38 



GONIDIA OF LICHENS. 



Trentepohlia are mixed in such a manner that they present them- 

 selves in the best condition for observation. The threads of the 

 hypha fix themselves to the alga upon some part, it being perfectly 

 indifferent whether the part be young or old, and put themselves 

 in contact with individual cells. Often one portion only of the 

 algal-filament, or one cell only, is touched by the hypha ; but more 

 frequently the hypha applies itself to the surface, follows all the 

 sinuosities of its contour, gradually throwing out lateral branches 

 which encompass, as with arms, portions more or less large of the 

 cells they touch. The vegetation of the hypha is stimulated by 

 contact with the Trentepohliar, the points of contact swelling and 

 extending, and its cells becoming shorter, produce numerous 

 branches, which ultimately surround the alga with a dense net- 

 work. The threads of the hypha often lie in the spaces where the 

 cells of the Trentepohlia join each other, from which there results 

 a constriction, causing a breaking up of the algal-filaments into 

 fragments of different length. These the hypha transforms into 

 gonidia, exactly similar to those in the adult thallus of the Ope- 

 grapha. 



There is nothing to be seen in any part of the thallus at any 

 period of its growth to indicate that the Trentepohlia may be pro- 

 duced by the hypha. On the contrary, the extreme irregularity of 

 the first connexion established between them excludes the possi- 

 bility of such a thing, besides which they vegetate in an inverse 

 way, and it is often the last-formed cell of the alga which is seized 

 on by the hypha. 



Verrucaria nitida (Schrad.) and Roccella phycopsis (Ach.) fur- 

 nish excellent illustrations of the same facts. 



Amongst exotic Lichens, Chiodecton nigrocinctum Montg., 

 Lecidea microsperma Nyl., Byssocaulon niveum Montg., and 

 Cwnogoninm, were examined, and contained gonidia of Trentepohlia, 

 different from those enclosed in our indigenous species. 



Ccenogonium Linkii Ehrenb. presents peculiar facilities for 

 studying this subject. The thallus is composed of articulated 

 filaments, of a greyish -yellow in the herbarium, surrounded and 

 held together by a network of colourless and much finer filaments 

 belonging to the hypha. M. Schwendener regards the arti- 

 culated filaments as those of an Alga allied to Cladophora, but 

 M. Bornet considers them allied to Trentepohlia, and says that 

 he has seen them invaded by the hypha from their base up- 

 wards, the lower cells being completely enveloped by a dense 

 network, while the upper ones were free. He has observed 

 also, in some instances, the hypha attacking the algal filament 

 a considerable distance above its base, leaving the inferior cells 

 untouched. 



Phyllactidium, Kiitz., he considers furnishes the gonidia to 

 Opegrapha filicina, Montg., the hypha of the lichen invading the 

 alga at an advanced stage of its developement. He also conducted a 



