THE GENUS POLYSIPHONIA. 277 
change in cases where examination proved possible. It seems most probable 
that the alga adheres largely by the action of its own gelatinous wall mem- 
brane, the formation of the dise giving increased surface for attachment. 
A modification of the disc oceurs in P. fastigiata. The rhizoids actually 
penetrate the tissues of the host, and at the tip the wall is thin and the cell 
is filled with granular contents. Ifa section of the Ascophyllum thallus be 
taken, it is noticeable that certain of the host cells near the tip of the rhizoid 
are darker in colour than their neighbours, and that similar dark cells are 
present at places where the rhizoid has passed at an earlier stage. A number 
of sections were taken, and the rhizoid-tip was found closely applied to certain 
darkened Ascophyllum cells. This suggested that the rhizoid attacked these 
cells, the swollen tip acting as an haustorium (Pl. 22. figs. 7,8). Rattray (48) 
believed that a partial parasitism must occur in the association, and Tobler- 
Wolf (55) observed the dark cells, but no definite statement of parasitic 
habit was made in either case. 
With a view to ascertaining whether the contents of the Ascophyllum 
cell had been changed by contact with the Polysiphonia, a chemical test 
was applied. Ascophyllum nodosum does not contain starch in the form in 
which it is usually found, but the food-store stains red when treated with 
a solution of vanillin in concentrated HCl. Sections were therefore placed 
for a moment in the solution, then washed and examined in water, and it was 
found that the contents of the ordinary Ascophyllum cells became bright 
red, whereas the dark cells gave no reaction, It is evident, therefore, that 
the Polysiphonia obtains food from the cells of the host which it attacks 
(PI. 22. fig. 8). 
Where there is an extensive growth of the parasite, the Ascophyllum 
frequently has a somewhat stunted and more rigid thallus in the immediate 
neighbourhood of the Polysiphonia. This, however, is probably due to some 
extent to the increased strain to which it is subjected, with an increased 
surface for the action of the waves, and is not solely due to the semi- 
parasitic habit of the Polysiphonia. As Rattray (loc. cit.) has pointed out, 
it is not absolutely necessary for the life of P. fastigiata that it should be 
fixed to any host, as it occurs, though comparatively rarely, growing on a 
rocky substratum. When this happens, however, its vegetative growth is 
less perfect, although it may be exposed to similar conditions with regard to 
depth and general environment. The fact only bears out the view that a 
direct advantage is gained by an epiphytic habit. 
In order to illustrate the degree of complexity which is visible in the 
atiachment organs of the genus, four types may be selected. 
1. P. urceolata. 
The young plant is usually attached to the substratum by two rhizoids 
which are developed by longitudinal proliferation of the base of the primary 
