622 



Unilateral illumination according to Clark. 



At 1.4 metre-candlepower only positive ciii-vatures are foniid, hut at 



each greater intensity there is a larger or smaller range of energy 



in which negative curvatures occur. Although the accuracy of the 



determination of the strongest light intensities was not very great, 



we may nevertheless say that, at all intensities from 5.5 CM. onwards, 



there is a range over which negative curvatures are present. At 



5.5 CM. this range is very small, the curvatures which occur are 



very feeble and a positixe one always precedes them. This range 



first increases at greater intensities and then diminishes again, but 



even at the greatest intensity employed, namely 20000 CM., it was 



possible to obtain a negative curvature after stimulation for about 



half a socond. If we, however, compare with this the values published 



by Clark for the appearance of a negative curvature, there is a very 



striiiing difference. For the first positive reaction the energy law is 



valid according to Clark, but not for the negative one. The great 



discrepancy between our figures depends on the phenomena at small 



intensities. For larger ones Clark agrees in finding the negative 



reaction at a constant amount of energy, but for feebler intensities 



he considers that a negative curvature occurs after much smaller 



amounts of energy. The cause of the discrepancy is Clark's method 



of working, as I have "been able to show by control experiments. 



A plant which executes a positive phototropic curvature assumes a 



})Osition in which its apex is stimulated by gravity. When the reaction 



caused by the last stimulus is stronger than the phototropic one, the 



plant assumes an upright position, vi'hich greatly resembles that due 



to a negative phototropic curvature succeeding a positive one. For an 



amount of energy from 500 — 2000 C M. S. Clark has mistaken this 



geotropic erection for negative phototropic curvatures.^) Had he made 



his plants, after illumination, rotate on a clinostat round a horizontal 



axis he would have seen no trace of a negative curvature. I desire to 



emphasize here, that in all my ex:periments control observations were 



made on a clinostat ; by this means alone it is possible to obtain 



1) Prof. JoST was so kind as to hiforni ine by letter, that Clark never rotated 

 his plants round a horizontal axis on a clinostat. 



