 grew rapidly on gelatine and showed a powerful liquefactive action 

 on the latter. Pierantoni (1910) similarly was able to grow the 

 yeast-like organisms from the mycetocytes of Icerya purchasi on a 

 gelatine medium. More recently, Brues and Glaser (1921) reported 

 that they were successful in cultivating the symbionts of the 

 Coccid Pulvinaria innumerahilis which occur, not in a definite 

 mycetome, but diffused in the fat-body of the insect. The 

 yeast-like bodies developed on suitable media into a separate 

 fungus of undetermined affinities which had the property of 

 secreting proteolytic, lipolytic and diastatic enzymes. It is evident 

 that the hosts in some unexplained manner are able to inhibit any 

 excess of multiplication of their symbionts, as the number of 

 mycetocytes remains very constant and does not appear to increase 

 beyond certain limits. Whatever role these organisms may play, 

 it would appear to be exercised upon materials contained either 

 in the blood or the fat-body of their hosts, since, as has already 

 been pointed out, they do not occur in direct relation with the 

 digestive canal. Plant-sucking Hemiptera absorb large quantities 

 of soluble carbohydrates in the cell-sap, which are eliminated to a 

 considerable extent as honey-dew. Buchner's hypothesis, that the 

 symbionts are able to assimilate atmospheric nitrogen, appears 

 to be based upon the assumed deficiency of nitrogen in the food of 

 these insects. There is, however, no evidence that plant-sucking 

 insects do not obtain all the proteins they require from the cell-sap, 

 and it appears more probable that the symbionts are in some way 

 concerned in dealing with an excess _pf carbohydrates present. It 

 is evident, therefore, that more definite information is required as 

 to the digestive physiology of the hosts before the role of their 

 symbionts can be elucidated. 



The cultivation of symbionts in vitro does not carry the problem 

 much nearer solution for the reason that their activities are often 

 very diverse, and no light is shed upon their essential functions 



