THE SPREAD OF DISEASE 211 



strated the fact that in many of the most destructive 

 diseases spores play but a small part, and in some instances 

 no part at all, in the perpetuation of such diseases. Of 

 course it has long been known that certain kinds of fungi 

 pfoduce sclerotia, or concentrated masses of mycelium, 

 which are capable, after a period of rest, of propagating 

 the species independent of spores ; again, some kinds of 

 fungi produce long strands of mycelium, which either 

 extend for a considerable distance under ground, as in the 

 case of Agaricus melleus, or in the humid forests of tropical 

 regions certain species of Marasmins often form a dense 

 tangle of fine threads, that might be mistaken for a growth 

 of Cuscuta, or dodder, on the branches of shrubs. These 

 strands of mycelium give origin at intervals to clusters of 

 sporophores, often at a very considerable distance from 

 the parent stock. 



Apart from such exceptions, all attempts to check the 

 extension af parasitic fungi injurious to cultivated plants 

 have turned on some method for destroying the spores ; 

 such lines of treatment being based on the assumption 

 that the plant to be protected was free from disease until 

 infected from without by means of spores. 



As previously stated, this assumption has been shown 

 to be too sweeping, and in reality in many instances 

 spores play a very subordinate part in the perpetuation of 

 a disease ; their part being taken by hybernating mycelium 

 present in the seed, tuber, bulb, or whatever portion of 

 the plant serves for the purpose of reproduction. More 

 than this, it has been proved that when a plant is once 

 infected, the mycelium present in the seed, tuber, etc., 

 grows along with plant, extends to the above-ground 

 portions where, in due course, spores are often produced ; 



