35 



I found presentation-times considerably shorter than those 

 observed by Mrs. Rutten-Pekelharing in the case of 

 experiments made under the saine conditions and with 

 the same plant. Distrusting thèse results I made control- 

 experiments and I asked some one else to stimulate some 

 vessels only among a number of marked vessels, of 

 wbich I had then to observe the reaction. The resuit was 

 that even in the non-stiniulated vessels I observed cur- 

 vations, in one case even 30 percent. After this expérience 

 the „just perceptible curvation" had to be more obvions, 

 should I note down a positive resuit. Having made a num- 

 ber of experiments relating to this point I thought myself 

 able to distinguish pretty surely the geotropical curvation 

 from the nutation, although I did not neglect the possi- 

 bility of mistakes on this point. 



Later experiments proved the necessity of more décisive 

 measures. 



Thèse experiments plainly showed that when the nega- 

 tively geotropical reaction subsides an opposite reaction 

 occurs. Generally this was a feeble reaction of 5 — 15 o/o of 

 the plants only, although hère a distinct curvation ofthe 

 top was seen, which quickly disappeared again. As some- 

 times thèse curvations were seen several times in succes- 

 sion in 25 — 80 o/o of the plants, it became necessary, to 

 trace the cause of this phenomenon. Further considération 

 of the figures, and observation of the reaction during 

 several hours and the behaviour of non-stimulated plants 

 induced me to look for an explanation in vigorous autono- 

 mie nutation. ') Hence we hâve to consider the nutations 



1) When occupying myself with this phenomenon, I happened 

 to read a paper by E. Pringsheim Jr., (1909) on phototropism, 

 in which a similar expérience is described in the case of photo- 

 tropism. Pringsheim décides in favour of a real negatively 

 heliotropic reaction. My opinion is that in the case communicated 

 by him the explanation has also to be sought for in natations. 



