562 Journal of Agricultural Research voi. xv, no. 10 



petiole, like the midrib, also shows normal conditions as regards the 

 amount and arrangement of vascular tissue. The fibers are more numer- 

 ous and of a larger diameter than those found in normal sections. The 

 secondary walls of these elements are often very thin and only very 

 slightly lignified. The smaller type of fiber, especially numerous in the 

 inner phloem, has its secondary walls heavily lignified, and the lumina 

 are sometimes filled with the granular deposit. Intercellular secretions 

 are evident in the peripheral region of the outer phloem and in some of 

 the cells of the pericycle. The inner phloem is, on the whole, normal^ 

 Plate 39, A, shows a disorganized phloem group with the wheel-shaped 

 arrangement of the parenchyma cells surrounding it. 



The diseased condition becomes more pronounced in the growing region 

 and especially in the nodal region of the upper part of the stem. Cross 

 sections through the upper three internodes show nearly all types of 

 pathological changes observed in this study. Plate 38, C, and Plate C, 

 figures I and 6, illustrate the formation of intercellular cavities between 

 the sieve tubes of an inner phloem group. The primary wall separates — a 

 fact not easily observed in unstained material — and the space resulting 

 from the separation of these cells is filled with a yellow substance, gummy 

 and plastic in nature. Similar changes occur in the outer phloem (PI. 

 38, B, and PI. C, fig. 2). In more advanced stages the formation of 

 intercellular spaces has progressed to the extent of causing bending in 

 and crushing of the phloem cells. In the final stage of disorganization 

 the phloem cells are almost always filled with a granular substance, while 

 the walls themselves have become cutinized or undergone similar changes 

 (PI. C, fig. 4). Lignification of either cell wall or cell content has never 

 been observed. 



The diseased condition is most severe in the nodal region of the upper 

 stem, and it is mostly the outer phloem that is affected. However, 

 cells of tissues other than the phloem exhibit pathological changes. 

 Plate 38, A, and Plate C, figure 3, show the effect of the disease in the 

 cells of the cortex and pericycle. The large cortical cells adjacent to 

 the fibers are filled with the granular precipitate, the walls themselves 

 having become cutinized, and the small triangular intercellular spaces 

 are filled with the yellow secretion. Intercellular spaces are seen to be 

 forming between the cells of the pericycle; they originate among the 

 outer fibers and gradually advance toward the cambium. The cells of 

 the pericy<;ie have become radially stretched and at the same time have 

 increased in number, forming a broad sheet of tissue between the fibers 

 and the primary phloem (PI. 42, A). In normal sections the primary 

 phloem groups, if not directly adjacent to the fibers, are almost always 

 separated by only one or two layers of cells. 



A different type of pathological condition is sometimes observed in 

 both stem and leaf sections. Plate 39, B, shows a section taken from the 

 internodal region of the middle part of the stem. Only the cells of the 



