252 BOTANICAL GAZETTE [octoper 
all the rhizoids are unicellular and unbranched; and, although 
an apparent tendency to branch may be observed occasionally, 
the lobes are not separated from the main portion by a wall, even 
the protoplasmic contents being undivided (Pl. XXV/, fig. 9), 
All the rhizoids eventually develop terminal clasping-disks. 
The first indication of such a structure may appear in plantlets 
four days old, but, as a rule, the primary rhizoids do not undergo 
modification until several days later. The disks begin asa sim- 
ple enlargement of the tip of the rhizoid (Pl. XX, figs. 48-50; 
AX, fig. 1), become deeply lobed, and assume a very irregular 
outline (Pl. XXT, figs. 52-56). The cell-contents of the rhizoid 
extend into the lobes, but no division takes place (Pl. XAIL 
fig. 8). Great variations in the length attained by the primary 
rhizoids occur both in plants grown under natural conditions and 
in laboratory cultures (P/. XX/, figs. 46, 48; XXII, fig. 1). The 
cause of such variations has not been determined, but it is prob- 
able that contact irritation may be the most important factor 10 
the formation of the disks. This view is supported by the fact 
that disks are sometimes produced on the sides of rhizoids when 
these come in contact with a firm substance (77. XXII, figs. 4s 
5). The length of the secondary rhizoids depends upo? the. 
distance of the parent cells from the substratum, and as rad 
contact is established broad clasping-disks are formed, whi 
mechanically cohering with one another and with the eee 
disk produce a very strong holdfast. The rhizoids are paler 
color than the rest of the plantlet, having less dense conten 
and fewer chromatophores. 
The rhizoids of Polysiphonia violacea never pe 
e latter 
host-plant; but at the point of contact the susface of ee de 
is often dark brown and cutinized, while the outer comic 
| tism occuss 
are destit _ But incipient parasi 
itute of chromatophores pices om nodosim™ 
in Polysiphonia fastigiata Grev., growing on Ascop here species: 
Stack. Gibson (5), in writing of the histology of this 
noticed that “the attachment of the epiphyte to 0 
very intimate. Root-filaments given off from the : 
frond penetrate deeply into the tissue of the host, a 
netrate the — 
