( 231 ) " 



Vicia sativa, that, al an intensity of 0.054 N.C. , the curvature began 



to appear after 3 hours and 45 minutes, whereas the same organ 

 in Vicia Faba, with light of the same strength, did not show any 

 curvature even after 48 hours. In none of these eases, therefore, 

 has an attempt been made to find the minimum period, during which 

 light of a given intensity must act on a plant in order to produce 

 a phototropic curvature. Later Figdok *) carried out similar experiments; 

 here only his conclusion can be mentioned, that the inferior limit of 

 phototropic sensitiveness is below 0.0003262 normal candle power 

 for seedlings of Lepidium sativum, Amarantus melancholicus ruber, 

 Papaver paeonijlorum and Lunaria biennis. 



Czapek 5 ) on the other hand has been engaged on a determination 

 of the presentation-time; by this he means the minimum period of 

 unilateral illumination, required for the subsequent production of a 

 phototropic curvature. For seedlings of Pkalaris and of Avena this 

 period is stated by him to be about 7 minutes, although he furnishes 

 no data as to the intensity of the light employed. Presumably the 

 author did not perceive the necessity of such data, because his in- 

 vestigation was almost wholly concerned with geotropism, where the 

 idea of presentation-time, without further specification, has a pretty 

 definite meaning, because we are concerned with the constant force 

 of gravity. 



The question, whether there is a connexion between this presentation- 

 time and the intensity of the light, was however close at hand. In 

 his further investigation, on the perception of phototropic stimuli, 

 Mr. A. H. Blaauw has also taken up this question in my laboratory ; 

 he has arrived at some very striking results, about which I wish to 

 make this brief preliminary communication. 



The experiments were performed with etiolated seedlings of Avena 

 sativa, the coleoptile of which is extremely sensitive to light stimuli, 

 as is well known since the investigations of Dakwin and of Rothekt. 



For the weaker intensities an Auer von Welsbach burner (incan- 

 descent gas light) was used; it was kept very constant by means of 

 a gas-pressure regulator. By placing the objects at varying distances 

 from the lamp, and, where necessary, by screening the light through 

 smoked glass, and further, by letting the light fall on a plate of 

 opalescent glass with a diaphragm, which in its turn acted as source 



1 ) W. Figdor. Versuche über heliotropische Emptindlichkeit der Pflanzen. Silz 

 ber. d. Math Natunv. Glasse d. K. Akademie der Wissensch VVien. Bd. Gil. 

 Abth. I 1893, p. 45. 



2 ) F. Czapek. Weitere Beitrüge zur Kenntniss der geotropischen Reizbewegunge». 

 Jahrbücher für wissensehaftliche Botanik. Bd. XXXII. 1898. p. 185, 



