AN ANALYSIS OF SYMBIONT REACTIONS 95 



physiologic modification, however, does not appear to be 

 impressed upon the germ plasm, as the young worms take 

 in food from the out-side hke any non-symbiotic turbellarian. 

 This response appears to be limited to a definite growth 

 period of the individual. 



The presence of special cells containing bacterial sym- 

 bionts in the Blattidii (cock-roaches) is another example of 

 morphologic response on the part of a host to the presence 

 of microsymbionts. We know nothing of the functional 

 significance of the "bacteriocytes" in these insects. The 

 microsymbionts are transmitted from one generation to 

 another in the germ cell. 



In the plant kingdom, the formation of root-nodules in 

 conjunction with the invasion of microorganisms represents 

 a specific illustration of a morphologic response in the 

 host. Wright^ distinguishes two types of bacteria which 

 produce root nodules on the soy-bean. These two types 

 are morphologically alike, but have different serological 

 and cultural characteristics. The nodules produced by 

 the two types have a different arrangement on the roots of 

 the host plant. In these plants the biologic association 

 of the symbionts has not developed to a point of absolute 

 dependence, although the association apparently is valua- 

 ble to both. 



The symbiotic relationships present in all the Uchens 

 represent one of the most significant examples of symbiosis 

 that we shall attempt to analyze. The group of lichens is 

 distinctive in that every member of it is a symbiotic com- 

 plex. The plant consists of algae symbiotically combined 

 with a fungus. The lichens are particularly significant 

 and instructive since the various species exhibit different 

 degrees of symbiotic relationship. In the simpler forms, 

 reproduction of the two symbionts is independent. It is 

 necessary for the offspring of the two symbionts to find 



* Personal communication. 



