1905] PEIRCE & RANDOLPH—IRRITABILITY IN ALGAE 329 
the less irritable germinating spores of marine algae induced the 
formation of very regular holdfasts. We may say then that the form, 
size, and even the development of holdfasts by sessile fresh-water 
algae depends upon the character, the degree of roughness, of the 
surface with which the zoospores come into contact. A uniform 
though slightly rough surface, like that of clean polished glass, will 
induce the formation of very symmetrical holdfasts. Coarsely or 
irregularly roughened surfaces induce the formation of irregular 
holdfasts. Extremely smooth surfaces fail to induce the formation of 
holdfasts, unless perhaps of the most rudimentary sort. It would 
seem clear then, that so far as the direction and kind of growth are 
concerned, contact acts as a stimulus; for the holdfast grows parallel 
with the surface, no matter how irregular this may be, and the organ 
produced corresponds to the surface on which it is formed, being 
simple or branched, symmetrical or irregular, according to the 
character of the surface. As to the effect of contact upon the rate 
of growth, we can say nothing so far as fresh-water algae are concerned. 
The measurements of growth rates among marine algae are given 
subsequently (p. 347), 
rom the observations thus reported, it is obvious that contact 
acts as a very important stimulus in the germination of the zoospores 
of sessile algae. The value of this to the plant is evident enough. 
Hf, however, we could induce algae ordinarily not sessile to form 
thizoids or other holdfasts by bringing them into contact with suitably 
toughened surfaces, and if we could find, in nature,3 algae ordinarily 
hot attached fastening themselves to rough surfaces occasionally, we 
should add materially to the significance of our observations. Both 
. these things one of us has done. Though reserving the subject 
ns : lurther study, we may report now that plants of a species of 
~°8yta which did not fruit and hence could not be determined 
Mile we were working upon them, formed rhizoids in our cultures, 
and were found to have formed rhizoids out of doors, wherever the 
ents were in contact with sufficiently rough material, clean cover 
pee 'rough-scrapings containing lime, diatoms, etc., and similar 
han part of our work was ended, at least for the time, CoLLINs’s paper in 
i Sitar - 1904) records the formation of holdfasts by Zygnema filaments 
Tock in a swiftly running stream. 
