108 



NUTRITIVE CONJUNCTIVE SYMBIOSIS 



A number of cases are known in which the mycorrhizal fungus 

 is partly ectotrophic and partly endotrophic. Such mycorrhizas 

 are said to be ectendotrophic. In most cases they probably represent 

 merely a difference in degree of parasitism. All ectotrophic mycor- 

 rhizal fungi are parasitic on the roots of the host plants. This leads 

 to a resistance to the parasite on the part of the host cells. In most 

 cases the fungus is able to penetrate between the cells but not to gain 

 an entrance to them, and so remains ectotrophic. If, however, the 

 fungus succeeds in entering the cortical cells of the root it becomes 

 ectendotrophic. 



There is a great deal of disagreement among investigators con- 

 cerning the relationship between mycorrhizal fungi and host plants 



Fig. 52. 



-Endotrophic mycorrhizas of Acer rubrum. Left, external appearance; 

 right, tangential section. 



and its significance. Some of this disagreement is undoubtedly due 

 to the fact that some investigators have worked mostly with dicoty- 

 ledonous trees while others have worked mostly with coniferous trees 

 or with herbaceous plants and the relationship probably differs 

 greatly in the dift'erent groups of plants or in different families within 

 a group. In the case of coniferous trees there is a considerable 

 amount of evidence that growth is much better where mycorrhizas 

 are abundant than where they are scarce or absent. This may be 

 due to the fact that an increased supply of nitrogen, and perhaps of 

 such elements as potassium, phosphorus and calcium, are obtained 

 by the tree through the fungi. Thus mycorrhizas may be of more 

 importance in the practice of forestry than was formerly thought. 

 A great deal more work will need to be done before we can speak 



