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No. 85. 



PHOTOTROPIC MOVEiMENTS OF LEAVES.— THE FUNCTIONS OF THE 

 LAMINA AND THE PETIOLE WITH REGARD TO THE PER- 

 CEPTION OF THE STIMULUS. 



By NIGEL G. BALL, M.A., 

 Assistant to the Professor of Botany in tlie University of Dublin. 



(Bead December IS. Printed December 2S, 1923.) 



In spite of a considerable amount of work which has been done from time to 

 time, the various factors involved in the orientation of leaves in response to 

 light are by no means clearly understood. This applies especially to the 

 question as to how far the lamina exerts a directing influence on the petiole, 

 and so far unanimity on this point has not been attained. 



Darwin (1) first showed that in experiments on leaves of Tropaeolum majus 

 and Ranunculus ficaria, in which the blades were covered with black paper, 

 the petioles became curved towards the light as completely as those of unprotected 

 leaves. Vochting (2), on the other hand, concluded that in the case of Malva 

 verticillata the movements of the petiole are controlled by the lamina. Darwin's 

 experiments on Tropaeolum were later confirmed by Rothert (3), who considered 

 that illumination of the lamina was without influence on the bending of the 

 petiole, and Krabbe (4) obtained similar results with Fuchsia and Phaseolus. 



Haberlandt (5) repeated the experiments of Darwin and Rothert o)i 

 Tropaeolum, but found that the correct diaphototropie position was not reached 

 as completely or as accurately by the leaves covered with black paper as 

 by those which were uncoverecl. He therefore concluded that while the main 

 movement of the leaves of Tropaeolum was effected owing to the phototropie 

 response of the petiole, the finer adjustments were controlled by the lamina. 

 In the case . of Begonia discolor and Monstera deliciosa he found that even 

 when the petioles were covered with tinfoil the leaves were able to reach 

 the correct phototropie position, and concluded that in these plants the lamina 

 is the sole percipient organ, and that it transmits the stimulus to the petiole. 

 Working with Phaseolus, however, he confirmed Krabbe 's (4) results, and found 

 that leaves in which the lamina was covered with black paper reached the 

 correct phototropie position as rapidly and as completely as uncovered leaves. 



More recently Wager (6) concluded that the "perception of light is located 

 not in the leaf -blade but in the leaf -stalk. " 



As a result of the experiments which have been carried out by Boysen- 

 Jensen (7), Paal (8), and others, on the transmission of the phototropie 

 stimulus in seedlings, phototropie reactions have lately acquired a new interest, 

 and further work on the movements of leaves seemed desirable. 



In the first experiments leaves of Oxalis macra Small were used. A number 

 of freshly-plucked leaves were fixed with the ends of the petioles in water 

 in small glass jars. The petioles passed loosely through holes in corks fitting 

 the necks of the jars, and were firmly fixed in these holes by means of 



SCIENT. PROC. K.D.S., VOL. XYIF, NO. 35. 3 G 



