D. Erratic Lichens
In certain lichens, there is a tendency for the thallus to develop excrescences of nodular form which easily become free and drift about in the wind while still living and growing. They are carried sometimes very long distances, and fall in thick deposits over localities far from their place of origin. The most famous instance is the “manna lichen,” Lecanora esculenta, which has been scientifically examined and described by Elenkin[921]. He distinguishes seven different forms of the species: f. esculenta, f. affinis, f. alpina, and f. fruticulosa-foliacea which are Alpine lichens, the remainder, f. desertoides, f. foliacea and f. esculenta-tarquina, grow on the steppes or in the desert[922].
Elenkin[921] adds to the list of erratic lichens a variety of Parmelia molliuscula along with P. ryssolea from S. Russia, from the Asiatic steppes and from Alpine regions. Mereschkovsky[923] has also recorded from the Crimea Parmelia vagans, probably derived from P. conspersa f. vaga (f. nov.). It drifts about in small rather flattened bits, and, like other erratics, it never fruits.
Meyer[924] long ago described the development of wandering lichens: scraps that were torn from the parent thallus continued to grow if there were sufficient moisture, but at the same time undergoing considerable change in appearance. The dark colour of the under surface disappears in the frequently altered position, as the lobes grow out into narrow intermingling fronds forming a more or less compact spherical mass; the rhizoids also become modified and, if near the edge, grow out into thread-like structures which bind the mass together. Meyer says that “wanderers” have been noted as belonging to Parmelia acetabulum, Platysma glaucum and Anaptychia ciliaris.
Fig. 121. Parmelia revoluta var. concentrica Cromb. a, plant on flint with detached fragment; b, upper surface of three specimens; c, three specimens as found on chalk downs; d, specimens in section showing central cavity (S. H., Photo.).
The most notable instance in Britain of the “erratic” habit is that of Parmelia revoluta var. concentrica ([Fig. 121]), first found on Melbury Hill near Shaftesbury, Dorset, and described as “a spherical unattached lichen which rolls on the exposed downs.” It has recently been observed on the downs near Seaford in Sussex, where, however, it seems to be confined to a small area about eight acres in extent which is exposed to south-west winds. The lichen is freely distributed over this locality. To R. Paulson and Somerville Hastings[925] we owe an account of the occurrence and origin of the revoluta wanderers. The specimens vary considerably in shape and size, and measure from 1 to 7 cm. in longest diameter. Very few are truly spherical, some are more or less flattened and many are quite irregular. The revolute edges of the overlapping lobes give a rough exterior to the balls, which thereby become entangled amongst the grass, etc., and movement is impeded or prevented, except in very high winds. Crombie[926] had suggested that the concentric plant originated from a corticolous habitat, but no trees are near the Seaford locality. Eventually specimens were found growing on flints in the immediate neighbourhood. While still on the stone the lichen tends to become panniform, a felt of intermingling imbricate lobes is formed, portions of which, in time, become crowded out and dislodged. When scattered over the ground, these are liable to be trampled on by sheep or other animals and so are broken up; each separate piece then forms the nucleus of new concentric growth.
Crombie[926] observed at Braemar, drifting about on the detritus of Morrone, an analogous structure in Parmelia omphalodes. He concluded that nodular excrescences of the thallus had become detached from the rocks on which the lichen grew; while still attached to the substratum Parmelia omphalodes and the allied species, P. saxatilis, form dense cushion-like masses.