V err near ia niargacea , Wahl., an Aquatic Lichen . 245 
perithecium may be regarded as a specialization in order to ensure the 
reproduction of the organism. A single spore floating in the water has 
a very remote chance of encountering a suitable algal cell under such con- 
ditions that germination, and enclosing of the alga by hyphae, could occur. 
Hence single spores appear to be of little use as agents of reproduction in 
the case of an aquatic lichen. On the other hand, a mass of interlacing 
fungal hyphae, exposing a large surface, would be more likely to entrap any 
free floating cells of Pleurococcus , and thus the requisite algal constituent 
of the lichen would be secured. I have frequently observed the algal cells 
in the meshes of such hyphae. 
Cultural Experiments. 
Hanging drop and Petri-dish cultures of the lichen were made, in 
various culture media. The results are on the whole inconstant, but the 
following general statements may be made : 
Dilute Knop Solution (o*i per cent.-o*4 per cent.) favoured the growth 
of the algal constituent. In some cases the algal cells grew out into short 
filaments of 3-6 cells, and occasionally branching occurred (Figs. 10, 11). 
In other cultures the algae of the thallus became so numerous that the 
hyphal network was completely hidden. Algae which became detached 
from the thallus underwent vegetative division, resulting in colonies in the 
form of flat plates. The constituent cells were often polygonal in shape, 
due to compression (Fig. 9). In most cases, the fungal constituent eventually 
disappeared. 
Agar and dilute Knop solution , Agar and 2 per cent. Glucose solution . 
Neither alga nor fungus appeared to be in a healthy condition whilst 
growing upon these media. In most cases the thallus turned pale and 
eventually died. 
In no instances did the lichen thallus as a whole thrive. An attempt 
was made to synthesise the lichen by sowing fungal spores among algal 
cells obtained from a culture in dilute Knop solution. The apparatus used 
was a modification of that employed by G. Bonnier in his researches on the 
Synthesis of Lichens (Bonnier, p. 15). The algae continued to grow for 
several months, but the conditions of the experiment were evidently 
unfavourable to the continued growth of the fungus, as in every case the 
hyphae formed on germination of the spores eventually died. 
Systematic Position and Nomenclature. 
There seems to have been a good deal of confusion regarding the 
synonomy of the lichen in question. After careful consideration of the 
original descriptions, and others to which I had access, I have concluded 
that the following are synonymous : 
1. Thelotrema margaceum , Wahl., in Ach. Meth. Lich. p. 30, 1803. 
s 
