VI TJie Web of Life 119 



between a small aquatic plant called Azolla and one of the 

 freshwater algse called Anabaena. On the under surface of 

 each floating leaf of Azolla there is a small opening leading 

 into a cavity in the substance of the leaf. In this cavity 

 a colony of the alga is always found. The alga may indeed 

 live independently in the water, but the Azolla is never 

 without its partner. We are not, however, certain as to the 

 precise meaning of the association ; and it may be doubted 

 whether we have really here as yet any appreciable measure of 

 true co-operation at all. Yet through such mere mechanical 

 associations or juxtapositions true parasitism and sym- 

 biosis must largely have arisen. 



One of the strangest kinds of reputed symbiosis is the 

 occurrence of fungi around the roots of certain plants. 

 Frank and others have shown that the root-tips of beeches, 

 birches, hazels, and the hke, are invested by a net of fungus- 

 threads. They suggest that the net acts as a sort of sponge 

 intermediate between the roots and the soil, and this idea 

 of the " mycorhiza," as the fungus network is called, receives 

 some corroboration from the fact that in heaths and some 

 orchids the fungus -threads actually penetrate into the sub- 

 stance of the root. The question is, however, under hot 

 discussion, the veteran arboriculturist, Hartig, stoutly main- 

 taining that we have here nothing beyond a mere parasitism 

 of the mould filaments upon the tree-roots. The controversy 

 is one which the student may profitably follow, summarising 

 (and where possible checking by actual observation) the 

 arg;uments on either side. 



