( 1T2 ) 



dissolved in water would, on the contrary, be able to j)enctrate to 

 a certain depth into tlie agar. 



Alter about ten days' diffusion, a time which I considered suffi- 

 ciently long in accordance witli that wanted for the diffusion unto 

 a considerable depth of diastase and trypsine, the surface of the 

 agar-plate was first cleaned with water and then with a solution of 

 sublimate; then the upper layer was removed by means of a sharp 

 platinum spatula. In this way the inner part of the agar might be 

 reached without its having any contact with the particles adhering 

 to the surface. Infection experiments performed with these deeper 

 layers caused as well the disease as the porcelain-filtrate. So, there 

 seems no doubt left but the contagium must be fit for diffusion and 

 consequently considered as fluid. 



The infection experiments were performed with the expressed sap 

 by injections with the syringe of Pravaz. The most proper place 

 for injecting is the stem, and in particular the youngest parts which 

 are still in growth. The nearer the place of injection is to the 

 terminal bud, the sooner its consequences show themselves. This is 

 evident from the experience that only those leaves are susceptible 

 of infection which are still in growth and in the phase of cell- 

 multiplication, ineristems being by far the most susceptible. By 

 maising use of this fact and injecting the virus cautiously quite 

 near to active meristems, I was of late enabled, three days since 

 the injection, already to observe the first symptoms of the disease, 

 whilst otherwise they must be waited for two or three weeks longer. 

 Full-grown leaves, and even leaves whose cells are still in the phase 

 of elongation but no more in that of multiplication, are unfit for 

 infection. 



As the quantity of virus, sufficient to produce a large number of 

 diseased leaves, is extremely small, and as the juice of these leaves 

 will serve to infect an unlimited number of plants, it is clear that 

 the virus must increase within the tissues. In accordance with what 

 is said before, this increase occurs in and with the dividing cells, 

 the full-grown tissues of the plant not allowing any such incrense. 

 This quality of the virus reminds, to a certain extent, of the action 

 of the cecidiogenous substances, which likewise exert their influence 

 in those p:\rts of plants only, which are still in a state of growth 

 and cell-multiplication. Out of the plant it seems impossible to bring 

 tiie virus to increase. This conclusion must be drawn from the fact 

 that bougie-filtrate mixed and long kept with a certain quantity of 

 the filtrate of juice of a healthy plant, not only does not increase 

 but even loses in viiulence in the same degree as if it had been 



