168 Gruenberg : Some aspects of the mycorhiza problem 



solution of iodine in potassium Iodide. The plant was covered over 

 with a light-tight box and left this way for several days. On re- 



examining the subepidermal regions for starch, no difference in the 



apparent abundance of this substance could be observed. That 

 the plant does not obtain its carbohydrate synthetically I feel 

 convinced. 



The only other direction from which it may come is the soil. 

 The constituents of the humus that may yield carbohydrates, the 



V 



means for their absorption, and the mechanism for their conversion 

 into starch remain to be determined. 



The question of the entry of the fungus into the rhizome is 

 another open problem. The seeds of orchids are notoriously indif- 

 ferent to all efforts to coax them into germination, and without 

 seedhngs free from the fungus it is hopeless to find the complete 

 analysis of the relation between the symbionts. In many myco- 

 rhizas there have been found hyphal connections between the endo- 

 tropic mycelium and external mycelia. But in very many cases 

 absolutely no trace of such connection can be found. In Coral- 

 iorhisa\\\Q mycelium of the cortical layers frequently sends hyphae 

 singly or in bundles into the trichomes on the papillae, but it is 

 not certain that these hyphae regularly reach the exterior. With 

 the exception of the trichomes, the epidermal cells are uniformly 

 free from hyphae. It has been supposed that the fungus gains en- 

 trance through the stomata of the rhizome ; it would be necessary 

 to find seedlings free from the fungus, and stomata through which 

 the hyphae could be seen to pass, I have searched for both and 

 have found neither. 



If the orchid is absolutely dependent upon the fungus for its 

 nutrition, how may the seedling develop without the fungus ? The 

 embryo of the orchid seed is undifferentiated, and the nutrients 

 stored within the seed could not carry the plant along very far. 

 But the ripened seeds are ordinarily free from fungus ; at any rate 

 fungal threads are not present in every seed, and it has been im- 

 possible to grow from the seeds any fungus that at all resembles 

 the fungus of the mycorhiza. Moreover, the scape, through which 

 any connection between the mycorhiza and the seed would have to 

 pass, is generally quite free from mycelial threads. 



The mycorhiza problem is not only of great interest theoret- 



