which take their nourishment through a central canal. 3. Those which 

 absorb through the exterior, but which also possess a central canal. 

 As a whole, his explanations are very imperfect, which accounts, 

 perhaps, for the fact that his treatise is at present almost entirelv 

 neglected. 



Georgi 1 made some valuable chemical examinations of lichens 

 and demonstrated the presence of oils, resins, gums, alkaline salts, 

 silica, and other substances. 



Acharius 2 seems to have been in great doubt as to the origin and 

 structure of lichens. For a time he even questioned whether they 

 were plants ; that they, perhaps, belonged to the polyps. It was this 

 doubt which led him to make more careful investigations concerning 

 these plants. He made an especially careful study of the structure 

 of the apothecia upon which his system 3 of classification is based. 

 Though this system was doubtless an improvement on previous ones, 

 it was subjected to severe criticism by Weber and other writers- 

 This led Acharius 4 to improve and enlarge his former work. The 

 illustrations of the structure of the apothecia and thalli are fairly 

 good, but they indicate that he must have used poor microscopes. 

 He gives only a few very crude illustrations of spores. One can 

 readily understand that this system must, of necessity, be very defi- 

 cient because no use w r as made of the spore-characters. 



Sprengel 5 made an attempt to classify lichens according to the 

 structure of the apothecia ; it proved to be very unsatisfactory ; it is 

 evident at least, that this author had no definite ideas in regard to 

 the " seed" and "fruit" of these plants. In a later work 6 Sprengel 

 adopts the essentials of the system of Acharius and introduces Ger- 

 man diagnoses of species. 



Of the numerous lichenographers of this period there are ye 

 three who deserve special mention, namely, Fries, Eschweiler and 

 Fee. Fries 7 devoted his attention to the purely systematic con- 



1 Georgi. Chemische Untersuchungen einiger Flechten. Act. Acad. Scien. 

 Petropol. pars. alt. 1779. 



2 Acharius, E. Anmiirkinger rorande Lafarterne. Kongl. Vet. Acad. Nya 

 Handl. 17: 1796. Prod. Lich. Suec. XVII. 1798. 



3 Acharius, E. Methodus Lichenum. 1803. 



4 Acharius, E. Lichenographia Universalis. Gdttingen. 1810. 

 "'Sprengel, K. Einleitung in das Studium der crvptogamischen Gewachse. 

 Halle. 1804. English translation. London. 1807. 



6 Sprengel, K. Anleitung zur Kenntniss der Gewachse. 2d Ed. Halle. 1817. 



7 Fries, E. Conspectus Lichenum. K. Vetensk. Acad. Handl. 323-324. 1821. 



