INTRODUCTION T(^ ORYPTOOAMIC BOTANY. 70 



ing them into three very natural groups, according as they 

 inhabit the water, the earth, or the air. The great tribes of 

 Gonfervce and seaweeds, for instance, with very few exceptions, 

 are entirely or periodically submerged, and live at the expense 

 of matter imbibed from the medium in which they grow. 

 The Lichens, though with more numerous exceptions, grow on 

 bodies from which they can derive no nutriment, but depend 

 upon supplies which come from the surrounding air, the ex- 

 ceptions of aquatic species being very few indeed ; while, on 

 the contrary, the Fungi are altogether dependent u^Don their 

 matrix, are frequently parasitic, and are not unfrequently con- 

 fined to a single species, genus, or natural order of Phcenogams. 

 The aquatic species are as rare as in Lichens, and more so than 

 the truly terrestrial among Alga?. That the three groups are 

 natural, it is quite impossible to deny. The question is, whe- 

 ther they are groups of precisely the same importance ; a 

 question, the difficulty of which has been felt by most authors 

 who have treated on the subject. Linnseus, for instance, and 

 Jussieu, considered Lichens as forming a part of Alga?, in which 

 they are followed by Fries, one of the best authorities upon 

 Cryptogams, and, above all Botanists, possessed of that tact 

 which grasps the real affinities of organisms, even before their 

 structure is accurately known. It is true that two of the 

 groups are easily and naturally divisible into more, but the 

 question is not as to subordinate groups, but as to the larger 

 and more comprehensive divisions. The real difficulty, indeed, 

 lies with the Lichens ; but this is far less than it was formerly, 

 since it has been proved that, in essential structure, and in 

 their secondary fruit, whether of sexual importance or not, 

 they are most closely related to Fungi. For my own part, 

 I am of opinion, that at any rate the nearest alliance of 

 Lichens is not with Alga?, but with Fungi ; there is not a single 

 instance amongst Algre, of ascophorous fruit, for such genera 

 as Lichina, Mastodia, &c. are evidently allied to Collema, 

 and, though the thallus of Collema is nearly identical in struc- 

 ture with Nostoc or Hormosiphon (Fig. 21), I consider this 

 merely an osculating point ; the true mode of increase in 

 Nostoc, which is now well known, connecting that genus rather 



