1905] PEIRCE & RANDOLPH—IRRITABILITY IN ALGAE 349 
mainly by the direction from which the light comes: the rhizoids are 
negatively phototropic, the plants positively. 
g. As in fresh-water algae, so also in sessile marine algae, the 
nature of the surface with which the spores come into contact very 
largely controls the nature of the attachment formed, a rough surface 
inducing the growth of a large and well-developed holdfast, a smooth 
surface causing proportionately less growth. 
10. Though the direction toward which the rhizoids ordinarily 
grow is determined at first by light, the character of the surface with 
which the rhizoid comes into contact still more strongly influences 
the direction of its growth. 
11. The direction, rate, and kind of growth of these germinating 
spores is strongly influenced by contact irritation. 
STANFORD UNIVERSITY, CALIFORNIA. 
LITERATURE CITED. 
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Borce, O., Ueber die Rhizoidenbildung bei einigen fadenformigen Chloro- 
phyceen. Upsala. 1894. 
4- Davenport, C. B., Experimental morphology. Part II. New York. 
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Wiss. Bot. 38:545-634. 1902 et seq. 
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Pflanze. Cohn’s Beitrage z. Biologie d. Pflanzen 6: 383-432. 1893. 
7- JENNINGs, H. S., Contribution to the study of the behavior of lower organ- 
: ‘sms. Carnegie Institution, Washington. 1904. 
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Pilzen. Jena. 1896. 
= ee, Willkiirliche Entwickelungsinderungen bei Pflanzen. _— sgh 
le EB, iz Untersuchungen zur physiologischen Morphologie der Thiere. 
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: MacDovear, D. T., Influence of light and darkness upon growth. Mem. 
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wn 
an 
12. Orry ; 
13. = S, F., Morphologie und Biologie der Algen 1. 1904. 
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~_—» A contribution to the physiology of the genus Cuscuta. Ann. 
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