Verrticaria margacea , Wahl., an Aquatic Lichen . 247 
septate, at different periods of the life-history. The fact that the spores 
become triseptate at a comparatively late period probably accounts for 
some of the discrepancies in the above-mentioned descriptions. V . 
margacea is repeatedly said to have unilocular spores, 1 although in a few 
instances 2 triseptate spores are said to be characteristic of V. margacea or 
its synonyms. 
It should be remembered in dealing with lichens that spore production 
is characteristic of the fungal constituent. The spores are incapable of 
reproducing the lichen thallus unless they are brought in contact with 
a suitable alga. Lichen spores are, therefore, more or less unreliable 
agents of reproduction. It seems not improbable that the late appearance 
of septa is a sign of degeneration. 
This goes to show that spore-characters of lichens are not always 
reliable as a basis of classification unless the complete life-history of the 
organism is followed out. 
In conclusion I have to express my gratitude to Professor West for 
his interest and assistance during the progress of the work, also to Dr. T. W. 
Woodhead for helpful suggestions. 
Summary. 
1. Verrticaria margacea is a crustaceous lichen attached to smooth 
stones in streams. The young thallus is pale green, but when older it 
appears dark olive-green or almost black. 
2. The algal constituent of the lichen is a form of Protococcus viridis, 
A g. The fungus forms a compact, pseudo-parenchymatous network, 
absolutely uniform in structure throughout the whole thallus. Three types 
of thallus structure have been observed : 
(a) In the young thallus the algae are disposed singly throughout the 
colourless fungal network. 
( b ) An older thallus shows the algae in the form of short chains at 
right angles to the surface of the thallus ; the chains are 
uniformly distributed throughout the fungal network (homoio- 
merous). 
(c) The chains of algae may be aggregated near the dorsal and ventral 
surfaces of the thallus. 
(A tendency towards the heteromerous condition.) 
3. The perithecia are immersed in elevations of the thallus, and are 
characterized by a firm, black outer investment. 
1 Leight. Lich. FI., p. 446. Mudd, Man. Brit. Lich., V. submersa , p. 286. Borr. : E. Bot. 
Suppl., No. 2768. 
2 Leight. Ang. Lich., p. 62. Mudd, Man. Brit. Lich., p. 294 ( Thelidium cataractum ). 
Lindsay : Blit. Lich., p. 306. 
