367 



Nevertheless experiments were extremely difficuU, especially at 

 high temperatures, owing to nutation and contai't curvatures. It is 

 found absolutely necessary to use only those plants, which at the 

 beginning of the experiment stood couipletely upright. For this reason 

 about 80 7o to 90 "/„ of the cultivated material had to be unques- 

 tionably condemned. 



It is now found that at 25° C, and it seems very desirable to 

 choose such a high temperature for these experiments, very slight 

 basal curvatures already occur at 100 C. M. S., when the upper 

 5 m.m. of the plant are covered with a cap. On api)lying more 

 energy these curvatures increase and are clearly visible, 2 hours, 

 after the beginning of the illumination with 300 to 1200 C. M. S. 

 They begin to be macroscopically visible after about half an hour. 



If more light is applied in each case with very short duration 

 of stimulus then the results are very uncertain. Sometimes very 

 faint positive curvatures, at other times negative ones occur. The 

 variability of the phenomenon is however so great and the curvatures 

 are so complicated by the fact that sometimes tirst a positive and 

 then a negative curvature arises, that it has not been possible to tix 

 the amount of energy of which it can be said with certainty that 

 the effect is positive or negative. The complications are probably 

 due in great measure to the fact that the different portions of the 

 base are not all equally sensitive, and various effects are thus super- 

 imposed. 



A further investigation with only a small zone illuminated might 

 be able to give an explanation of this. It is certain that with the 

 application of 30000 C. M. S. during a short period no strong 

 phototropic curvatures of the base occur. It need cause no surprise 

 that voN GuTTENBKRG, VAN DKR WoLK and WiLSCHKE all give these 

 values since they did not apply this amount of energy as I did in 

 a short period of time, but in much longer ones (von Guttenberg, 

 e. g. in an hour). Then the tone-phenomenon also occurs for the 

 base and a large amount of energy applied ovei- a considerable time 

 no longer yields a negative but a positive effect. 



To gain an idea of the sensitiveness of the most basal zones, the 

 quantity of light was determined, which applied in a short time 

 so as to eliminate tone, produced positive curvatures when the 

 uppermost 12 mm. of the coleoptile had been covered with a cap. 

 This limit was found to lie at about 400 C. M. S. The curvatures at 

 500 C. M. S. were the strongest, at more than 800 C. M. S. they were 

 weaker, whilst from KJOO to 2400 C. M. S. even negative ones occurred. 



If a comparison is now made of results which have been obtained 



