PLANT IN RELATION TO ITS BIOLOGICAL ENVIRONMENT. 211 



types of vegetation are sometimes of such a nature that one may be 

 able to take advantage of the waste products of another. This is 

 notably true of a number of bacteria and moulds, and the common 

 occurrence of a regular succession of forms of these lower organisms 

 illustrates this point. The remarkable relation of the nitrifying organ- 

 isms to each other serves as an example to show how one species not 

 only may pave the way for the entrance of another, by oxidizing 

 a compound that is directly poisonous to the associated organism, but 

 by this very process of oxidation may provide it with essential means 

 of subsistence. 



A study of the relation of the higher plant to various fungal species 

 is full of promise, both from a practical and a theoretical point of view. 

 The cultivation of orchids has been facilitated by the discovery of 

 the general nature and mode of infection of the germinating seedling 

 with the so-called symbiotic fungus. One of the most interesting, and 

 from a theoretical point of view important, instances of the relation 

 of an orchid with a fungus is that of the Japanese species Gastrodia 

 elata. We are fully acquainted with its remarkable life -history, and 

 it is one of the very few instances in which we are sure of the identity 

 of the infecting fungus. Gastrodia is a terrestrial species which forms 

 underground tubers. These only produce flowers when they have 

 been infected with the rhizomorph of Agaricus {Armillaria) melleus, 

 which is often parasitic on the trees amongst which the orchid grows. 

 (Figs. 44 and 45.) 



There are many theories (or rather hypotheses) as to how the 

 association of the root and fungus to form a mycorrhiza is brought 

 about, and as to the relation to each other which subsists between the 

 two symbionts. But the examples that have been investigated leave 

 no room for doubt that the relation is essentially one of parasitism. 

 The degree of infection, however, is checked by the root, in some 

 way not as yet fully understood ; I shall return to this later and shall 

 attempt to show that it is not as exceptional as it may perhaps appear 

 to be. For the present it will suffice to say that, while in many cases 

 the fungus seems to thrive, but fails to penetrate the deeper tissues of 

 the root, in others it invades the whole plant, without, however, doing it 

 any harm. In others, again, certain tissues of the host plant have 

 obviously and definitely the power of destroying it after it has infected 

 them and thriven for a while in their interior. But the mycorrhizal 

 association itself is always beneficial, and often indispensable. Inter- 

 ference with the fungal symbiont is, at least in some instances, a main 

 reason for intolerance of a calcareous soil, though the influence of the 

 calcium carbonate may be very indirect. Thus it would seem that for 

 certain calcifugal heaths the inimical influence of the chalk is due 

 to its facilitating the development of certain bacteria, which in their 

 turn are prejudicial to the mycorrhizal fungus of the roots of the 

 heaths. More investigations are urgently needed, especially on lime- 

 enduring heaths, e.g. Erica carnea. 



The influences which enable a parasite to obtain entrance into 



