OP ARTS AND SCIENCES. 17 



summarized in a series of papers published in the Quarterly Journal 

 of Microscopical Science,* and to those papers the reader is referred 

 for further information upon the subject. This summary closes with 

 a brief account of Treub's experiments of 1873 in the culture of lichen- 

 spores in the presence of unieelhilar alga;, and presents in a concise 

 form all the evidence which conspired to establish beyond all con- 

 troversy the first of Schwendener's hypotheses, that a lichen is a 

 compound organism consisting of a fungus parasitic upon a living uni- 

 cellular or filamentous alga. The grounds for this hypothesis were 

 originally merely the remarkable similarity in outwai'd aj^pearance 

 which was seen to exist between the gonidia of lichens and certain of 

 the lower algce, but the proof of the identity of these two similar organ- 

 isms was to be found only from simple and synthetic cultures. As 

 early as 1867, Famintzin and Barenetzky f had published the results 

 obtained by them from the culture of the chlorophyllaceous gonidia of 

 Physcia, Evernia, and Cladonia, which showed that from these gonidia 

 were produced unicellular algfe identical in form and method of repro- 

 duction with Kiigeli's genus Cystococcus ; while from cultures made 

 by Itzigsohn,t about the same time, of the gonidia of Peltigera, were 

 produced Gloeocapsa monococca, Kiitz., and Polycoccus punctiformis, 

 Kiitz. Later, Woroniu § confirmed these results by proving that 

 from the zoospores emitted by the gonidia of Parmelia pulverulenta, 

 Ach., were produced algoe identical with those gonidia. In 1871 the 

 Schwendener hypothesis received further confirmation from the ex- 

 periments of Reess,|| who sowed the spores of CoUema on Nostoc colo- 

 nies, and produced what might easily be considered a mature Colle- 

 maceous lichen, and two years later appeared the results of Bornet'slf 

 experiments in more than sixty genera of lichens, from which he drew 

 the following conclusions: — 1. Every lichen gonidium may be re- 

 ferred to some definite species of alga. 2. The relations of the hyphae 

 and gonidia are such that neither could have arisen from the other, 

 and the theory of parasitism alone can explain. them. 



It was reserved however for Treub, by the synthetic culture of 



* W. Archer, Quart. Jour. Micr. Sci., Vol. XIII. p. 217, and Vol. XIV. p. 115. 



t Famintzin and Barenetzky, Memoires de I'Acade'mie Impe'riale des Sciences 

 de St. re'tersbourg, Se'r. VII. Tom. XI. No. 9. Botanische Zeitung, 1867, p. 169. 



\ Itzigsohn. Bulletin de I'Academie Iniperiale des Sciences de St. Pe'ters- 

 bourg, Tom. VI. Botanische Zeitung, 18G8, p. 185. 



§ Woronin, Annales des Sciences Naturelles, Se'r. V. Tom. XVI. 



II Reess, Monatsb. d. K. Akad. d. Wiss. zu Berlin, 1871. 



H Bornet, "Sur les Gonidies des Lichens," Comptes Rendus, Tom. LXXIV. 

 No. 12. 



VOL. XXV. (n. s. xvii.) 2 



