THE "BLACK KNOT." 



223 



found twisted together in bundles (Fig. 3, a), wliich in gen- 

 eral are parallel to one another, and run from within outwards. 

 They extend down into the stem for a short distance helow 

 the knot, but not more than a few inches. The threads in 

 the knot, as they come near the surface, branch more and 

 more, and become black, and at the surface form a granular 

 mass. The granulations can be seen with the naked eye ; 

 and, when examined under the microscope, each granulation 

 is found to contain a cavity (Fig. 1), in which are a number 

 of sacs intermixed with white threads. In the sacs are eight 

 spores (Fig. 2), each of which consists of two parts, one being 

 smaller than the other. The whole cavity opens outwards, 

 and it is easy to see how the spores reach the air. 



When spring comes, the 

 threads which were concealed 

 in the stem below the knot 

 begin to grow again, and cause 

 a new swelling just below the 

 old knot. In a few weeks 

 they will have grown to such 

 an extent as to burst through 

 the bark, and appear on the 

 surface in spots which are 

 green-colored, not like the col- 

 or of a leaf, however, but a 

 very dark, deep green. A 

 microscopic examination of 

 the greenish knots which are 

 found late in the spring shows on the surface a number of 

 threads, on which are borne small bodies (Fig. 3, 5), which are 

 also spores ; and, to distinguish them from the other spores, 

 they are called conidial spores. As the conidial spores ripen 

 and fall off, we find beneath the surface the beginnings of the 

 cavities and sacs which are to contain the winter spores. 

 The knot grows rapidly blacker and rougher, until it assumes 

 the appearance of the specimen before you. 



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

 opment of the black knot, it remains to state the conclusions 

 which may be drawn from the knowledge obtained by means 

 of the microscope. In the first place, we may say that the 

 disease is caused by a fungus. Why caused ? you perhaps 



Fig. 3. 



