CLUSTBE-CXTPS. 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 leayes 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 cyhndrical, with 

 an evident tendency to curvature ; the mouth is 

 serrated, but not spUt 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 Bcestelia lacerata, that on pear-leaves as 

 BcBstelia ccmcellata, and the one on the leaves of 

 the mountain-ash as BcesteUa comuta. 



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 aucwparia consist of minute 

 globules intermixed with wool-like fibres. On 

 c 2 



