, STRUCTURES PECULIAR TO LICHENS 



143 



of smaller or larger size is thus gradually built up on the stalk filament or 

 filaments, and is ultimately detached by the breaking down of the slender 

 support. 



a. Scattered Soredia. The simplest example of soredial formation 

 may be seen on the bark of trees or on palings when the green coating of 

 algal cells is gradually assuming a greyish hue caused by the invasion of 

 hyphal lichenoid growth. This condition is generally referred to as " leprose " 

 and has even been classified as a distinct genus, Lepra or Lepraria. 

 Somewhat similar soredial growth is also associated with many species of 

 Cladonia, the turfy soil in the neighbourhood of the upright podetia being 

 often powdered with white granules. Such soredia are especially abundant 

 in that genus, so much so, that Meyer\ Krabbe^ and others have maintained 

 that the spores take little part in the propagation of species. The under 

 side of the primary thallus, but more frequently the upright podetia, are 

 often covered with a coating of soredia, either finely furfuraceous, or of larger 

 growth and coarsely granular, the size of the soredia depending on the 

 number of gonidia enclosed in each " head." 



Soredia are only occasionally present on the apothecial margins: the 

 rather swollen rims in Lobaria scrobiculata are sometimes powdery-grey, and 

 Bitter' has observed soredia, or rather soralia, on the apothecial margins of 

 Parmelia vittata; they are very rare, however, and are probably to be ex- 

 plained by excess of moisture in the surroundings. 



b. IsiDlAL Soredia. In a few lichens soredia arise by the breaking 

 down of the cortex at the tips of the thalline outgrowths termed "isidia." 

 In Parmelia verructilifera, for instance, where the coralloid isidia grow in 

 closely packed groups or warts, the upper part of the isidium frequently 

 becomes soredial. In that lichen the younger parts of the upper cortex 

 bear hairs or trichomes, and the individual soredia are also adorned with 



hairs. The somewhat short warted 

 isidia of P. subaurif era' m.z.y become 

 entirely sorediose, and in P- farinacea 

 the whole thallus is covered with isidia 

 transformed into soralia. The trans- 

 formation is constant and is a distinct 

 specific character. Bitter" considers 

 that it proves that no sharp distinction 

 exists between isidia and soralia, at 

 least in their initial stages. 



c. Soredia as Buds. Schwen- 

 dener* has described soredia in the 



MiiHi'iMli 



Fig. 82. Usnea barbata Web. Longitudinal 

 section of filament and base of "soredial" 

 branch x 40 (after Schwendener). 



^ Meyer 1825, p. 170. ^ Krabbe 1891. ^ Bitter 1901. * Schwendener i860, p. 137. 



