264i EEY. J. M. CROMBIE ON" THE ALGO-LICHEN HYPOTHESIS. 



produced by the other, and the theory of parasitism can alone 

 explain it satisfactorily. It is in this way only, he maintains, 

 that we can account for the gonidia of very diverse Lichens being 

 almost identical, as well as for the marked differences between 

 the gonidia of other Lichens, of which the thallus and fruc- 

 tification are identical. Thus, also, he thinks, can only be 

 rightly explained the origin of dead gonidia found in the interior 

 of Lichens, as well as the occurrence in the same thallus of 

 dissimilarly coloured gonidia. 



But not to dwell further, at present, upon the conclusions 

 at which, upon these and similar grounds, the supporters of 

 Schwendenerism have arrived, we proceed to notice what would 

 naturally suggest itself as heing a very simple solution of the 

 prohlem. At first sight it might seem that the connexion 

 between the hyphse and the gonidia (if any such existed) could 

 readily be proved by cultivating lichens from their spores, and 

 tracing the subsequent evolution of the thallus at every stage of 

 growth. If the gonidia were proper organs of the lichen, we 

 should thus be able to observe when and how they first made 

 their appearance ; but if, on the contrary, they were foreign 

 bodies, then evidently they would nowhere present themselves in 

 the thallus thus obtained. Indeed Schw T endener himself (in ' Die 

 Algentypen,' &c.) rightly observes that the question cannot be 

 settled by hypotheses or isolated one-sided observations, but, as 

 De Bary had previously advised, by numerous and carefully con- 

 ducted experiments in the culture of lichen-spores, lichen-gonidia, 

 and unicellular Algae. In this way only, he says, can it be 

 definitely established whether the germinating lichen-spore de- 

 velops gonidia or not, and whether such free gonidia-groups as 

 he takes to be algals can form from themselves a hypha-bearing 

 thallus or not. Accordingly, to say nothing of earlier attempts, 

 numerous others have been made since the promulgation of 

 Schwendenerism, in the way of lichen-cultivation. Most of the 

 experiments thus instituted, as will presently appear, proceeded 

 upon an erroneous principle; while all of them confessedly 

 failed in producing gonidia and much less a perfect thallus. The 

 only legitimate cultures made were those of Treub (' Onderzoek- 

 ingen over de Natuur der Lichenen,' 1873), in so far at least as 

 these relate to the spores of certain species of Phjscia, liamalina, 

 and Lecanora, which, in the manner long before indicated by 

 Tulasne, were sown upon various moistened substrata. Nay, not 



