Observations on Squamosis and Exanthema of the Citrus. 147 
histologically identical with those formed in gummosis, but are even found, 
to my knowledge, on the shoots of young trees affected by this malady. 
This form of exanthema need not, therefore, be further studied. 
In the development of the erumpent pustule we meet with a character 
not observed in gummosis. Owing to the pressure from beneath, the 
epidermis ruptures and the cortical cells begin to proliferate, the proliferating 
cells, at first sub-epidermal, gradually extending deeper into the cortex 
until they finally make up all the tissue exterior to the last formed row of 
pericyclic fibre bundles. With the beginning of the proliferation or coinci- 
dent therewith, a sickle of susceptible tissue is laid down by the cambium 
and gum pockets are formed, in the manner described in our histological 
study of gummosis, between the medullary rays, which themselves are 
rarely involved in gummous degeneration. When large pustules are pro- 
Text-fig. 3. Cross-section through sickle of pathognomonic tissue showing development of gum 
pockets in rows, and separated from one another by more or less perfectly formed ligneous cells. 
duced the sickle of susceptible tissue laid down may become relatively 
thick at its middle point, and contain several rows of gum pockets more or 
less perfectly separated by a fillet of tissue not affected by gummosis 
(PI. IX, Fig. 4 ; Text-fig. 3). The growth in thickness of the sickle of sus- 
ceptible tissue is thus clearly due to a period of normal or subnormal 
growth being rapidly followed by the further laying down of gummo- 
genetic tissues. Such a peculiar development as this, and one in every 
respect homologous, may be observed infrequently in gummosis. 
When the shoots affected by exanthema show typical gummosis 
characters (PI. IX, Fig. 1), normal wood may re-form over the gum pockets, 
but I have not observed it laid down de novo over the pathognomonic 
sickles. Lignification of the cells of the susceptible tissue (sickle formation) 
begins with the cessation of the conditions favourable to gummous degenera- 
tion, but is less perfect than in squamosis or gummosis. 
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