APPENDIX D. lix 



Rev. M. J. Berkeley says l : ' No vegetable productions are 

 more liable to variation than Lichens, a circumstance which 

 makes their study very difficult ; and, without a knowledge 

 of the fruit, it is almost impossible to distinguish species 

 accurately. Not only is the crust liable to put on various 

 forms by the over-production of some of its constituent 

 parts, but even where these are in a normal condition, the 

 degree of division of its lobes, the difference of colour, the 

 obliteration of the margin of the apothecia, the exposure of 

 those which have a true excipulum partially covered by the 

 crust, the greater or less crowding of the fruit, the reduction 

 of compound forms to simple, and many other circumstances, 

 induce variations which can only be appreciated by the prac- 

 tised student. The tropical Verrucartce, for instance, as- 

 sume forms so different, that, without a comparison of the 

 fruit, it is almost impossible to come to any correct judg- 

 ment, and in these the Lichenoid character is sometimes com- 

 pletely obliterated by the non-development or evanescence of 

 the crust' 



Algae. After speaking of the extreme variability of the 

 simpler unicellular Algae, Mr. Berkeley also says 2 : ' When 

 we come to the articulated Algae, amongst which the dis- 

 tinctions of species are often slight, an increased degree of 

 caution is requisite. A very short acquaintance suffices to 

 show the immense difference of diameter which may exist in 

 threads of the same mass, and in the same threads the pro- 

 portion of length and breadth in the articulations, are quite 

 as variable. Species, therefore, evidently of the most close 

 affinity, cannot be safely separated from mere consideration 

 of relative proportion without any other characters. Even 



1 Loc. cit, p. 1 66. 



2 Introd. to ' Cryptogam. Bot.,' 1857, p. 418. 



