28 hijcci'ton. 



CHAPTER VI . 



Infection. 



After the germiuatiou of the smut spores had been observed for a large 

 number of species, the mode of entrance into the host-pknt and the furtiier 

 development there until spores were again produced, was carefully studied 

 by a number of botanists. Among the most prominent investigators were 

 De Bary, Kuehn, Fischer von Waldheim, Wolf?, and Brefeld. They determined 

 the mode of infection in certain species, but a number still remain in which 

 the exact method of germination and infection has yet to be discovered. 



It was generally found that infection occurred in the seedling stage of 

 the host-plant by means of the spores, which germinated and produced their 

 conidia. The young seedlings may be infected either by the smut-spores 

 attached to and so-rni with the seed, or they may be in the soil. While the 

 spores of some species of smuts remain in the soil in an inactive condition, 

 there are others, such as the flag smut, in which the infection is principally 

 from the soil. BrefekP showed in his experiments with loose smut of oats 

 that seed sown in an infected mixture of held soil and fresh horse-dung yielded 

 between 40-50 per cent, of smut ; but his''' recent similar experiments with 

 the loose smut of wheat and barley for three years in succession only yielded 

 negative results. The reason for these negative results lies in the fact that 

 infection does not occur in the seedling stage, but through the flower, as we 

 shall afterwards see. It is principally owing to the patient and long contiimed 

 investigations of Brefeld that our previous views regarding the modes of 

 infection have had to be considerably modified, and, in accordance with this, 

 the methods of treatment have been placed on a surer foundation. 



Infection and Contagion. 



When Meyen contended that the smut is not a contagious disease but is 

 inherited, he simply meant that it was not due to any external parasite, 

 but was inherent in the plant itself. Infection and contagion were in his 

 time regarded as synonymous, but now the latter term is generally applied 

 to those infectious diseases in which direct contact is necessary to produce 

 them. There is no necessity for the plants to be actually in contact in order 

 to contract the disease caused by the smut fungi, but the spores or germs are 

 conveyed to the plants, and so we speak of infection. Infection does not 

 consist in the mere penetration of the germ-tube of the spore or conidium 

 through the epidermis into the tissues of the host-plant, but the germ tube 

 must grow and develop at the expense of the living cells until it reaches the 

 apex of growth, and there become associated with the growth of the host, 

 so that finally fresh spores are formed. It is well known that the germ- 

 tube of the smut fungi may penetrate inside a plant, and there die oi? without 

 producing any injurious effect, and for convenience we might use the term 

 inoculation to signify merely that the spores or germs had been applied to 

 an otherwise healthy plant, while the normal infection would consist in the 

 entrance and growth of the germ-tube within the tissues of the plant and the 

 final production of spores. The period which elapses between the infection 

 and the production of spores through the multiplication of the fungus inside 

 the plant might also be called the " incubation period." Thus, in the case of 

 stinking smut, the incubation period extends from the infection of the 

 seedling to the production of spores in the ovary, and in the case of the 

 American corn smut, where infection is strictly confined to the young parts 



