on species of Tolypothrix . 657 
show them in the fully-developed condition. These external 
branches are generally somewhat narrowed down at the point 
where they pass through the algal sheath, expanding again 
as soon as they enter the water. Except for the absence 
of the regular constrictions, they in every way resemble 
the internal mycelium ; transverse walls occasionally occur 
in their course. They are frequently branched and ramify 
in all directions in the water. The vegetative cells of the 
internal mycelium, from which they arise, are in no way 
especially modified. 
When these hyphae come into contact with another healthy 
algal filament they frequently penetrate its sheath, and give 
rise to a parasitic mycelium within (Figs. 10, 11). Such 
infecting hyphae (‘ Ansteckungshyphen ’) are one of the chief 
means of propagation of the Fungus, and constitute a strong 
point of resemblance to the genus Ancylistes , in which they 
occur abundantly. It should, however, be remarked that 
these hyphae were often seen to come into contact with 
a healthy filament of the Alga without attacking it. In some 
cases the hyphae arising from the parasite all grow out in 
one and the same direction, as though there were some 
stimulus regulating their formation and direction of growth. 
The thick-walled heterocysts of the Alga (which in this 
species occurred in groups of 4-9 together), again presented 
a considerable obstruction to the passage of the Fungus, and 
not rarely seemed to form an unsurpassable barrier. Further, 
the Alga protects itself by the formation of thick transverse 
walls some little way in front of the momentary position 
of the Fungus 1 . Apparently these also form considerable 
obstacles to the growth of the latter, and in some few cases 
the Fungus was observed to emerge from the filament at 
such a point and to come in again on the other side (Figs. 
13, 14), finding it easier to pass through the thick sheath 
than through the protecting wall formed by the Alga. To 
the difficulty of passing through a heterocyst must also be 
1 Such protecting-walls were also, but rarely, seen in the case of the first- 
described species. 
