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 

 found immediately opposite on the other; and hence 

 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. 



Ha 



