STUDIES OF IRRITABILITY IN PLANTS. 

 By GEORGE JAMES PEIRCE, Professor of Botany and Plant Physiology. 



III. 



THE FORMATIVE INFLUENCE OF LIGHT. 



Introduction. 



IN 1906 1 I published a paper, under the above title, recording the results 

 of a series of experiments on the influence of the direction of illumina- 

 tion upon the shape of certain plants. The most striking result reported 

 was that Anthoceros plants grown from the spore on a disc revolving in a 

 horizontal plane, and therefore receiving fairly equal amounts of light on 

 all sides successively, showed no trace of the usual dorsi-ventral form and 

 structure of the thallus but were radial in structure, cylindrical or conical 

 in form. Anthoceros jusijormis, Aust. and A. Pearsoni, M. A. Howe, both 

 native here and growing within a short distance of this laboratory, gave the 

 same results; but the spores of the Marchantiaceous liverwort Fimbriaria 

 (Asterella) Californica and of the fern Gymnogramme triangularis did not, 

 under the same conditions, give rise to plants round in section. To this 

 extent their dorsi-ventrality failed to show itself rhizoids grew equally in 

 all directions from their thalli or prothalli, respectively but the plants were 

 thin plates, though curiously crumpled, as the figures showed. I did not 

 understand this difference in result and have tried in various ways to ascer- 

 tain the reason for it. I have not yet reached a satisfying explanation, but 

 some of the results of these succeeding experiments are interesting enough 

 to record now. 



THE APPARATUS: A MULTIPLE CLINOSTAT. 



The apparatus used in the experiments of Czapek 2 , which suggested 

 mine, consisted essentially of the expensive form of clinostat, the only one 

 generally known and used in botanical laboratories. My experiments were 

 carried on with cheap clocks, modified as described by Ganong 3 . Such ap- 

 paratus is, however, unreliable. Indeed, cheap apparatus may be the most 

 expensive. Although cheap apparatus may perhaps be well enough for an 



1 Annals of Botany, XX, 449-465, 1906. 



2 Czapek, F. Weitere Beitrage zur Kenntniss der geotropischen Reizbewegungen. 

 Jahrb. f. w. Bot, XXII, 261, 1898. 



3 Ganong, W. F. A laboratory course in Plant Physiology, pp. 120-1, New 

 York, 1901. 



