224 BOARD OF AGRICULTURE. 



ask. The word " cause " is not used in natural science to sig- 

 nify ultimate, but proximate cause. Ultimate causes are dis- 

 cussed in theology and philosophy ; but they do not properly 

 fall within the scope of natural science. When we say that 

 a disease is caused by a fungus, we simply mean that the man- 

 ifestation of symptoms which we collectively call the disease 

 is invariably preceded by the presence and growth of the 

 fungus. To illustrate : the formation of the black knot is 

 invariably preceded by the presence of the fungus known to 

 botanists under the name of Sphceria morhosa, and the on- 

 ward growth of the mycelium of the fungus in the healthy 

 stem of the plum is followed without fail by the swelling and 

 blackening characteristic of the knot. On the one hand, we 

 never find the fungus unless accompanied by the knot. On 

 the other, we never find the knot unless accompanied and also 

 preceded by the fungus. If you examine the slightly swollen 

 branches of the choke-cherry in spring, before the bark has 

 cracked open, you will find the threads of the fungus already 

 in the stem ; and, later in the season, you will certainly find 

 the characteristic swelling and blackening. If the fungus 

 were only found with the knot, we could not say that it was 

 the cause of it. As the knots grow old, there is usually a num- 

 ber of insects and fungi found in or on them. They cannot, 

 however, be considered the cause of the knot, as they are 

 found in other excrescences as well. It is because the fungus 

 constantly precedes, as well as accompanies, the knot, that we 

 are entitled to say that it is the cause of the knot. 



Next, as to the means to be taken to prevent the spread of 

 the disease. Our knowledge of the habits of the fungus 

 throws light upon this point. First, we have seen that the 

 threads of the fungus extend in the stem some inches beyond 

 the knot itself, and these threads will, the next season, be fol- 

 lowed b}'" a new knot. Hence, in cutting away the knots, one 

 should cut several inches — to be safe, we will say about ten 

 inches — below the knots. The way the disease iiicreases in 

 a plaiit once attacked is by the onward growth of the myce- 

 lium. The next question is, how to prevent its spreading to 

 other trees. The spreading is produced by the growth of 

 the spores, one kind of which ripens, as we have seen, in mid- 

 winter, and another in early summer. The spores are all light, 

 and easily blown about, and, when they fall upon other trees, 



