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



cuticle which invests the thallus of some species of Collema, or 

 rather of Leptogium, Fr., is a modification only of the anasto- 

 mosing filaments, as can be proved from the structure of some 

 allied plants. 



What has just been stated may be considered a description of 

 the ordinary structure of Collema and Leptogium, but in Collema 

 nigrum we find each frond corresponding to a single nostoc-vesi- 

 cle, which becomes invested with a cellular cuticle, and has ex- 

 ternal to this the characteristic anastomosing filaments, which, 

 with those of other similar fronds, go to form the filamentous 

 substratum or kind of thallus upon which the fronds of this spe- 

 cies are situated. In the true Lichens is to be traced a very 

 similar structure, only that instead of nostoc-vesicles we find 

 groups of cells very nearly resembling those of the genus Pleu- 

 rococcuSj Meneghini, and around these cells, which increase in 

 number by continual subdivision, anastomosing filaments or mo- 

 difications of them become developed, just as takes place in Col- 

 lema nigrum', indeed so great is the resemblance between the 

 small fronds of that species and a state I have found of Biatora 

 vernalis, as to have at first made me suppose they were imme- 

 diately allied to each other. 



From the above then it is clear, that the gonidia of a Lichen 

 are the analogues as regards their functions of the nostoc-vesicles 

 of Collema, and this view enables us to understand what pre- 

 viously appeared an anomalous character in these organs. The 

 gonidia are in fact the essential part of the whole structure, 

 and can scarcely be considered as gemnue, except when under 

 certain circumstances they put on that character, just as ordinary 

 cells do in other plants. 



The other elements of the Lichen-thallus may without difficulty 

 be believed to represent modifications of the anastomosing fila- 

 ments of Collema, which no doubt they are. 



It is thus shown that between Collema and the true Lichens 

 there subsists a close though not an immediate affinity, the 

 essential part of the former being represented by the genus 

 Nostoc, and of the latter by the genus Pleurococcus. 



There are other plants bearing considerable external resem- 

 blance to those we have been describing, but which are repre- 

 sented, as respects their essential structure, by other genera of 

 the lower Algae. Among such may be mentioned Synalissa vul- 

 garis, Fr., first gathered in this country by Mr. Borrer, who found 

 it growing upon St. Vincent's Rocks : externally this little plant 

 much resembles a Collema, but an examination of its internal 

 substance under the microscope exhibits to us a structure very 

 like that of the genus Coccochloris : a number of single cells (or 

 binate, when undergoing subdivision) are scattered throughout 



