Verrucaria margacea , Wahl ., an Aquatic Lichen . 243 
although the fungus dominates from a reproductive point of view, the alga 
plays a prominent part in determining the general aspect of the thallus, 
responsible as it is for the green colour, even from the earliest stages. 
Reproduction. 
1. Vegetative. I have been unable to gain any definite information as to 
the production of true soredia. On gently sweeping the surface of the 
thallus with a camel’s-hair brush, a large number of fragments are obtained, 
having a structure identical with that of the ordinary lichen thallus (Fig. 6). 
These are frequently somewhat circular in outline and may be true soredia, 
but only in one or two instances have such fragments been observed in direct 
connexion with the upper surface of the thallus. 
Any detached portion of the thallus appears to have the capacity of 
continuing its existence. 
2. The Perithecia } The perithecia are flask-shaped bodies wholly 
immersed in elevations of the thallus. When mature, each perithecium 
communicates with the exterior by means of an ostiole which is surrounded 
by the black outer investment of the perithecium. This black coat is very 
characteristic of the perithecium. It surrounds the perithecium on all sides 
except the base, and is readily visible through the transparent thallus which 
covers it. The development of the perithecium proceeds in the following 
manner : 
From the base of the thallus numerous hyphae grow vertically and 
obliquely towards the substratum. By their growth they usually push 
upwards that portion of the thallus above their origin, so that it appears as 
a small protuberance. The hyphae branch profusely, septa soon appear, so 
that a pseudo-parenchymatous tissue, in the shape of a broad, smoothly 
conical mass, is produced beneath the thallus (Fig. 7). The cell walls of the 
outermost hyphae of the cone become impregnated with a dark brown 
material, the deposition commencing at that portion nearest the base of the 
thallus and gradually extending towards the base of the cone-shaped mass 
of hyphae. In old specimens this deposit is so abundant that the perithecial 
investment appears quite black. If perithecia are being formed in close 
proximity, the dark brown tissues of each may become continuous, so that 
a dark interrupted stratum is formed beneath the crust (Fig. 12). 
During these changes in the outermost hyphae of the cone important 
changes are proceeding in the inner hyphae. These never become impreg- 
nated with the brown material. A cavity appears in the centre of the mass, 
which gradually increases in size. From the hyphae of the upper surface of 
this cavity short colourless threads (periphyses) are formed, and these hang 
1 A good deal of work has been done with a view to determining whether sexual organs are 
formed. The evidence, however, appears to be too scanty to admit of a definite statement on this 
point. 
