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 im 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- 
rococcus, Meneghini, and around these cells, which merease 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 m these organs. ‘The 
gonidia ave in fact the essential part of the whole structure, 
and can scarcely be considered as gemme, 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 Algz. 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 Coccoch/oris : a number of single cells (or 
binate, when undergoing subdivision) are scattered throughout 
