Vlll INTRODUCTION. 



Zygncmacece, and meet it by the assertion that, as far as our 

 experience goes, this suggestion is not supported, and the 

 evidence produced is far from satisfactory. The larger and 

 more complex hypothesis of the combination of Algse and Fungi 

 in the production of Lichens, has been combated elsewhere,* 

 and to this we have nothing to add, save that we have since 

 encountered no facts or arguments which would induce us to 

 modify that opinion. Even should hereafter the terrestrial 

 Nostocs be united with the Collemacece, the saxicolous Stigo- 

 nemece be relegated to the Sirosiphei, and the corticolous 

 Chroolepidce be transferred to some other relationship, the true 

 aquatic Alga? would still remain in sufficient number to interest 

 the student, and maintain for some time to come the distinctive 

 group of Fresh-Water Alga3. 



* " Grevillea," Vol. vii., p. 102. 



