The Attaching Discs of the Ulvaceae. 
BY 
E. MARION DELF. 
With Plate XLV and three Figures in the Text. 
T HE thallus in the Ulvaceae consists of a somewhat delicate green frond, 
terminating below in a narrow and very often short stalk or stipe, 
which is attached to stones, wood, or, less often, to other Algae by means 
of a tough disc-like expansion. 
This attaching organ was described by Thuret 1 in 1878 as a small 
disc formed by the interwoven tubular prolongations of the cells of the 
thallus. Any cell of the thallus was said to have the power of forming the 
tubes, with the possible exception of those cells which become zoosporangia. 
Just above the disc the thallus is distinctly thicker than higher up, and 
Thuret noticed that in some cases the tubes were woven together in dense 
strands or bundles, several such strands occurring side by side in one disc. 
Some material of Ulva latissima (Harvey), preserved in formalin, was 
accidentally obtained growing upon a fragment of the frond of some other 
seaweed of the Furcellaria type. Hand sections showed that the tubular 
prolongations of the cells (or disc filaments, as I shall call them) had in 
some places displaced the small peripheral cells of the supporting thallus, 
in a way which suggested either a parasitic or a saprophytic habit. Later 
on, in fresh material from Rottingdean, a frond of Ulva was found attached 
to part of a thallus of the Polysiphonia type of structure, and here the 
disc filaments had actually penetrated into the cells of the host, just as is 
done by the intracellular hyphae of some parasitic Fungi. Other Ulva 
plants were collected which were attached to small chalky stones, and 
the discs of these were detached, carefully scraped, and washed free from 
grit as far as possible ; they were then fixed in weak Flemming solution 
and embedded in the usual way. The microtome sections obtained from 
this and also from the parasitic material were stained with Kleinenberg’s 
haematoxylin and eosine. Hand sections and preparations of teased-out 
hyphae were also examined. 
A vertical section through the disc of a thallus of Ulva at its point of 
attachment to the supporting seaweed showed the tubular prolongations of 
1 Thuret: Etudes algologiques, 1878. 
[Annals of Botany, Vol. XXVI. No. CII. April, 1913.] 
