35 



nourishment from the substratum (host-plant), at least not sufficient 

 to produce any injurious effects. (For further particulars see n.) 



II. NUTRICISM. 



This establishes a connecting link between antagonistic symbiosis 

 and mutualistic symbiosis. Tubeuf defines it as that form of symbio- 

 sis in which one symbiont nourishes the second symbiont without re- 

 ceiving any food supply in return ; that it is, so to speak, a one-sided 

 symbiosis. Tubeuf recognizes two forms of nutricism. In the first 

 the saprophytic fungus living in contact with the growing root-tips 

 of higher plants supplies soluble organic food substances, the fun- 

 gus acting in the manner of a transfer agent. In the second the fun- 

 gus lives within the root-cells of the host-plant. The author ad- 

 mits that the fungus is at first nourished by the host, but that it finally 

 dies and is absorbed. The second form is undoubtedly mutualistic 

 symbiosis, as will be explained in the ,next chapter. I will there- 

 fore, limit myself to a discussion of the first case ; it must be re- 

 membered that it is very frequently difficult to decide what is merely 

 nutricism and what is mutualistic symbiosis. 



The most common form of nutricism is met with in the occurrence 

 known as mycorhiza (fungus-root). This is the association of a 

 fungus and the root of some higher plant. It is frequently met 

 with among the Cupuliferae growing in humus-bearing soil. The 

 hyphal fungus forms a network about the young root-tip of the 

 growing tree ; its function is to supply the tree with organic food 

 substances taken from the soil ; in a certain sense it supplants the 

 function of the hair-cells which are wanting in the mycorhiza. The 

 fungus does not receive any marked benefit from this association, but 

 it has been conclusively proven, experimentally, that the tree is very 

 materially benefited. The hyphae of the fungus always remain on 

 the outside of the root and this form is, therefore, known as ectotrophic 

 mycorhiza. In the endotrophic mycorhiza of Orchids the fungus 

 enters the parenchyma cells of the root where the hyphae develop a 

 mycelial network ; too little is known concerning this phenomenon 

 to give any correct explanation of it ; it is at present impossible to 

 state whether it is a case of antagonistic symbiosis, nutricism or 

 mutualistic symbiosis. In regard to the ectotrophic mycorhiza we 

 are certain that it is not a form of antagonistic symbiosis. 



In Cycas rcvolnta we find a form of symbiosis which is un- 



