638 Bulletin 81. 



about Cincinnati in regions which had previously been free from 

 the disease. Other cases of a similar nature might be mentioned, 

 and although the disease may have been native to a large area, it 

 appears to have attracted attention only after the cultivated plants 

 upon which it thrives had been generally grown, and the condi- 

 tions for its rapid dissemination had become favorable. At 

 present the disease is confined to North America, no case of its 

 occurrence in Europe or foreign countries having been reported. 

 It is found throughout the United States, but is rare in the ex- 

 treme south. 



Description. — The superficial characters of the parts attacked 

 by this disease are familiar to nearly every one who is in any de- 

 gree interested in the extermination of the pest. The term ' ' black 

 knot" is very expressive, for the affected parts swell and present 

 the form of more or less elongated knots which turn a deep black 

 during the fall and winter. According to Humphrey* the newly 

 forming knot can often be seen in the fall. It then appears " as 

 a slight swelling of the branch, arising near an old knot, or inde- 

 pendently." It is generally in the spring, however, that the 

 knots can first be plainly seen. Affected portions commence to 

 swell as soon as the active flow of sap begins, and as the swelling 

 increases the bark is ruptured and a granular growth soon fills the 

 crevices. This newly formed substance is light yellowish brown 

 in color, and a tinge of green can sometimes be seen. The brown 

 shade deepens as the season advances. During the latter part of 

 May and early in June the swellings hav^e enlarged to such an ex- 

 tent that they are frequently three to four times the diameter of 

 the twigs upon which they are found. The bark very soon dis- 

 appears, and the knot is found to be composed of a mass "in 

 which all distinction between wood and bark has been lost. In 

 the knot we find bast fibres, wood cells, and dotted ducts ; but 

 the prevailing tissue consists of a collection of dotted, rectangular, 

 parenchymatous cells, with very thick walls which closely resem- 

 ble the walls of the medullary rays."t The knot also gradually 



*Eighth Annual Report of the Mass. State Agric. Exp. Station, p. 205. 

 fFarlow, Bull, of the Bussey Institution, Harvard University. Part V. 

 p. 446. 



