S. Schwendener. 



ix 



cells containing chlorophyll, which form an essential constituent of the 

 thallus, the " gonidia," as they had been termed. It had long been known 

 that the gonidia closely resemble certain free-living organisms which had 

 been described as Algte. Inasmuch as the gonidia were assumed to be 

 developed from the colourless filaments of which the thallus mainly consists, 

 the view was held that many of the simpler Algee were in reality nothing 

 but the gonidia of Lichens, which had become free and had continued so to 

 live. With regard to the rest of the thallus, the similarity between the 

 colourless filamentous tissue and the mycelium of Fungi had been recognised, 

 as well as that between the spore-bearing fructifications of the Lichens and 

 those of the Ascomycetous Fungi. But no definite idea as to the nature of 

 the Lichen-thallus had been reached. Then it began to be realised that 

 there might be another, inverse, interpretation of the nature of the gonidia 

 which might lead to some satisfactory conclusion. The position was well 

 stated by De Bary in the following passage taken from his 'Morphologie 

 der Pilzen, Flechten, etc.,' 1866. Speaking of certain Lichens, he says : 

 " The Lichens in question are either the fully developed fructifying states of 

 plants, the incompletely developed forms of which have hitherto been 

 regarded as belonging to the algal groups IsTostocacese and Chroococcacese — 

 or these Nostocacese and Chroococcaceee are typical Algae, which acquire the 

 form of Lichens, because they are invaded by certain parasitic Ascomycetous 

 Fungi, the filamentous mycelium of which penetrates into the developing 

 thallus, and often becomes attached to the coloured cells." In the last of 

 his papers published (1868) in Naegeli's ' Beitrage,' Schwendener, as the 

 result of his observations on the gonidia, expressed himself strongly in 

 favour of the latter of the above alternatives, and promised a further 

 statement on the subject as soon as it became possible for him to resume his 

 work, which had been interrupted by his removal to Basle. This promise he 

 redeemed by the publication, in 1869, of. his celebrated pamphlet " Die 

 Algentypen der Fleehtengonidien," in which he adduced convincing evidence 

 that the gonidia do not originate in the thallus as developments of the 

 filamentous tissue ; but, on the contrary, are Algee which have become 

 imprisoned in, or invaded by, the mycelium of a Fungus, forming the 

 thallus, in which, as it grows, the algal cells multiply by division. This led 

 to the remarkable inference that a Lichen is a composite, not a simple, 

 organism, consisting of Alga and Fungus living together in a relation which, 

 on the whole, is one of mutual advantage ; an altogether new biological 

 conception, subsequently designated " symbiosis " by De Bary. This con- 

 clusion aroused the most lively opposition from the professed lichenologists. 

 For years the famous controversy raged, conducted with much acuteness and 

 no little acrimony, nor has it even now completely died out. It may be 

 added that such new facts as have since been discovered, notably, the 

 observations of Stahl (1877), have contributed to strengthen the position of 

 the Schwendenerian theory, and that it has long been almost universally 

 accepted. 



