1905] PEIRCE & RANDOLPH—IRRITABILITY IN ALGAE 331 
unless it has happened to land upon particles of dirt or other solid. 
Germination, then, follows enforced cessation of locomotion; growth, 
so far at least as the formation of a holdfast is concerned, follows and 
is dependent upon contact irritation. 
Let us turn now to the marine algae. 
II. MARINE ALGAE. 
MATERIAL AND METHOD. 
The material consisted of fresh fruiting plants brought in almost 
daily from the Bay of Naples. These plants, when not used imme- 
diately, were put into well-lighted aquaria, supplied with running 
sea-water, where they remained healthy for days after they had 
ceased to be used. The plants were Dictyopteris (Halyseris) poly- 
podioides, Dictyota dichotoma (two varieties at least of this extremely 
variable form), C ystoseira barbata, C. erica marina, among brown 
algae; and Laurentia obtusa, Polysiphonia (sp.?) among the red. 
I tried to get zoospores of Ulva, Cladophora, and other green algae, 
for the sake of comparison with the behavior of the zoospores of 
fresh-water algae; but whether owing to the season of the year (Octo- 
ber, November) or to other causes, I do not know—at all events, I 
failed to get sufficient numbers to justify any conclusions. The 
Spores used were of two sorts, the fertilized eggs of the two species of 
Cystoseira, and the non-sexual spores and tetraspores of Dictyopteris, 
a ka Polysiphonia, and Laurentia. Their behavior is essentially 
s r. 
BEHAVIOR OF THE SPORES. 
Anjluence oj light upon their escape—I began my work with 
D ‘ct yo pleris (Halyseris) polypodioides and the two species of Cysto- 
“ara. Clean fruiting branches, as free as possible from diatoms and 
other organisms, were put in small dishes about 4° diameter and 
a depth, filled with fresh sea-water. To reduce evaporation the 
dishes were covered either with glass caps or clear glass plates. The 
dishes were set on a broad shelf inside a northwest window about 
13" below the lower edge of the glass. The light fell therefore 
ae wnat obliquely upon the cultures. At noon on sunny days 
Paty the screens, made of sheeting, in order to prevent the sun 
shining directly upon the dishes during the afternoon. I found this 
