i6o 



ment (Usnea~). The radial structure predominates in Cetraria, and 

 in the genera above it. The question may therefore arise as to- 

 whether the fruticose or the foliose lichen-thallus presents the 

 highest lichen-type. The answer to this question depends upon 

 whether we consider the mechanical or the physiological function as 

 of prime importance. In the study of Alectoria, Bryopogon, Usnea, 

 in fact of all typically fruticose thalli ( Sphaerophorus, podetia 

 of Cladonia) we find that the mechanical adaptation is highly spec- 

 ialized ; the physiological adaptations are, however, not corre- 

 spondingly developed. In living organisms the physiological func- 

 tion takes precedence over the mechanical function, which suggests 

 the reason why the Collemaceae and Pannariaceae are here placed 

 higher than the Parmeliaceae. 



Some of the general characters of the family are the following : 

 The apothecia are discoid with thalloid exciple ; but in Urceolaria 

 and Pcrtusaria they are more or less immersed in the thallus and can 

 therefore not be said to have a true thalloid exciple ; the thallus is 

 usually greenish in color, though there are marked exceptions, as in 

 Alectoria and Bryopogon ; and most of the Evernias are tinged with 

 yellow ; the spores are quite variable as to the number of septa but 

 quite constant as to size and absence of color ; they are medium in 

 size but vary greatly in form. Urceolaria is the only genus with 

 multilocular dark spores. The evidence of the spore-characters is 

 in favor of the theory that the spores degenerate as the lichen be- 

 comes more highly specialized ; this is also well illustrated in the 

 genus Parmelia. The higher forms are quite constantly sterile or 

 with very few apothecia, while the lower are constantly apothecium- 

 bearing ; it is evident in this genus that the occurrence of the soredia 

 increases with the decrease of the apothecia. 



The symbiotic algae are typically Cystococcus hiimicola Nag. 

 They seem to reach their maximum size in the fruticose forms as 

 Evernia, Alectoria and Bryopogon ; their maximum increase in num- 

 ber is reached in Parmelia and other foliose forms and in the cen- 

 tric thallus-lobes of Cetraria. 



The fungal portion of the Parmeliaceae indicates a polyphyletic 

 derivation from the Patellariaceae ; but which groups of the Patellar- 

 iaceae represent the ancestral forms from which the various lichen 

 genera were derived is not definitely known. The spermagonia are 

 quite numerous in some of the genera, particularly in Parmelia and 



