12 



and dead organic matter, commonly decaying. And, on this basis, we 

 may divide the fungi into two classes, those which derive their nour- 

 ishment from other livmg things, and those which live on the remains 

 of dead organisms. The latter, known as corpse-plants ov saprophytes , 

 include the moulds, toad-stools and many other fungi. But the 

 first named group is that which at present interests us. since it con- 

 tains the various groups mentioned at the beginning of this sketch, 

 which live on or in tiie bodies of other living plants at their expense, 

 and cause extreme weakeningor even the death of the affected plants- 

 Such fungi are known as parasites, and the plants they attack are 

 called their Jiosts. This distinction between saprophytic and para- 

 sitic fungi is a very useful one, but no sharp line can be drawn 

 between the two groups, since some fungi seem to be able to live 

 either as parasites or as saprophytes, while it is probable that very 

 many pleomorphic fungi are parasites in some of their forms, and 

 saprophytes in other stages of their life-cycle. 



Finally, we may notice the interesting fact that any given para- 

 sitic fungus is usually restricted in its capacity for harm to a single 

 host-i)lant or to a few closely related ones ; though, on the other 

 hand, closely related fungi may attack plants of widely different 

 relationships. Thus, the mildew of the lettuce and that of the onion 

 are very closely related fungi, yet neither mildew can attack the host- 

 plant of the otlier, since the structural resemblances are few and the 

 relationship remote between the lettuce and the onion. 



From the above facts we may derive a few important piinciples for 

 our guidance in attempts to avoid or check the ravages of fungi 

 among plants cultivated for use or beauty. Since the mycelium of a 

 parasitic fungus grows usually within the tissues of its host-plant, it 

 is too late to try remedies after a plant is once infected. It is true 

 that a few fungi are superficial in growth, and a treatment may per- 

 haps be found which shall destroy such parasites without harm to the 

 host. But in most cases our aim must be to fortify exposed plants 

 against infection by the timely application of protective solutions or 

 mixtures, which shall prevent the germination of the spores which 

 fall upon the plant so treated. Some progress has been made in 

 this direction and some results have been reached which justify 

 hopes of ultimate general success in largely avoiding the present 

 enormous annual losses resulting from fungous diseases. 



The treatment which now gives promise of most general applica- 

 bility and efficiency is the spraying of the plants with a solution of 



