40 4 
Delf. — The Attaching Discs of the Ulvaceae . 
the superficial cells passing obliquely down to the disc, where they spread 
out and appear to become closely interwoven with one another. When 
they reach the host, the tips of the filaments swell up and become closely 
appressed to the surface, often appearing to form a kind of pseudo-paren- 
chymatous investment ; this is shown diagram - 
matically in Text-fig. i. 
Microtome sections through the discs of 
fronds which had been attached to stones showed 
that wherever the filaments had reached the ex- 
terior their tips had become swollen and multi - 
nucleate ; often definite cell-divisions appeared 
to have taken place, two or three successive 
segments having been cut off from the swollen 
part of the filaments (PI. XLV, Figs. 5, 6). These 
larger segments were often surrounded, especially 
on the outside, by the cut ends of many minute 
filaments. Some of these had their origin in 
peripheral cells of the thallus, as seen in PI. XLV, 
Fig. 8, at a; others appeared to have reached the 
periphery from within, and to have found no 
room for expansion at the end of their course ; 
it is possible that some may also have formed 
by the branching of larger filaments, such as that 
shown in PI. XLV, Fig. 1. The longitudinal section 
of such a disc is seen in Text-fig. 2, and a small 
part taken from the region A is shown in detail 
in Pl. XLV, Fig. 7. 
The region marked B is part of the disc of 
another frond, formed, as it appears, from an 
outgrowth of the disc filaments, but this capacity 
for forming new fronds from the tissue of the 
disc was seen better in a hand section, which is 
represented diagrammatically in Text-fig. 3. 
On 'teasing out the filaments from thick 
sections, it was found that they pursue a some- 
what sinuous course through the lower part of 
the thallus, often bending on themselves, and the 
pointed awl-shaped tip suggests an active boring 
movement, as though the penetration of the tangled filaments were 
against distinct resistance (PI. XLV, Figs. 2, 3). In some cases the 
filaments appeared to have branched, as in PI. XLV, Figs. 1, 4, but these 
were very difficult to isolate, owing to the delicate nature of the tubes. In 
one case, however, no fewer than ten branches had been given off (PI. XLV, 
Text-fig. i. Longitudinal 
section through attaching disc 
of Ulva : outline, and cells of 
host, drawn with camera lucida ; 
filaments of the disc shown much 
simplified, and somewhat dia- 
grammatically. x 100. 
