330 BOTANICAL GAZETTE [Novewner 
material. We have never seen rhizoids on floating filaments or on 
the free floating parts of attached filaments. The size, form and 
direction of growth of the rhizoids were greatly influenced by the 
nature of the surface which the cells of Spirogyra touched, clean glass 
inducing the formation of strikingly regular crenate disks on the ends 
of the rhizoids like those formed on the same material by Oedogonium. 
Dirty glass, on the other hand, induced the formation of proportionally 
irregularly branching holdfasts. Thus our observations confirm the 
generally unknown observations of BorRGE (3), who in 1894 reported 
his studies of a species of Spirogyra which formed rhizoids. Borcr’s 
paper contains a list of earlier authors who mention having seen 
rhizoid-bearing Spirogyra.+ 
From the fact that on extremely smooth surfaces like the surface 
film of clean water or of wet neutral gelatine, no true holdfasts form, 
and that on sufficiently rough surfaces even such a plant as Spirogyra 
may form organs of attachment, it is clear that the growth of rhizoids 
or holdfasts represents the reaction of a plant to the stimulus of 
contact. The germination of a zoospore of a sessile alga is influenced 
by contact; but contact, though it stimulate the zoospore to form an 
organ of attachment, is not needed to cause the zoospore to germinate. 
The zoospore may remain in motion for a long, but at present 
unknown, period of time. So long as it continues to move about, it 
does not germinate. When its locomotion is stopped, germination 
begins. When a zoospore reaches a very smooth but impenetrable 
sheet of gelatine interposed between it and the source of light falling 
upon a culture-vessel, it loses its cilia, ceases to move, and surrounds 
itself by a cellulose wall—it germinates; but unless it has stopped 
upon a particle of dirt on the surface of the gelatine, it puts out at 
best only a rudimentary hypha-like process, a rhizoid, not @ gore 
and this it does after germination has begun. Similarly, if the f : 
progress of a zoospore toward the light be opposed by 1 _— 
reached the top of the water in which it was found, the zoospor lose 
cease to move, will lose its cilia, will surround itself with a cellu : 
wall, will germinate; but it will not form a holdfast or even a gee 
4 Subsequent papers by BorcE figure other species of Spirogyr with nik ee a 
holdfasts (Die Algen der ersten Regnelschen Expedition. Archiy for Bota e 
1903). 
