142 Butler^ — A Study on Gummosis of Pr unus and Citrus, with 
this probably only occurs when the disease begins in the trunk, for if it starts 
in the smaller limbs these latter are soon girdled and chlorosis sets in, 
followed later by progressive death from the apex downwards (PI. VII, 
lower figure. Squamosis on limbs has practically destroyed one half the 
tree). When the disease begins in the trunk or large JLimbs chlorosis appears 
very much later and is much less intense ; in these cases the foliar appear- 
ance of diseased trees is not very indicative. In fact one might say that 
squamosis under these conditions, may be present in a grove without the 
affected trees showing, to the casual observer, any pathognomonic sym- 
ptoms, a scaling of the bark, though abnormal in the Citrus , being sufficiently 
common in trees in general not to excite any particular fears, especially 
since the disease is not correlated with definite environmental conditions. 
Squamosis may be observed in light soils and heavy soils, in dry soils 
and moist soils, but apparently never when the conditions are favourable 
for the development of gummosis ; it is, however, a form of this latter 
malady, and from this fact we are necessarily led to the conclusion that 
its development must also depend on a growth and water relation. 
Minute. The characteristic of squamosis, i. e. the scaling of the bark, 
has already been briefly mentioned. I shall now describe in greater detail 
the symptoms of the disease. 
Squamosis first appears, as has already been mentioned, as a rounded 
or an irregular exfoliation of the bark, about an inch or somewhat less 
in diameter, which is pushed out by the growth of the subjacent tissues 
upon which it stands in slight alto rilievo. The detached bark soon dies 
from the periphery inwards-, and curls more or less. The curling of the 
dying bark reveals the subjacent cortical tissues, which appear rugose, 
white, or white with a yellow tinge, and somewhat mealy. When the bark 
finally falls a more or less highly developed pustule will be found occupying 
the centre of the exfoliated area. The centre of the pustule is navicular, 
and gum infrequently oozes from it in small amount. Not infrequently the 
pustular outgrowth, especially after the disease has attained a certain develop- 
ment, becomes less prominent and is replaced by a general swelling of the 
sub-epidermal cortical layers which causes the bark to flake off in large 
strips (PI. VIII, upper right-hand figure). In Florida the shoots are also 
affected by squamosis, but in California this* form of the disease is confined 
to water sprouts coming from the neighbourhood of diseased tissues. The 
shoots are discoloured subepidermally into rather well-defined shield-like 
maculations somewhat raised above the surface. With age these dis- 
coloured maculations become indurated, darker in colour, and further raised 
above the normal surface of the bark. The epidermis ruptures around the 
diseased areas, which then present, even more markedly than before, a 
scuteliform appearance. When the shoots are more seriously affected the 
maculations lose their definiteness of outline, become larger, and the 
