ISO 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 lf 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. 1.) Lamarck names it sphceria 

 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- 

 platanus) 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 sphaeria. 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. 



