Journal of Agricultural Research voi. xxiv, No. 3 
The progressive lignification of the 
phloem tissue can best be studied in 
sections taken through the distal stem 
region. Here one usually finds a com¬ 
plete series of changes from the first 
cell response to complete disorganiza¬ 
tion of the elements. Before there is 
any detectable evidence of lignification 
the development of the vascular tissue 
shows a deviation from its normal 
course, which is indicated by a more 
irregular maturing of the xylem than 
is found in healthy plants (PI. i, B). If 
such a section is stained with phloro- 
glucin and hydrochloric acid, one notices 
upon close observation that the phloem 
cells centrifugal to the depression in 
the cambium show a slight amount of 
lignification in parts of Sie wall. 
The cells of the pericycle in this 
region differ also from the normal type 
in having a greater radial diameter. 
As lignifeation progresses the entire 
cell wall or entire groups of cells become 
affected, while at the same time the 
radial elongation of the pericyclic cells 
of the outer phloem and the paren¬ 
chyma of the perimedullary zone of 
the inner region becomes more pro¬ 
nounced (PI. 3). The cells which first 
show lignification are commonly found 
adjacent to the fibers, but now and 
again lignification in a phloem group 
may start at the center and extend in 
a centrifugal direction. 
Prior to lignification of the phloem, 
to be tested microchemically, a swell¬ 
ing of the walls of the diseased cells 
takes place. Ferrous sulphate and po¬ 
tassium ferrocyanid at this stage impart 
a deep blue color to the walls, indicat¬ 
ing the presence of large quantities of 
pectic substances. The swelling of the 
walls extends from the region of the 
fibers centrifugally (PI. 4, B). Grad¬ 
ually the cells lignify and process!vely 
cells and groups of cells are withdrawn 
from active participation in conduction. 
In severe cases most or all of the 
primary phloem becomes destroyed (fig. 
3; PI. 5). In the initial stages of ligni¬ 
fication of certain phloem groups the 
primary walls of adjacent cells swell 
and separate. The intercellular space 
