FUNGI 25 



immunity. It is also an established fact that in many 

 instances certain forms or varieties of a cultivated plant are 

 much more resistant to disease than others. This idea 

 has been clearly established by Dr. Cobb in connection 

 with wheat and its fungous enemy (Pucdnia graminis). 

 Unfortunately, it has also been demonstrated that varieties 

 that have been proved to be comparatively immune in one 

 particular district are equally or even especially susceptible 

 to the disease when removed to another locality ; or forms 

 that have behaved in an encouraging manner for a certain 

 period of time have afterwards lost this power. Never- 

 theless, it is encouraging to know that in particular districts 

 certain varieties are more disease-proof than others, and in 

 all probability further investigation of this phase of the 

 subject will result in the development of disease-resisting 

 varieties. This peculiarity turns on some, probably slight, 

 modifications, morphological or physiological. But it is 

 always well to remember that however much the host-plant 

 may succeed, aided or unaided, in guarding itself against 

 its foe, fungi, on the other hand, have proved themselves 

 equally capable of adaptation to existing circumstances. 



Evidence forthcoming supports the idea that, by careful and 

 continued selection, disease-proof varieties of plants may be 

 produced and perpetuated. 



Care should be taken to prevent the spread of disease 

 directly by human agency. This too frequently occurs, 

 especially in the case of those fungi whose mycelium spreads 

 in the soil ; also resting-spores and sclerotia. These are 

 carried from place to place by the soil adhering to shoes, tools, 

 cart-wheels, etc. Summer-spores are also diffused by adhering 

 to clothing. 



