272 REV. J. M. CROMBIE ON THE ALGO-LICHEN HYPOTHESIS. 



types, in ' Flora,' 1877, pp. 358-9 ; from which it is evident that 

 while some gonidia and gonimia bear a resemblance to certain 

 algals, others are so different as never to have found a place 

 amongst these latter. This had already been observed by Koerber 

 in his valuable essay ' Zur Abwehr der Schwendener-Bornet'- 

 schen Flechtentheorie ' (1874), where, towards the close of his 

 argument, he says that if the gonidia were all true Alga?, they 

 would all have been met with in the free state, whereas this is 

 by no means the case ; the gonidia, for instance, of Ncetrocymbe, 

 Phylliscum, Melanomia, and others, having not yet been found 

 elsewhere than within the lichen-thallus *. Ny lander also, and 

 more distinctly, affirms (Z. c. p. 356) that the gonidia of lichens 

 do not in nature at the same time occur within the thallus, and 

 living free without it. This disposes of an alternative which was 

 at one time put forward in opposition to Schwendenerism, viz. 

 that many of the presumed algals of the hypothesis were pro- 

 bably only erratic gonidia vegetating separately. Numerous 

 observations, however, entirely homologate Nylander's statement ; 

 so that we may regard it as well established that the gonidia 

 of lichens in their different types and forms are nowhere seen 

 save within the lichen-thallus f, and therefore cannot be Algae. 

 Hence, as there is no fungus-mycelium, so neither is there any 

 algal-colony in the lichen. 



From the considerations now adduced, it is evident that the 

 three propositions laid down by Koerber in the essay referred to 

 are correct, and rest upon even better grounds than he himself 



* Professor Cobn (in "Conspectus Familiarum Crypt." &e., in ' Hedwigia,' 1 872, 

 p. 17) also affirms that algals from which Usnca, Cladonia, &c. could proceed 

 are not known. As also directly hearing upon this point, Mr. Archer (in Quart. 

 Journ. Mic. Sc. n. s. vol. xiii. p. 234) puts the very pertinent question, " Why do 

 not several other aerial types, quite as accessible to an intruding parasite as 

 other species, play their part as gonidia-formers?" 



t This of course does not apply to the " soredia," or the rounded pulveru- 

 lent eruptions which frequently occur on the cortical stratum, and which con- 

 sist of gonidia intermingled with medullary hyphae protruded along with them. 

 These, being but loosely adherent to the surface of the thallus, are readily dis- 

 seminated outside the lichen, though occasionally, as in Alcctoria nidulifera and 

 the specimens exhibited of Usnca ccratina\&r. scabrosa, they originate propagula 

 while still adherent to the thallus. In these soredia the gonidia constitute 

 a " syngonidium," and are in reality fragments of the thallus serving, like 

 bulbils or gemmae, for the propagation of species which occur only in a sterile 

 condition. 



