100 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



Another property developed by the Bacillus radicicola in 

 the sjonbiotic state, is that of "fragihty." This is especially 

 prominent in the final morphogenic state — the large globu- 

 lar body. In an ordinary bacteriological preparation in 

 which the nodule is crushed on a sUde, the large bodies are 

 practically never seen, undoubtedly, due to their fragile 

 nature. In histological sections, the large globular form is 

 difficult to demonstrate with the ordinary technique. The 

 organism of Rocky Mountain spotted fever and other 

 Rickettsia bodies exhibit similar fragility. All bacterial 

 symbionts do not necessarily exhibit the same degree of 

 fragility, but on the contrary, many micros ymbionts ap- 

 pear to retain their hardier properties. 



It is a difficult matter to explain this variation in fragiUty. 

 In general, it is safe to assume that fragility is a conse- 

 quence of symbiosis; the greatest degree of fragihty is 

 associated with absolute symbiosis. Other factors, how- 

 ever, must be considered in any attempt to explain fra- 

 gihty. It must be recognized that the chemical constitution 

 and the physiologic activity of the s>Tnbionts may be 

 different in every symbiotic complex. We must also ad- 

 mit that there may be different "degrees" of symbiosis, as 

 exemphfied in the three groups of Uchens. When a sym- 

 biosis is as "loose" as it is in the lower hchens, the individ- 

 uals still retain their hardier characteristics, and are 

 readily capable of an independent existence. On the other 

 hand, when the symbiosis is as "close" as it is in the higher 

 Uchens, the symbionts have lost their hardier characteris- 

 tics and are no longer able to maintain an independent 

 existence. 



The "degree" of symbiosis determines, in a large measure, 

 the independence of the symbionts. This circumstance 

 has a bearing on the difficulty in cultivating microsym- 

 bionts in artificial culture media (see Meyer, '25). In some 

 forms in which the symbiosis is not so close, the microsym- 



