2o8 THE LIVING ORGANISMS OF THE SOIL [chap. 



has been attributed, but the question still requires further 

 investigation. 



The existence of mycotrophy has certain interesting 

 applications in practice; there are many plants which 

 can only be cultivated with difficulty in gardens ; for 

 example, some of the orchids, ericas, lilies, and others, 

 generally plants which must be grown in leaf-mould, 

 peat, or other material rich in humus. Yet humus 

 alone is not always sufficient for the purpose, the peat 

 or leaf-mould has often to be obtained from a particular 

 place; other materials, though equally rich in humus 

 and possessing similar mechanical properties, prove 

 quite unsuitable. It is easy to surmise that this effect, 

 confined in the main to mycotrophic plants and humic 

 soils, may easily be due to the absence of the proper 

 fungus from the soils found to be unsuitable. 



It has also been shown that the difficulty usually 

 experienced in raising seedlings of exotic orchids, 

 which die off in great number just after they have 

 germinated, may, to a large extent, be obviated by 

 mixing with the medium in which the seeds are sown 

 a little of the material in which the parent plants are 

 growing. The young seedling is found to develop 

 mycorhiza at a very early stage, and then only will 

 grow properly. 



" Finger-and- Toe" 



On many soils, particularly those of a sandy nature, 

 the turnip crop is often almost wholly destroyed by the 

 disease known as " finger-and-toe," " club," or " anbury." 

 Cabbages and other cruciferous crops are equally at- 

 tacked ; so much so, that in gardens which have become 

 infected it is practically impossible to raise crops of 

 this nature. The disease is caused by an organism, 

 Plasmodiophora brassicce, belonging to the slime fungi, 



