The Structure and Life-history of Verrucaria 
margacea, Wahl., an Aquatic Lichen. 
BY 
ETHEL M. POULTON, M.Sc., 
University Scholar , University of Birmingham. 
With Plates XTII and XIV. 
IT ERRUCARIA MARGACEA is a crustaceous lichen found adhering 
to smooth stones in running water. The material examined was 
obtained chiefly from a clear, shallow stream in Sutton Park, Warwick- 
shire, where the lichen was first observed in 190a by Mr. W. H. Wilkinson. 
It was subsequently discovered independently by Professor G. S. West, at 
whose instigation I undertook the study of the organism. The lichen also 
occurs in great abundance in the streams of Cannock Chase, Staffordshire. 
External Characters. 
When young, Verrucaria margacea appears as very small, almost trans- 
parent membranous patches, circular in outline, and pale green in colour 
(PI. XIII, Fig. 1). The crust at this stage is continuous and perfectly 
smooth. The small thalli are firmly adherent to the substratum, but may 
be peeled from the stone by means of a sharp knife. Increase in area 
eventually occurs and neighbouring thalli frequently coalesce to form a 
more or less extensive thallus of irregular contour (Fig. 2). A definite 
ridge usually marks the place where such union has occurred. The 
mature thallus is of a dark olive-green colour, and is very conspicuous 
when adherent to light-coloured stones. On darker stones the lichen is 
scarcely recognizable. Some of the older specimens examined were almost 
black, and the crust was often much wrinkled. The thallus is typically 
covered with perithecia which appear as small black spots. They have 
been observed on thalli as small as 2 mm. in diameter. 
The perithecia are arranged in no definite order. Occasionally a slight 
zoning is observed, but this is merely a coincidence. The youngest peri- 
thecia are situated nearest the periphery of the thallus. In no case do the 
perithecia extend to the extreme edge of the thallus ; there is invariably 
a zone devoid of perithecia, and pale green in colour. In many specimens 
[Annals of Botany, Vol. XXVIII. No. CX. April, 1914.] 
