464 Peirce.— Studies of Irritability in Plants. 
are the fruit of their bodies. Yet there are unchanging factors in the 
environment. These also contribute to the repetition of parental char- 
acters in the offspring. A knowledge of these definite forces, which we 
can recognize and the effects of which we can study, will help us to solve 
the problem of heredity. 
Stanford University, California. 
J 'une , 1906. 
Publications Cited. 
Campbell, D. H. (’05) : Mosses and ferns. 2nd Ed. New York. 
Czapek, F. (’98) : Weitere Beitrage zur Kenntniss der geotropischen Reizbewegungen. Jahrb. 
f. wiss. Bot., xxxii. 
Farmer, J. B. (’03) : On stimulus and mechanism as factors in organization. New Phytologist, 
ii, Nov. and Dec. 
Ganong, W. F. (’01) : A laboratory course in plant physiology. New York. 
Klebs, G. (’96) : Die Bedingungen der Fortpflanzung bei einigen Algen und Pilzen. Jena. 
Mac Dougal, D. T. (’03) : Influence of light and darkness on growth and development. Mem. 
N. Y. Bot. Garden, II. New York. 
Peirce, G. J. (’03) : Text-book of plant physiology. New York. 
(’04) : Certain undetermined factors in heredity and environment. Amer. Naturalist, 
xxxviii. 
(’06) : Anthoceros and its Nostoc colonies. Bot. Gazette, xlii, July, 1906. 
Peirce, G. J., and Randolph, F. A. (05) : Studies of Irritability in Algae. Bot. Gazette, xl, 1905. 
Pfeffer, W. (’91) : Symmetric und specifische Wachsthumsursachen. Arb. bot. Inst. Wurzburg, i. 
(’94) : Handbuch der Pflanzenphysiologie, ii, 1. 
Reinke, F. : Entwickelungsgeschichtliche Untersuchungen iiber die Dictyotaceen des Golfes von 
Neapel. Nova Acta Leopold. Acad., 1, 1878. 
Rosenvinge, K. (’89) : Influence d 5 agents ext^rieurs sur l’organisation polaire et dorsiventrale des 
plantes. Rev. gen^r. de Bot., i. 
Vochting, H.- (’93) : liber den Einfluss des Lichtes auf die Gestaltung und Anlage der Bliithen. 
Jahrb. f. wiss. Bot., xxv. 
Winkler, H. (’00) : Einfluss ausserer Factoren auf die Keimung der Eier von Cystoseira barbata. 
Ber. d. Deutsch. Bot. Ges., xviii. 
EXPLANATION OF FIGURES IN PLATE XXXV. 
Illustrating Professor Peirce’s Paper on Irritability in Plants. 
Fig. 1. Anthoceros ftisiformis. Spores germinating fourteen days after sowing, light only from 
direction of arrow, x 84. 
Note direction of growth of germ-tube. Plants a and b shaded by others. 
Fig. 2. Fimbriaria Californica. Spores germinating thirteen days after sowing, light only 
from direction of arrow, x 84. 
Note directions of germ-tube, cross- walls, and rhizoid. 
Fig* 3* Fimbriaria Californica. Group of two spores at 2.15 P.M., thirteen days after 
sowing, light only from direction of arrow, x 84. 
Fig. 4. Fimbriaria Californica. Same group of two spores twenty-one hours later, x 84. 
