CLUSTER-CUPS. 19 



spicuous except on account of its size, for it is the 

 largest of all that we have had occasion to notice. 



The third species occurs on the under-surface 

 of the leaves of the mountain-ash. The peridia 

 are clustered on a rusty orange-coloured spot 

 which is visible on the upper surface (plate II. 

 figs. 18, 19). They are long and cylindrical, with 

 an evident tendency to curvature ; the mouth is 

 serrated, but not split up into threads, as in the 

 species found on the hawthorn. There will often 

 be found instead of well-developed peridia, what at 

 one time were regarded as abortive peridia, forming 

 a thickened orange or rust-coloured spot, studded 

 with minute elevated points. These spots are 

 clusters of spermogones, which organs are described 

 in detail in our second chapter. The clusters and 

 spores are of a brighter reddish brown than in 

 either of the other species. All are remarkably 

 distinct, and perhaps the most curious and interest- 

 ing of any that we have passed in review. To 

 botanists, the species found on the hawthorn is 

 known as Rcestelia lacerata, that on pear-leaves as 

 Rcestelia cancellatci, and the one on the leaves of 

 the mountain-ash as Rcestelia cornuta. 



Dr. Withering observed the spore-spots on the 

 leaves of the mountain-ash, but was evidently 

 puzzled to account for them. He writes (in his 

 Arrangement of British Plants), " The spots on 

 the leaves of Sorbus aucuparia consist of minute 

 globules intermixed with wool-like fibres. On 



c 2 



