72 SYMBIOGENESIS 



We are further told by the Encyclopaedia Britannica : 

 The resistance of lichens is extraordinary ; they may be cooled to 

 very low temperatures and heated to high temperatures without being 

 killed. They may be dried so thoroughly that they can easily be heated 

 to powder yet their vitality is not destroyed but only suspended : on 

 being supplied with water they absorb it quickly by their general surface 

 and renew their activity. 



Their geographical range is more extended than that of any other 

 class of plants, appearing as they do in the coldest and warmest regions 

 on the dreary shores of Arctic and Antarctic seas and in the torrid 

 valleys of tropical climes, as well as on the greatest mountain elevations 

 yet attained by man, on projecting rocks even far above the enow line. 



The eminent power of meeting the most varying con- 

 tingencies of life possessed by genuinely symbiotic organisms 

 is thus again brought home to us, and it is an additional 

 reason for rejecting the nitrogen theory tentatively put forward 

 by Prof. Keeble. This power is identical with that which 

 resists disease generally, and it justifies the view here expressed 

 that symbiosis is the true antithesis to disease, just as, 

 inversely, cessation of symbiosis is the beginning of all patho- 

 logical developments. "He liveth long, who liveth well." 

 True, the symbiotic association in the lichens is sometimes apt 

 to degenerate into parasitism, the fungus killing the algal 

 cells. Sometimes lichens will live parasitically on others. We 

 must remember that in the lichen we have but a particular 

 case of symbiosis, suited to particular conditions. The main 

 line of evolution was by way of non-attached biological 

 symbiosis. Generally speaking, however, the ubiquity and 

 dominance of the lichen is clearly associated with its 

 strenuousness. 



Cora pavonia which in tropical regions is found growing on the 

 bare earth is unique because the fungal element is also found growing 

 and fruiting wholly devoid of algse, whilst in the ascolichens the fungal 

 portion seems to have become so specialised to its symbiotic mode of life 

 that it is never found growing independent. (Italics mine.) 



Again we see that there is no need in genuine symbiosis to 

 surrender essential potentialities as in the case of parasitism. 

 This instance also again points to the fact that originally there 



