Medical Officers of the Army of India.

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     A transverse section taken still lower down a white zone, shows the inner
hypodermal ring now present and encircling the whole circumference of the root,
while the layer giving rise to the packing cells is now external to the inner
hypodermal ring.

     A transverse section further down still, shows the packing cells beginning to
disappear, and mixed up with cells of the outer hypodermal ring, resembling the
outermost cells of that ring in a brown zone. The section in fact, except for the
few packing cells still present, is just like a section through a brown zone.

     A longitudinal section made through the junction of a white zone with the
brown zone above it, and extending downwards to the brown zone below, explains
how the different appearances of transverse sections at various levels of the white
zone are brought about, and also how the white zone fades away inferiorly in an
indefinite manner, in contrast to the abrupt way in which it ends above. In a
longitudinal section, as in figure 5 of Plate III, the inner sclerenchymatous hypoder-
mal ring C, traced from above downwards, ceases abruptly just above the upper
margin of the white zone and begins again a little below the upper margin of the
white zone.

     The gap thus left between the upper and lower ends of the broken inner
hypodermal ring is occupied by the actively dividing outer cortical layers giving
rise to the packing cells, which latter have their focus of formation in this gap,
overhung by the torn outer hypodermal ring. The actively dividing cortical
layers traced downwards pass external to the lower continuation of the inner
hypodermal ring, and become continuous ultimately with the clear ring of cells,
separating the inner from the outer hypodermal ring. The dense inner layers of
the packing cells become continuous with the inner cells of the outer hypodermal
ring. At the upper end of the white zone the actively dividing cortical layers
give rise to packing cells only, but as the layers are traced downwards, the
thickness of the mass of packing cells diminishes, they gradually become less
irregular in shape, more like the outer cells of the outer hypodermal ring with
which they are mixed up, and ultimately are entirely replaced by the latter in the
lower zone below.

     Owing to the looseness of the texture of the packing cells, an extraordinary
amount of air is entangled in their interstices, and it is this which gives the
white appearance of the zone to the naked eye. This air is exceedingly difficult
to get rid of, especially from the inner layers of the packing cells, and it is the
presence of this air in the inner layers of the packing cells which causes the
appearance of the black band seen in figures 4 and 5 of Plate III.

     The development of the white zones appears to the writer to lead to the con-
clusion that they are in reality specialised lenticels of unusually large size and

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