640 BULI^ETIN 81. 



The large majority of the knots found upon Morello cherries 

 seem to form in the following places : in the crotches of younger 

 limbs, at the union of the consecutive growths of two seasons, and 

 upon small spurs which commonly bear the fruits. I have seen some 

 the cases in which it appeared as if entrance had been gained in the 

 axil of a leaf, or about the bud situated at its base. The swell- 

 ings frequently form in the crotches of limbs that sometimes are 

 four or five years old, but younger branches sujBFer to a very great 

 extent. The knots which form at the union of the growth of 

 different seasons may generally be found upon wood that 

 is from one to three years of age. The center of the swelling is, 

 asa rule, near the point of union. (See also page 646.) I have 

 as yet failed to find in autumn or winter a well developed knot 

 which was formed upon the growth made during the preceding 

 spring and summer. They have invariably appeared upon older 

 wood, and generally in the positions mentioned above. (See Fig. 3 

 page 645.) 



During May and June the newly formed knots are covered 

 with a velvety growth, but this soon disappears. The 

 knot then becomes dark and by winter has become perfectly 

 black. It is then covered by innumerable, smal .levations 

 which project from its irregular granular surface. At this'season 

 the knot is generally hollowed out by insects which have devel- 

 oped within, and so there is often not much more than a mere 

 crust remaining. During the fall and early winter many of the 

 knots become more or less coated with a substance which is yel- 

 lowish white, and other colors may also be seen. This appear- 

 ance is not a development of the knots, but is caused by a fungus 

 which is parasitic upon them. 



Cause. — It is only in recent years that the true cause of the 

 black knot has been demonstrated. Formerly the trouble was 

 generally supposed to be due to the work of insects, and this 

 opinion is still held by many, although scientists have shown 

 plainly that insects are not responsible for such swellings. Other 

 theories were also freely advanced, such as a diseased condition 

 of the sap, abnormal condition of the plant's "constitution," 

 the character of the soil in which the trees were growing, and 

 some form of mechanical injury. But such ideas were not so 

 commonly accepted. 



