PARASITISM v. SYMBIOSIS 133 



some carbohydrate food, at first at any rate, but in return for k this the 

 plant acquires mineral substances in solution, which the fungus absorbs 

 from the soil. A considerable number of flowering plants are unable to 

 thrive unless their roots become infected in this way. (Italics mine.) 



Evidently we have here a relation of immense mutual advan- 

 tage, entailing immense benefits also to the biological community 

 at large. The relation involves sacrifices and risks such as the 

 frailty of life everywhere entails. The sacrifices on the part 

 of the fungus do not appear to be excessive, if we consider its 

 low status and the concomitant inability to serve in more perfect 

 ways. Having lost chlorophyll, and, hence, the secret of the 

 essential photosynthetic industry of plants, the fungus has, in 

 its own interest, to be useful as best it may and as its powers allow 

 it to be. The status of the higher plant, fortunately, is in itself 

 a certain guarantee against excessive exploitation of " helpers " 

 the conspicuous industry and vigour of which (relative to their 

 lowliness) is the best testimonial to the forbearance of their 

 " employers." Obviously the benefits conferred by the relation 

 upon the flowering plant and the world of life generally are very 

 great. The benefits which the fungus receives may be greater 

 than we at present know. The fungus certainly stands in urgent 

 need of Carbohydrates, which an adequate exchange relation 

 with the higher plant can best supply. The fungus pays the price 

 for what it receives. I would here point out that though the 

 " employment " of the fungus be characterised by considerable 

 degrees of compulsoriness, it is a very different thing to be con- 

 strained to industry, to mutual aid, and to " fair " service by 

 strenuous and cross-feeding organisms, so modestly and symbio- 

 tically disposed as our higher flowering plants, from being forced 

 to yield to totally one-sided " diabolical " exploitation, as, for 

 example, in the case of the crab parasitised by Sacculina. In 

 this sense, too, I am willing to believe that service was at first 

 compulsory or obligatory. After all, service is the most beneficial 

 necessity the sweetest of luxuries. But service is a different 

 matter from slavery, and there is a vast difference in results. 

 The " beautiful example of two-sided parasitism," therefore, 

 on due analysis, emerges as an example of Symbiosis, with mutual 

 service and general avail towards life well accentuated in strong 

 contrast to what results from Parasitism. 



So in the case of the leguminous plants, the associated bacillus of which, 

 when provided by the pea or the clover with carbohydrate food, is able to 



