292 DISEASES OF FIELD & GARDEN CROPS. [CH. 



corded a number of curious experiments made with diseased 

 potatoes. In those experiments it is shown that not only 

 will diseased tubers sometimes produce perfectly sound 

 plants, but that slices of diseased tubers, inserted within 

 the substance of sound ones at the time of planting, will, 

 in some instances, have no ill effect, but that the roots of 

 the sound plant may grow in the putrid remains of the 

 diseased one and still remain untainted. If the perennial 

 mycelium is alive, the result is generally, but not invari- 

 ably, different. The common mushroom spawn, as sold 

 by nurserymen in " bricks," the white fungus threads we 

 everywhere see when the earth is upturned, and the 

 white felt so commonly noticed amongst fallen leaves, is 

 perennial spawn. The spores of fungi are so extremely 

 delicate that a slight variation of heat, dryness, or 

 moisture often destroys them at once ; but when spores 

 have once germinated and produced spawn, this spawn 

 does not so readily perish, but may rest for a long time 

 in a hibernating state. This is proved in the case of 

 the familiar fairy-rings of our lawns and pastures, 

 which sometimes are not seen for many years, as the 

 subterranean spawn is awaiting suitable conditions of 

 warmth and moisture to cause it to produce the perfect 

 fungi. 



Here we must not forget that the virus of the disease 

 may exist in some form which has not yet been detected 

 by our microscopes. Before the higher powers of the 

 microscope were used no one suspected the presence of 

 motile zoospores with vibrating cilia ; and if we could use 

 still higher powers, it is not unreasonable to imagine that 

 some other condition of the parasite, at present quite 

 unknown to and unsuspected by us, might be brought to 

 light. The fungus may exist in inconceivably fine dust- 

 like particles, or in the condition of a mucous fluid. 

 Because we are acquainted with a certain number of 

 curious facts regarding fungi, it does not follow that we 

 know all. We have long suspected that the virus of this 



