THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL. 



Vol. YII. 



Washington, D. C, June, 1886. 



No. 6. 



A Eesiime of the Algo- Lichen 

 Hypothesis.* 



BY F. H. KNOWI.TON, B. S. 



In the light of modern science it is 

 hai'dly necessary to say that one of the 

 most interesting biological problems 

 of the day, in so far at least as relates 

 to vegetable morphology anel physi- 

 ology, is connected with the theory 

 usually known under this name, or, 

 as it might more correctly be called, 

 the Algo-Fungal-Lichen Hypothesis. 

 To prove this we have but to turn to 

 the now extensive literature of the 

 subject or glance for a moment at 

 the extended discussions to which 

 it has given rise. 



The object of the present paper is 

 to bring before the Society the re- 

 sults of some recent European in- 

 vestigations, particularly those of 

 Rev. James M. Crombie {Jotir. 

 Linn. Soc.^ xxx., pp. 259-283), 

 and to sum up briefly the princi- 

 pal arguments used in defence of 

 the autonomy of the plants called 

 lichens. 



I will first give, very briefly, the 

 history of this hypothesis, and the 

 causes which lead to its adoption 

 by so many of the Continental in- 

 vestigators. 



If we cut a thin, transverse section 

 through the thallus of a lichen, and 

 examine it under a moderately high 

 power of the microscope, we shall 

 find it to be readily distinguishable 

 into several parts or layers ; an up- 

 per and under cortical layer of 

 long, slender, and closely interlac- 



*Read before the Washington Microscopical Societj' 

 May nth, 1886. • 



ing cells, the hyphce, and a central 

 layer, or irregular chain, of larger 

 green cells, the gonidia. Now, the 

 problem to be solved is, What is the 

 origin of the gonidia, and in what 

 relation do they stand to the thallus ? 

 If it can be shown that they have 

 their origin outside of the organism, 

 and are subsequently entrapped, then 

 parasitism has proved its case ; but if, 

 on the other hand, it can be proved 

 that they have their origin in the hy- 

 phal layer, and are self - developed 

 organs of the thallus. then, of course, 

 this hypothesis must go to the wall. 



As might be supposed, in pre- 

 microscopical days nothing was either 

 kno^vn or written on the subject, for, 

 as a matter of fact, the gonidia were 

 not discovered until 182^, when they 

 were made out by Wallroth. In 

 Koerber's dissertation, De Gonidiis 

 Lichenum, published four years later 

 (1839), they were treated more fully 

 than in any previous work, but noth- 

 ing authentic was adduced as to their 

 genesis. Bayerhofler seems to have 

 been the first to give any explanation 

 of the matter, and in 18^1 he stated 

 that the ' threads of the fibrous stratum 

 swell up at the top, which swellings 

 afterward become the male gonidia.' 

 This was confirmed, with slight 

 modification, by Speerschneider in 

 1853, by Schwendener in 1859, as 

 also by De Bary in i86=^. 



This view of the genesis of the 

 gonidia from the hyphas was for 

 some time the accepted explanation, 

 but in 1868 Prof. Schwendener, re- 

 viewing the original notion on this 

 subject, towards the end of a paper 

 entitled Unterschung-cn iibcr den 



