56 POPULAR HISTORY OF LICHENS. 



dye species are richer in colorific matter at certain seasons, at 

 which they are consequently uniformly collected, than at 

 others. The mode of growth in some species, and its fitful 

 and irregular character in all, is also a fertile source of 

 alteration in the form of the thallus. In some foliaceous 

 species, growing equally from a common centre, it some- 

 times happens that the centre dies and falls away, while 

 the circumference is vigorously extending itself in an exoge- 

 nous manner, that is, by the production of new peripheral 

 rings. The growth of these rings or zones depends chiefly 

 on moisture ; the development of the thallus is retarded or 

 stopped during drought, but is immediately promoted by 

 the slightest rain or dew ; and in proportion as the hy- 

 grometric condition of the atmosphere varies, so do these 

 rings exhibit irregularities in form and size. Such abnor- 

 malities have doubtless formed the basis of many of the 

 "fairy-rings," described as existing on old trees and rocks ; 

 they are of common occurrence in Parmelia saxatilis and 

 its variety omphalodes. Frequently this decaying or dead 

 centre becomes the nidus of a new thalline growth, origi- 

 nating from the gonidia of the old thallus ; in such a con- 

 dition we have a fine example of simultaneous growth and 

 decay in the same individual. The gonidic element of the 



