264 Professor J. Reynolds Green [Feb. 9, 



full development of this relationship is only gradually brought 

 about. 



The advantages of the symbiosis appear on the whole to be greater 

 on the side of the mistletoe than on that of its host. During the 

 louger portion of the year the latter is in full foliage, and no doubt 

 the invader derives considerable nutriment from it by the passage of 

 the latter into the sinRers from the parenchyma and the bast of the 

 host. It must, moreover, draw from the wood of the latter the water 

 and the mineral salts which it needs for the working of its own 

 chlorophyll apparatus. The mistletoe, however, is an evergreen plant, 

 while its host loses its leaves as autumn gives way to winter. The 

 advantage then becomes transferred to the host plant, which derives 

 supplies of nutrient material from the mistletoe, the latter being able 

 to elaborate it from the raw materials it absorbs on the one hand from 

 the air, and on the other from the wood to which its sinkers are so 

 closely connected. 



Instances of symbiotic association between members of the higher 

 and of the lower plants are not at all infrequent. One of the most 

 curious relationships is exhibited by the young roots of a number of 

 our woody plants, some shrubs, and other trees. Among the former 

 we find large numbers of the Heaths and Rhododendrons, while the trees 

 are represented by the Firs, Oaks, Beeches, Willows and Poplars, and 

 many others. The roots of these plants when taken up carefully 

 from the soil are found to be covered with a dense feltwork of fungal 

 hypliEe, constituting, in some cases, a mantle of considerable thick- 

 ness. The filaments of the fungus not only cover the outside of the 

 roots but penetrate into the cortex and ramify at first between the 

 cells, later on entering them and branching copiously in their in- 

 terior. The thickness of the mantle varies a good deal in different 

 cases, but its composition is much the same in all. From it delicate 

 threads or solitary hyphae extend outwards and ramify among the 

 particles of soil, appearing almost exactly like the long root-hairs 

 with which plants are usually supplied. There is a constant setting 

 up of this symbiotic relationship going on in the soil ; as a young 

 root grows out it finds itself in mould which is permeated by hyphal 

 threads or contains spores which are ready to produce them. The 

 young hypha makes its way into the young root, forcing itself be- 

 tween certain of the cells, and once inside it ramifies freely, being 

 nourished by the juices of the root. As long as the latter continues 

 to grow or even to exist the mycelium accompanies it, becoming 

 larger and thicker during its development, and sometimes forming a 

 considerable mass of hyphae in which the root is enveloped. 



The symbiosis in this case is not at first apparent. The advan- 

 tages appear to be altogether on the side of the fungus, which 

 evidently thrives very luxuriantly at the expense of the nutrient 

 materials which it extracts from the cells of the cortex of the root 

 into which we have seen it penetrating. The loss of this particular 

 nutrient matter is, however, of very small relative importance. The 



