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Wisconsin State Horticultural Society. 



ori the surface in spots which are green colored, not like the color 

 of a leaf, however, but a very dark, deep greeu. A microscopic 

 examination of the greenish knots which are found late in the 



Fig. 12. Fig. 13. 



spring shows on the surface a number of threads, on which are 

 borne small bodies which are also spores; and to distinguish them 

 from the other spores they are called conidial spores (Fig. 13 B). 

 As the conidial spores ripen and fall off, we find beneath the sur- 

 face the beginnings of the cavities and the sacs which are to con- 

 tain the winter spores. The knot grows rapidly blacker and 

 rougher. 



Having given as briefly as possible an account of the develop- 

 ment of the black knot, it remains to state the conclusions which 

 may be drawn from the knowledge obtained by means of the 

 misroscope. In the first place, we may say that the disease is 

 caused by a fungus. Why caused? you, perhaps, will ask. The 

 word "cause " is not used in natural science to signify ultimate, 

 but proximate cause. Ultimate causes are discussed in philosophy 

 and theology, but they do not properly fall in the scope of nat- 

 ural science. When we say that a disease is caused by a fungus, 

 we simply mean that the manifestation 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 



