282 THE WONDER OF LIFE 



Tillandsia usneoides, the so-called ' vegetable horse-hair ', 

 which is common on trees, and it is almost certain that this 

 colour-resemblance has protective value. 



Ambrosia. — In the tunnels made by various beetles 

 (e.g. species of Xyloterus and the hke) in the bark and 

 wood of trees there is a lining of Fungus, which produces 

 special spherical ' ambrosia ' cells, serving as food for the 

 insects. This association appears to be useful to both 

 organisms : the insects are fond of the ' ambrosia ' , and 

 its growth makes up for the frequently poor nutritive 

 quahty of the wood ; the fungi profit because the larvae 

 carry them in their borings into the sapwood, where they 

 get the best food and have at the same time a good supply 

 of air. The association has been carefully studied by Prof. 

 Neger, who regards it as a genuine symbiosis. It is much 

 commoner in warm and tropical zones, where the boring 

 insects often do much harm both by their own operations 

 and by introducing the fungi, most of which seem to be 

 related to the Ascomycete genus Endomyces. The matter 

 may become more compUcated — wheels within wheels 

 again — ^when weeds begin to grow in the fungus garden 

 in the form of yeasts and Bacteria and the hke which further 

 infect the wood, but are not of any use to the beetles. 



Neger found the same ' ambrosia-cells ' inside the gaUs 

 made by certain mites {Asfhondylia). The cavity of the 

 gall is lined by a layer of fungus threads, among which are 

 the special ' ambrosia cells ' which the developing mites 

 eat. Alter the mites have departed, the spores of the 

 fungus are produced on the outer surface of the gall. 

 Here, then, there is a triple combination of flowering-plant, 

 mite, and fungus. 



Plants Turning the Tables. — Even the worm will 



