110

Scientific Memoirs by

section, and in longitudinal section differing only in the greater length of the
cells.

     The transverse diameter varies from 20" to about 67", the longitudinal
from 36" to 92". The intercellular spaces show the same peculiar appearance
as in the cortex. There are also present the same kind of large secretory (?)
cells, with dark staining contents, as in the cortex.

     Sections taken through a white zone of a root show, so far as the greater part
of the cortex and the stele are concerned, the same structure as sections through a
brown zone.

     There is, however, a considerable difference in the outer cortical region, which
may best be explained by tracing the course of development that leads to the
appearance of the white zones.

     In the region of the root, where a white zone is about to develop, the inner
sclerenchymatous hypodermal ring is absent. The outer cells of the cortex in the
region, corresponding to the inner margin of the inner hypodermal ring, form
several layers of actively dividing cells. These cells are thin walled, oval or
circular, and continuous internally with the ordinary cortical cells. Externally
these actively dividing layers of cells produce a crescentic mass of cells, the
inner three or four layers of which are made up of cells also in a state of sub-
division and of the same shape as the cells of the underlying originating layers,
only with thicker and coarser walls. As the cells of the mass are pushed further
out, they develop large intercellular spaces and assume very irregular shapes,
as shown in figure 7 of Plate III. Under a high power they are seen to be finely
muriculate. The whole mass reminds one of the packing cells of a lenticel.

     The crescentic mass of packing cells is at first bounded externally by the
outer hypodermal ring, and at each horn of the crescent by the ends of the in-
complete inner hypodermal ring. Ultimately the pressure of the mass of rapidly
formed cells increases to such an extent that the outer hypodermal ring, and
overlying epiblema if present, are ruptured, and the mass of cells appears on the
surface as a white patch.

     The above description applies to the appearance of a transverse section
taken just a little above the upper margin of a white zone, and represented in
figure 4 of Plate III.

     A transverse section taken just below the upper margin of a white zone,
shows the layers of actively dividing cortical cells encircling the entire circum-
ference of the root and giving off packing cells all round, which externally are
separating from each other and falling away. There is no trace of the inner
hypodermal ring.