258 BIONOMICS 



by soredia. They are all peculiarly liable to be broken and portions of the 

 thallus scattered by the combined action of wind and rain. 



Peirce 1 found that Ramalina reticulata (Fig. 65), of which the fronds are 

 an open network, was mainly distributed by the tearing of the lichen in high 

 wind. This takes place during the winter rains, when not only the lichen is 

 wet and soft in texture, but when the deciduous trees are bare of leaves, at 

 a season, therefore, when the drifting thalline scraps can again catch on to 

 branch or stem. A series of observations on the dispersal of forms of long 

 pendulous Usneas was made by Schrenk 2 . In the Middle and North Atlantic 

 States of America these filamentous species rarely bear apothecia. The 

 high winds break and disperse them when they are in a wet condition. They 

 generally grow on Spruces and Firs, because the drifting filaments are more 

 easily caught and entangled on short needles. The successive wetting and 

 drying causes them to coil and uncoil, resulting in a tangle impossible to 

 unravel, which holds them securely anchored to the support. 



D. ERRATIC LICHENS 



In certain lichens, there is a tendency for the thallus to develop excres- 

 cences 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 escnlenta, which has 

 been scientifically examined and described by Elenkin 3 . He distinguishes 

 seven different forms of the species: f. esculenta, f. affinis, f. alpina, and 

 i. fntticulosa-foliacea which are Alpine lichens, the remainder, f. desertoides, 

 f. foliacea and f. csadcnta-tarquina, grow on the steppes or in the desert 4 . 



Elenkin 3 adds to the list of erratic lichens a variety of Parmelia mollius- 

 cnla along with P. ryssolea from S. Russia, from the Asiatic steppes and 

 from Alpine regions. Mereschkovsky 5 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 6 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 threadlike structures which 



1 Feirce 1898. * Schrenk 1898. 3 Elenkin 1901. 4 See Chap. X. 



8 Mereschkovsky 1918. 6 Meyer 1825, p. 44. 



