Mr. G. H. K. Thwaites on the Gonidia of Lichens. 221 



the gelatinous substance of the plant, and most thickly towards 

 the periphery of the cylindrical branches of the fronds. Each 

 cell is found, upon a careful inspection, to be surrounded by its 

 definite ainouut of gelatine, and to be situated at the extremity 

 of an ultimate ramification of the numerous somewhat anasto- 

 mosing filaments which pervade the whole mass of the plant 

 (PI. YIII. A. fig. 2). The genus PauUa, Fee, a species of which 

 {Paulia perforata, Mont. ^ISS.) has, at the request of ^Ir. Berke- 

 ley, been kindly sent for my inspection by Dr. INIontagne, pos- 

 sesses an internal structure precisely similar in character to that 

 of Synalissa. The asci of Synalissa vulgaris contain numerous 

 perfectly spherical sporidia : I could not detect any apothecia in 

 Dr. Montague's specimen of Paulia. The genus Lichiaa is im- 

 mediately allied to Stigonema {Ephebe, Fr.), and the whole struc- 

 ture is very different from that of Paulia, as I have ascertained 

 from the examination of freshly-gathered specimens of the former 

 recently sent me by Prof. Harvey. 



Whilst writing on this subject, I may mention another very 

 interesting plant, which, in the texture of its frond and character 

 of its fructification, exhibits some analogy to Collema. I allude 

 to Mastodia tessellata, Flor. Ant., for a sight of specimens of 

 which I have been indebted to the kindness of Professor Harvey 

 and Mr. Berkeley. The essential structure of this plant is re- 

 presented by the genus Ulva (especially Ulva crispa), but it pos- 

 sesses apothecia containing asci, though the latter organs appear 

 to have escaped the observation of the excellent botanists who 

 described the plant, owing to the sporidia so soon becoming free. 



We have thus then offered to our view plants which, judging 

 from their external appearance alone, would be arranged together 

 in one undivided group, and even in some cases in the same 

 genus, exhibiting nevertheless totally different types of structure. 

 They are as follows : — 



1 . The Lichens proper ; 



2. Collema, Leptogium, &c. ; 



3. Synalissa and Paulia ; 



4. Mastodia; 



represented respectively, as regards their essential fundamental 

 structure, by the genera Pleurococcus, Nostoc, Coccochloris and 

 Ulva {U. crispa), which are usually placed very near together in 

 a natural arrangement ; but the circumstance of their each im- 

 pressing a character, upon being a bond of union, as it were, to 

 plants higher in the scale of vegetation, will doubtless, if well 

 considered, furnish a key to the proper arrangement of species 

 closely allied to and of equally low development with them. 



It is highly interesting to observe in these lower plants a 

 typical character of essential structure binding together nume- 



