This view can hardly be accepted; isidia as well as soredia are typical of certain species and are produced regularly and normally in ordinary conditions; both of them are often present on the same thallus. It is not denied, however, that their development in certain instances is furthered by increased shade or moisture. In Evernia furfuracea isidia are more freely produced on the older more shaded parts of the thallus. Zopf[513] has described such an instance in Evernia olivetorina (E. furfuracea), which grew in the high Alps on pine trees, and which was much more isidiose when it grew on the outer ends of the branches, where dew, rain or snow had more direct influence. He[514] quotes other examples occurring in forms of E. furfuracea which grew on the branches of pines, larches, etc. in a damp locality in S. Tyrol. The thalli hung in great abundance on each side of the branches, and were invariably more isidiose near the tips, because evidently the water or snow trickled down and was retained longer there than at the base.
Bitter[515] has given a striking instance of shade influence in Umbilicaria. He found that some boulders on which the lichen grew freely had become covered over with fallen pine needles. The result was at first an enormous increase of the coralline isidia, though finally the lichen was killed by the want of light.
Isidia are primarily of service to the plant in increasing the assimilating surface. Occasionally they grow out into new thallus lobes. The more slender are easily rubbed off, and, when scattered, become efficient organs of propagation. This view of their function is emphasized by Bitter who points out that both in Evernia furfuracea and in Umbilicaria pustulata other organs of reproduction are rare or absent. Zopf[513] found new plants of Evernia furfuracea beginning to grow on the trunk of a tree lower down than an old isidiose specimen. They had developed from isidia which had been detached and washed down by rain.
VI. HYMENOLICHENS
A. Supposed Affinity with other Plants
Lichens in which the fungal elements belong to the Hymenomycetes are confined to three tropical genera. They are associated with blue-green algae and are most nearly related to the Thelephoraceae among fungi. The spores are borne, as in that family, on basidia.
Fig. 86. Cora Pavonia Fr. (after Mattirolo).
The best known Hymenolichen, Cora Pavonia ([Fig. 86]), was discovered by Swartz[516] during his travels in the W. Indies (1785-87) growing on trees in the mountains of Jamaica, and the new plant was recorded by him as Ulva montana. Gmelin[517] also included it in Ulva in close association with Ulva (Padina) Pavonia, but that classification was shortly after disputed by Woodward[518] who thought its affinity was more nearly with the fungi and suggested that it should be made the type of a new genus near to Boletus (Polystictus) versicolor. Fries[519] in due time made the new genus Cora, though he included it among algae; finally Nylander[520] established the lichenoid character of the thallus and transferred it to the Lecanorei.