THE GENUS POLYSIPHONIA. 29] 
Rhizoids, produced by the elongation of pericentral siphons, penetrate 
into the tissues of the host, the latter being almost invariably Ascophyl- 
lum nodosum. Procumbent branches creep along the thallus of the host, 
and send out rhizoids at intervals. Tips of the rhizoids swollen, with 
thin walls, closely applied to cells of the host which have become dark 
in colour. Dark cells also present in the thallus of the host in the area 
surrounding the path taken by the invading rhizoid. Reproductive organs. 
Tetraspores are borne in swollen terminal branches. Antheridia bright 
yellow in colour, borne in winter and spring in elongated clusters at 
the tips of the branches. Cystocarps sessile, egg-shaped, replacing one of 
the apical dichotomous branches. 
The protoplasmic connections between the central and pericentral 
siphons are very well seen in young plants of P. fastigiata, although the 
maintenance of the connection between these siphons throughout the life 
of the plant has been the subject of much debate. Harvey Gibson (27. 
p. 129 et seg.) has shown that the continuity of the protoplasm is not 
maintained in the older parts of the plant. The pit between the siphons 
becomes closed by the development of a cellulose plate, and, although 
fine granular striæ are visible, maceration shows that these are due to a 
fringe of fine threads which arise along the margin of the plate. They 
are quite independent of the protoplasmie contents of the canal. 
Polysiphonia usually attacks its host in the depression from which a 
new shoot will arise. This crevice forms a suitable substratum for the 
early stages of development of the plant, and rhizoids are soon produced 
by the elongation of single siphons which penctrate into the tissue of the 
host-plant. Tobler-Wolff (55) suggests that the reason P. fastigiata selects 
Ascophyllum as its host is the fact that the outer cells occur in chains in this 
species, and penetration is thus made more simple. This, however, seems 
an inadequate explanation of the phenomenon, as the epiphyte has been 
recorded on other Fuci. The cells which have been attacked by the 
Polysiphonia are dark in colour, and do not re-act when treated with vanillin 
in concentrated HCl. The ordinary Ascophyllum cells contain food-store, 
which turns red in the presence of the vanillin, and the failure of the dark 
cells to react shows that food must have been extracted from them. 
Tobler-Wolff (55) in describing the dark cells says:—“ Es war von 
vornherein anzunehmen, dass eine so tief eingreifende Befestigung—die wohl 
in dieser Weise für keinen anderen Fall bekannt ist—nicht ohne Schädigung 
des Substrates vor sich gehen kann. Tatsächlich sieht man auf entspre- 
chenden Schnitten in der unmittelbaren Umgebung der Rhizoiden fast 
immer eine Anzahl Zellen mit auffallendem Inhalt. Er ist glasig, von einem 
rötlichen Braun, das von dem hellen gelbbraunen Ton der anderen Zellen 
stark absticht. Zudem sind diese Zellen meist aus dem Gewebeverband 
losgelöst ; sie sind abgestorben.” 
Y2 
