( ni ) 



monly the discoloration first nianit'ests itself near the nerves of the 

 leaf by a considerable increase of chlorophyll ; afterwards the inter- 

 spaces between the dark green spots are affected by a bleaching- 

 process, which mostly does not go farther than to give a yellow 

 hue to the sickened parts, but which in some cases, causes variega- 

 tion. The dark green spots grow in the beginning more intensely 

 than the rest of the leaf, thus becoming blown-up protuberanc^es 

 rising from the upper surfiice of the leaf. This, however, occurs 

 oftener with artificial infection experiments than on the tobacco fields, 

 where the diseased leaves remain flat. The third phase of the disease 

 consists in the locally dying of little spots irregularly spread over 

 the leaf; they get soon brown, are very brittle and even the culling 

 of the leaf may change them into holes. They make the leaves 

 valueless as wrappers of cigars. 



Professor Adolf Mayer pointed out in 18SG that this disease is 

 contagious. He pressed the sap out of diseased plants, filled with 

 it little capillar tubes, put them into healthy plants and after two 

 or three weeks he found these to be likewise attacked by the disease. 



In 1887 I endeavoured to solve the question whether any parasite 

 might be found as cause of the disease. It was clear that, if this 

 should really be the case, there could only be thought of bacteria, 

 for microscopic observations had not indicated the least traces of 

 microbes. Tiie bacteriologie culture-methods proved that aerobic 

 bacteria could not come into consideration for they failed as well 

 in the tissue of the healthy as of the diseased plants, and the 

 same holds good regarding the anaërobics. So, it was certain that 

 here was an instance of a disease caused by a contagium fixum. 

 This consideration induced me in 1897 to conduct new infection 

 experiments in order to become better acquainted with the charac- 

 teristics of the contagium. The chief results of these experiments 

 are the following. 



In the fii'st place it was proved that the sap pressed from diseased 

 plants and filtered through very dense porcelain was absolutely 

 devoid of bacteria, without losing of its virulence. Attonipis made 

 to point out in the filtrate ac'robics or anaërobics again did not 

 give any result. 



In order to answer the question whether the virus ought to be 

 considered as corpuscular or as dissolved or liquid, some parenchyma 

 of diseased plants rubbed fine was spread over agar-plates and then, 

 left to diffusion. A virus, consisting of discrete particles, would 

 needs remain on the surface of the agar and consequently be in 

 the impossibility of rendering the agar virulent; a virus, really 



