Nov. 15, 1920 Pathological Anatomy of Potato Blackleg t> 2 7 



The elements of the xylem are normal in size, though occasionally 

 they appear smaller. The lumen is reduced; the walls are thicker and 

 more strongly lignified. Even in unstained sections and without the mi- 

 croscope the xylem appears to be darkened. The discoloration often ex- 

 tends to the stem apex and into the petiole, but it is most pronounced in 

 the underground parts of the stem where the external symptoms are most 

 striking. Usually the cell wall alone is discolored, but sometimes a 

 brown, gummy deposit is found in the lumen of the cells, especially of 

 the larger vessels. In typical cases, only the primary xylem is affected; 

 in advanced stages, however, a part of the secondary xylem may also 

 show the browning of the walls. This discoloration of the elements of the 

 xylem is not necessarily a symptom limited to blackleg, since it is asso- 

 ciated with numerous other pathological disturbances and is commonly 

 observed in plants which are suffering from excess of water. 



The phloem fibers are more abundant. They, too, show a general in- 

 crease in wall thickening and intensity of lignification. The secondary 

 wall often is so thick as completely to fill the lumen (Pi. 57, C; 58, A); it 

 is distinctly layered and contains numerous simple pits. 



While one occasionally finds sclereids in the cortex of the under- 

 ground stem of the normal plant, there is nothing that could compare 

 with their relative abundance in plants suffering from blackleg. These 

 sclereids are typical parenchyma cells with strongly lignified secondary 

 walls (PI. 57, C; 58, A). They are either scattered or form solid masses 

 of tissue, often completely replacing the pith and part of the cortex. 

 The transformation of pith cells into sclereids is most pronounced in the 

 apical stem region and in the petiole. In the midrib and in the stem 

 region close to the base, where the browning of the xylem is most pro- 

 nounced, relatively few sclereids are found. 



In the small parenchyma cells of the perimedullary zone similar changes 

 occur. The cells show at first pectic degeneration, which is followed by 

 lignification. The peripheral pith cells, especially in the interfascicular 

 region, are sometimes completely transformed so that they form a 

 sclerenchymatous sheath on the inner face of the vascular tissue. 



The phloem elements are mostly normal at the base of the stem but 

 show increasingly advanced pathological changes toward the apex and 

 in the petiole. The cell walls are swollen, occasionally necrotic. The 

 cells of the pericycle undergo similar changes which are more severe and 

 are noticeable even in the lower stem regions. 



Plants which are infected early but do not succumb to the attack of the 

 parasite very readily show the most typical and pronounced symptoms. 

 In plants in which the course of the disease has been a rapid one, relatively 

 few changes are exhibited. It will be understood, however, that indi- 

 vidual plants vary and that the environment, the age of the plant, and 

 its physiological constitution will in a large measure determine the degree 

 of anatomical changes in tissues and organs. 



