176 M. C. COOKE ON THE DUAL-LICHEN HYPOTHESIS. 



destroy Lichens are few in number, and quite different in their rela- 

 tionships from those which prey on Fungi. 



There is also amongst the Diptera a family called Mycetophilida, 

 with like propensities, as evidenced by the genera Mycetophila, 

 Boletina, Mycetobios, Bolitophila, and yet there are none so equally 

 destructive or parasitic on Lichens. 



These insects must have come to a sounder conclusion than some 

 men, that Lichens are not Fungi, with the addition of an innocuous 

 green Alga. Surely if they had the same grateful food to offer, 

 these insects would patronise the Lichens ; but as they attack all 

 kinds of Fungi, and do not attack Lichens, insects certainly have 

 not yet become converts to the Schwendenerian hypothesis. 



In concluding these comparisons, the only inference which ap- 

 pears feasible is, that all these differences are so many reasons 

 against the identity of Fungi with Lichens. There must be a most 

 essential difference between them, beyond the gonidia, for the 

 gonidia are not sufficient causes to produce all these divergencies. 

 It is quite erroneous to state, as some have done, that the gonidia 

 constitute the only difference between Lichens and Fungi, whereas 

 the presence of gonidia is only one out of the many differences which 

 exist between them. 



Are Gonidia a part of the Lichen-structure ? — The thalli of 

 Lichens enclose within them peculiar cellules, forming a sub-cortical 

 layer, which are sub-globose, of a greenish colour, and to these the 

 name of gonidia has been given. They frequently burst through, 

 and appear on the surface, in the form of powdery masses. The 

 lichenologist believes, and as he thinks has good reason for believ- 

 ing, that they form part of the plant itself. One says, " They may 

 be regarded as intermediate in function between the vegetative and 

 reproductive cell, assuming the offices and partaking of the char- 

 acters of both."* The justly celebrated Tulasne considered them to 

 be parts of the Lichen, and also to perform important functions. 

 On the other hand, the theorists contend that they are no part of 

 the plant, but that they are a form of Algge, upon which the residue 

 of the Lichen is parasitic. The grounds of the belief are suppositi- 

 tious, and the proof is still defective. 



In the first place, it is argued that they are free cells, resembling 

 in size, form, and colour certain low forms of unicellular AlgaB, and 



* Lindsay, "Loc. Cit.," p. 41. 



