DECOMPOSITION OF THE SYCAMORE. 89 
in considerable numbers during the spring and summer 
seasons. These long resist the common agents of disso- 
lution, like those of the holly, by means of the im- 
penetrable varnish that is spread over them. This, 
however, wears off, and they decay ; but their destruc- 
tion is at times accelerated by a small excrescent sub- 
stance, which fixes on the leaf, breaks the surface, and 
admits humidity. It appears in the form of a small 
black speck, and, when ripe, discharges a yellow 
powder from the centre ,* but as soon as one speck, 
which is the vessel containing the capsules, has fixed 
itself on one side of the leaf, a similar one will be 
i found immediately opposite on the other; and hence 
j it is well named by Lamarck the two-fronted uredo 
(uredo bifrons).* This I believe to be peculiar to the 
laurel and the holly. 
The leaf of the elm in autumn may commonly be 
observed marked with dark-colored blotches, which are 
the “plague spot” of its destruction. These leaves 
remain in large proportions uninjured through the win- 
ter months ; but when spring arrives, the spots become 
matured, the surface cracks, and the capsules discharge 
; their seeds. Lamarck names it sphaeria xylomoides, but 
mentions another as a more early observer. At these 
spots the decay of the leaf generally commences. 
Most persons must have observed that the upper 
surface of the leaves of the sycamore (acer pseudopla- 
tanus) is blotched with dark-colored spots (xyloma 
acerinum) in autumn. This leaf is detached by the 
earliest frosts, and falling to the ground the spots com- 
mence their operations by corroding away the portions 
of the leaf that surrounds them, but continue attached 
themselves, appearing as raised, shining, vermicular 
lines. This has been mentioned by Lamarck and 
others, and is only now noticed to point out the vari- 
* Without close examination, this plant appears to be a uredo ; but 
it is in fact a sphaeria. Uredo differs from sphaeria chiefly in the vessels 
not containing the capsules in cells, but loose. Hoffman observes, 
that both sphaeria and uredo discharge pollen from an orifice ; but, 
if the summit of this plant be cut off, the capsules are obvious. 
H2 
