CLASSIFICATION OF SYMBIOTIC PHENOMENA 255 



Sjnnbiosis — The living together of dissimilar organisms. 

 I. Disjunctive symbiosis. 



1. Social — (plant communities). 



2. Nutritive. 



a. Antagonistic — (Herbivorous mammals and 



plants; ants and fungi: ambrosia beetles and 

 fungi; etc. 



b. Reciprocal — (Pollinating animals and flowers). 

 II. Conjunctive symbiosis. 



1. Social — (Lianas; epiphytes; Azolla and algae; etc.)- 



2. Nutritive. 



a. Antagonistic i.e. parasitism — (Ectotrophic my- 



corhizas; insect galls; plant diseases due to 

 animals or plants; animal diseases; etc.). 



b. Reciprocal i.e. reciprocal parasitism, — (Endo- 



trophic mycorhizas; lichens; root tubercles; 

 leaf tubercles of Rubiaceae; etc.). 



Our ability to correctly place any given symbiotic phenome- 

 non in this classification depends entirely upon our knowledge 

 of the phenomenon and it is not impossible that some of the 

 examples I have given will need to have their status changed 

 when we understand them better. I have no doubt that some 

 readers will not be ready to agree that I have correctly placed 

 ectotrophic mycorhizas, for some of the very recent text books 

 have retained the obsolete view that the mycorhizal fungi aid 

 their hosts in absorbing mineral salts from the soil. But it is 

 pretty generally agreed among recent workers on ectotrophic 

 mycorhizas that they represent simply instances of parasitism 

 of fungi on the roots of higher plants. Endotrophic mycorhizas, 

 on the other hand, are somewhat comparable to the root-tuber- 

 cles of legumes. None of the mycorhizal fungi, probably, are 

 capable of fixing nitrogen ; but their relation to the host is similar 

 to that of tubercle bacteria, since, while the fungus is at first 

 parasitic on the host, eventually, in the majority of cases at 

 least, the higher plant becomes parasitic on the fungus. These, 

 therefore, are instances of reciprocal parasitism. 



Probably no one will dispute the status of lichens but some 

 may object to speaking of lichens as symbiotic phenomena at all, 



