DECOMPOSITION OF THE SYCAMORE. 



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 pe- 

 culiar to the laurel and the holly. (See Plate 4. 

 Fig. 2.) 



The leaf of the elm in autumn may commonly 

 be observed marked with dark-coloured blotches, 

 which are the " plague spot" of its destruction. 

 These leaves remain in large proportions unin- 

 jured through the winter months; but when spring 

 arrives, the spots become matured, the surface 

 cracks, and the capsules discharge their seeds. 

 (See Plate 4. Fig. I.) Lamarck names it sph&ria 

 xylomoides, but mentions another as a more early 

 observer. At these spots the decay of the leaf ge- 

 nerally commences. 



Most persons must have observed that the upper 

 surface of the leaves of the sycamore (acer pseudo- 

 platamis) is blotched with dark-coloured spots 

 (xyloma acerinum) in autumn. This leaf is de- 



* Without close examination, this plant appears to be a uredo ; 

 but it is in fact a sphseria. Uredo differs from sphaeria chiefly in 

 the vessels not containing the capsules in cells, but loose. Hoff 

 man 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. 



