ECOLOGICAL 173 



ments now cease. By growth the tissue soon becomes wonderfully 

 strong and durable. The tendril has done its work, and has done it 

 in an admirable manner." 



MYCORHIZA. — As a striking example of mutually beneficial living 

 together, we take Mycorhiza (i.e. fungus-root), where a partner- 

 ship has been established between a fungus and the root of 

 some plant, usually green. The association occurs in two grades of 

 intimacy. In some cases, as in the beech, the fungus forms an 

 external network around the roots; in other cases, as in orchids, 

 the fungus penetrates into the interior. In a few plants, as in Vanilla, 

 the fungus is both outside and inside. 



The external root-fungi (ectotrophic) may form a feltwork so 

 dense that the roots of the plant do not come into contact with the 

 soil; but there are many gradations. Sometimes, as in beech, the 

 fungus is a soil mould, allied to Penicillium ; sometimes the threads 

 are very like those that form the mycelium of truffles and mush- 

 rooms. The difficulty of identification is due to the fact that the 

 partner fungus does not normally form reproductive organs, which 

 are more characteristic features of a fungus than its vegetative 

 system. Yet recent skilful culture has elicited reproductive develop- 

 ments, thus showing that Boletus, common in larch-woods, supplies 

 their mycorhiza; and the like for other well-known fungi, common 

 in forests. 



The internal fungoid (partners endotrophic) seem to be more 

 specialised types, not ordinary soil fungi; they send their threads 

 into certain cells of the root, where they often form coils around the 

 nuclei. Special suctorial structures are often developed. When 

 predominantly external root-fungi send hyphae into the tissue of the 

 root, they usually spread between the cells, not into them. 



An external association between fungoid hyphae and roots is so 

 very common, often occurring in more than half the species of a 

 random collection of flowering plants, that it seems useful to try to 

 restrict the term mycorhiza to cases where the feltwork is pro- 

 nounced and constant, or where it is associated with some peculiarity 

 of root growth, or where the seedlings do not flourish in sterilised 

 soil. But even when these conditions are insisted on, the number of 

 external root-fungi is very large. What used to be regarded as a 

 peculiarity of forest-trees, such as beech and pine, is now known to 

 be an exceedingly common linkage. 



Very interesting experiments on internal mycorhiza in Orchids 

 were made by the French botanist, Noel Bernard, too early lost to 

 science. He found that the well-known difficulty or impossibility of 

 growing imported orchids from seed was due to the fact that germin- 

 ation does not take place, or does not succeed, except in the presence 

 of certain fungi, which must sometimes be of a particular species 



