70 SYMBWGENESIS 



The alga is wisely conservative its wisdom, as we shall 

 presently see, bringing length of days in her hands and, 

 though adapting itself wonderfully to the changed condition of 

 life, it yet retains what is essential, or might become essential, 

 at any moment to its survival. Genuine adaptation thus 

 always guards against parasitic atrophy of vital functions, 

 though continually sacrificing the lower for the higher, and 

 husbands those potentialities essential for continuous progress. 



What is the usefulness, both domestic and biological, of 

 this symbiotic co-ordination and subordination presented by 

 the case of the lichen ? 



Lichens are able to live in situations where neither the 

 alga nor fungus could exist alone. The alga is protected by 

 the threads (hyphffi) of the fungus, and supplied with water 

 and salts and, possibly, organic nitrogenous substances, and, 

 in turn, it manufactures photosynthetically carbohydrates, the 

 surplus of which it yields to the fungus. 



This form of relationship (says the Encyclopaedia Sritannica) is 

 now known in other groups of plants, but it was first discovered in the 

 lichens. 



Lichens are frequently found in the most exposed and arid situa- 

 tions. In the extreme polar regions these plants are practically the 

 only vegetable forms of life and possess the capacity of resisting extremes 

 of warmth, cold and drought without destruction. On a bare rocky 

 surface a fungus would die for want of organic substances, and an alga 

 from drought and want of mineral substances. The lichen, however, 

 is able to grow as the alga supplies organic food material and the fungus 

 has developed a battery of acids which enable it actually to dissolve the 

 most resistant rocks. It is owing to the power of disintegrating by both 

 mechanical and chemical means the rocks on which they are growing 

 that lichens play such an important part in soil production. (Italics 

 mine.) 



And thus, again, we see the enormous and far-reaching 

 use of symbiosis. We see how domestic and biological 

 symbiosis are inter-connected and dovetail, as it were, into 

 each other, just as do Bio-Economics and Bio-Chemistry. We 

 see how many great biological functions are performed 

 simultaneously and pari passu with the proper solution of 



