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 interesting of any that we have passed in review. To botanists, the species found on the hawthorn is known as _Rœstelia lacerata_, that on pear-leaves as _Rœstelia cancellata_, and the one on the leaves of the mountain-ash as _Rœstelia 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 examining many of them in different states, I at length found a small maggot in some of the younger spots, so that the globules are probably its excrement, and the fibres, the woody fibres of the plant unfit for its food.” We now-a-days smile at such simple and singular conjectures. It affords evidence of the manner in which the speculations of one generation become follies in the next.
Only two species of cluster-cups are described in Withering’s Flora under the genus _Lycoperdon_: one of these is now called _Æcidium compositarum_, and is found on various composite plants; the other includes the species found on the wood-anemone and that on the moschatel, and also probably a species of _Puccinia_ on the wood-betony.
To render this chapter more complete, though of less importance to the microscopist, we may allude to the other two genera comprised within this order. _Peridermium_ is the name of one genus which contains two British species found on the leaves and young shoots of coniferous trees. In this genus the peridium bursts irregularly, and does not form cups, or horns, or fringed vessels. The most common species is found on the needle-shaped leaves of the Scotch fir (Plate II. fig. 27), and also on the young twigs, in the latter instance larger and more prominent than in the former. The elongated peridia burst irregularly at their apices without forming teeth (fig. 28).