244 THE LIVING ORGANISMS OF THE SOIL [chap. 



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 only then 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, 

 PlasmodiopJiora brassica, belonging to the slime fungi 

 and forming spores which may remain dormant in the 

 soil for some time, certainly for two or three years. It 

 has long been known that the best remedy against finger- 

 and-toe consists in the application of lime; and as far 

 back as 1859, Voelcker showed that soils on which this 

 disease is prevalent are deficient in lime ; and in many 

 cases in potash also. Later researches have only served 

 to emphasise the fact that the disease is associated with 



