COMPOUND ORGANISMS 193 



attracted to the root by substances which 

 have a food value for its hyphse, just as para- 

 sitic fungi are induced to enter the bodies 

 of their victims. But in a mycorhizal 

 association the cells of the root control the 

 degree of invasiveness which the fungus can 

 manifest, and not only so, but they often 

 proceed to actually digest the fungus itself 

 after it has flourished within them, and at 

 their expense for a while. 



We have here, then, a beautiful example 

 of two-sided parasitism, in which the final 

 balance of profit very clearly lies with the 

 flowering plant. It is practically certain that 

 the fungus obtains some carbohydrate food, 

 at first at any rate, but in return for 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. This is especially true 

 of orchids. Indeed, one of the great diffi- 

 culties experienced in raising these plants 

 from seed has been solved by supplying the 

 young seedling, during its germination, with 

 the fungus appropriate to it. And so close 

 has the degree of association between fungus 

 and flowering plant become, both in orchids 

 and in many other plants, that neither 

 can grow properly in the absence of the 

 other. 



Now this intimate mycorhizal relationship 

 is found to exist in all the flowering sapro- 



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