MICROSCOPICAL STRUCTURE OP LICHENS. 167 



it has been proved to produce abundant motile swarm-spores as 

 well as aplanospores. The gonidium we are considering could 

 not be Protococcus or Pleurococcus (old style) because these 

 both divide vegetatively and the gonidium of our type does not 

 divide in this manner. I ventured two years ago to suggest that 

 the gonidium in question is a species of Chlorella, for the process 

 in the formation of daughter cells — reduced zoogonidia — in 

 free Chlorella and in the gonidium is to all appearances precisely 

 similar (7). The late Prof. West examined some of the material 

 I was working on, and in his opinion " the alga is nearer to 

 Chlorella than anything else " he was acquainted with. 



When the investigation of the constituents of the lichen thallus 

 was planned with Somerville Hastings, its purpose was that of 

 accumulating data bearing on the frequency of the penetration of 

 the algal cells by the fungus hyphae, for it had been asserted by 

 recent authors in America and Russia that the fungus of the 

 lichen thallus is parasitic upon the alga, and in support of their 

 statements, the penetration of living algal cells by the fungus was 

 the most important (3). 



Figures and photomicrographs which have been published for 

 the purpose of demonstrating actual penetration of living algal 

 cells are by no means convincing. In many cases they indicate 

 defects in the methods of illumination. 



In the investigation no single instance was found of what 

 could be regarded as a clear case of penetration, such as those 

 which have been figured (2.) Elfving writing in 1913 says : 

 " The formation of haustoria upon the hyphae, which grow into 

 the lumen of gonidial cells as represented in popular textbooks, 

 was extraordinarily rare ; in my material only on a single 

 occasion have I seen such a haustorium." 



It is contended that before a theory relating to the parasitic 

 nature of the fungus of the lichen thallus can be firmly estab- 

 lished, on the basis of penetration, an approximation of the 

 extent to which penetration takes place must be made. Pene- 

 tration possibly takes place occasionally, but until this condition 

 is proved to be the rule rather than the exception, the theory 

 of parasitism has little to support it. 



The question of penetration can be decided by the microscope 



