1900.] on Symbiosis and Symbiotic Fermentation. 265 



tree is vigorous, usually, indeed, in proportion to the development of 

 the fungus, a fact which shows that the advantages of the connection 

 must at any rate be mutual. 



Observation of the structure of the young root shows us that 

 unlike noimal terrestrial roots, it does not give rise to any root-hairs. 

 If the mycelial mantle is prevented from developing, as may be done 

 by cultivating a plant from seed by the method of water-culture, or 

 if the fungus is removed from a root on which it has become estab- 

 lished, the plant has no power of flourishing and very speedily dies. 

 If the mycelium is suffered to play its part this neglect to produce 

 root-hairs is not attended with any ill-results. In fact the mycelium 

 takes up the function of absorbing water which is discharged in 

 other terrestrial plants by the root-hairs. These filaments we have 

 seen ramify in the soil, coming in contact with its ultimate particles 

 just as the root-hairs do ; they are in communication also with the 

 interior of certain cells of the cortex, in which they break up and 

 form a kind of network. They thus serve for the supply of water 

 and dissolved mineral matter to the tree, which in turn supports them 

 by the elaborated nutrient substances they derive from the cells in 

 which they end. 



A case of a more complete alliance is afforded by the great group of 

 Cryptogamous plants known as the Lichens, the two constituents of 

 which are of relatively equal systematic position, and which enter into 

 the composition of the symbiont organism in almost equal amounts. 



The Lichens are of very wide distribution and show extraordinary 

 diversities of form. Many of them appear as incrustations on stones 

 or wood, or the bark of trees, and resemble mere discolorations of 

 the surface ; others are thin and papery, somewhat resembling leaves 

 in their texture, though not in their shape, the latter being very 

 irregularly lobed and corrugated. Some again are of more sturdy 

 habit resembling miniature shrubs, while yet others are fleshy or 

 gelatinous cushions of very irregular form. 



The thallus of a lichen, when cut, so as to show its section, is 

 found to be composed of two constituents ; a green or sometimes 

 blue-green alga, imbedded in the midst of a mingled mass of hyphal 

 fungal filaments. The order in which these two constituents are 

 arranged is very varied, the alga being sometimes fairly evenly dis- 

 tributed throughout the thickness of the body or thallus, and in 

 other cases confined to a particular region of it. There is a great 

 variety possible in the species which form the lichen ; a particular 

 alga may co-operate with different fungi and a particular fungus with 

 different algae. 



The gradual formation of a lichen by the actual growing together 

 of its two constituents can often be observed. The green cells of 

 Protococcus which are found in such quantity on the bark of elm 

 trees, forming a green dusty-looking surface over the brown bark, 

 are frequently found attached by or associated with hyphal filaments 

 of some fungus, which coil round and enclose them. As the al^al 



Vol. XVI. (No. 94.) t 



