X INTRODUCTION. 



was pointed out by Mr. R, Brown, botanists were at a loss 

 to define the respective orders, though very seldom indeed 

 puzzled as to whether a genus were Rosaceous or Legumi- 

 nous. If it be thus difficult to define groups among highly 

 organized plants, it can be no matter of wonder that when 

 we come to the Cryptogamia, whose structure is so much 

 more simple and uniform, and whose forms are still more 

 sportive, the difficulties become vastly increased. But it 

 fortunately happens that these difficulties are much more 

 formidable on paper than in the field. Thus, while the 

 system-maker, in his study, may puzzle his brains with the 

 fruitless task of attempting to express in words a diagnostic 

 which shall include every species of the class Alg^, and, 

 at the same time, exclude every denizen of the allied 

 groups. Fungi and Lichenes ; the student, roaming through 

 the fields or along the sea-shore, finds no difficulty what- 

 ever in recognising a sea-weed, as distinct from a mush- 

 room or a Lichen. The search into structure and affini- 

 ties among the works of creation is something like that 

 after first principles. We can distinguish and analyse up 

 to a certain point : there we are stopped by that invisible 

 and intangible, but impassable veil, behind which the 

 Creator hides his operations. At this point we must rest 

 satisfied with diffiirences which we can see, but which we 

 caimot know or define. Dismissing, therefore, specula- 

 tions on the exact limits between Alg.e and all other 

 tribes, let us proceed to consider the subject more imme- 

 diately before us, namely, the habit, structure, geographi- 

 cal dislrihution and uses of these plants. 



The name Alg^, under which the Lichens were for- 

 merly included, is now limited by botanists to that large 

 group or Natural Class of Cryptogamic or (lowerless 

 plants, which forms the ])rincipal and characteristic vege- 



