Lichen. This generic term became unsatisfactory in proportion as 

 the number of known forms increased. There were, no doubt, botan- 

 ists before Micheli who recognized this defect in lichen classification, 

 but this author was the first to point it out to the botanical world. 

 Micheli went, perhaps, to the opposite extreme and made too many sub- 

 divisions. He divided lichens into thirty-eight orders, basing this 

 division upon the external appearance and consistency of the thallus, 

 the position of the apothecia (recej>tacula jloriim), and the soredia 

 (semina). Nineteen of these orders, representing one hundred 

 species, were illustrated. 



This step in advance was, however, more apparent than real, be- 

 cause of the fact that but little progress had been made in studying 

 the morphology of these plants. It cannot be denied, however, that 

 Micheli made good use of the simple microscope in the study of 

 lichens. 



The next prominent lichenologist was J. J. Dillen, 1 who made a 

 considerable change in the system of Micheli. He grouped the 

 lichens with mosses and subdivided them into three classes. These 

 classes were again subdivided into orders, series and divisions, ac- 

 cording to the structure of the thallus, and the structure and position 

 of the apothecia. 



After Micheli and Dillen other botanists did not hesitate to propose 

 other arrangements, none of which were, perhaps, equal in value 

 to those of the two eminent workers mentioned. These attempts 

 were, however, indirectly productive of good results, because, in order 

 to establish new systems, it was found necessary to make more care- 

 ful observations in regard to the gross, as well as to the minute anat- 

 omy of the thallus and apothecia. 



Hill, 2 who classed lichens with mosses, divided all the known 

 species into six genera — Collema, Usnea, Platysma, Cladonia, 

 Pyxidium and Placodium — whose limitations have been retained in 

 part up to modern times. Adanson 3 classed lichens with fungi and 

 separated them into nine genera. Linne's 4 system of lichens was in 

 a certain sense retrogressive, since he grouped all these plants under 

 the generic term 'Lichen (see Tournefort). This group was, 



I Dillenius, J. J. Historia Muscorum. London. 1763. 



2 Hill, J. A. History of Plants. London. 1751. 



3 Adanson, M. Families des Plants. Paris. 1763. 

 'Linne", C. Species Plantarum. Holmiae. 1753. 



