17 



preliminary 'warming for 1, 2, 4, 6, 12 and 2-i hours and 

 sometimes also even after 8, 18 and 48 hours. 



The températures, at which déterminations were roade, 

 lay between 0° C. and 40° C, with intervais of 5° C. 

 Between 35° C. and 40° C. experiments were further made 

 at 37° C, 38° C. and 39° C, because the presentation-time 

 hère increases extraordinarily fast. At 40° C. the presen- 

 tation-time had already risen to 260 minutes after only 

 one hours' stay in the thermostat and therefore no déter- 

 minations were carried out at higher températures. Nor 

 was the presentation-time determined below 0° C, as the 

 results expected would not hâve been worth the trouble. 



Ail the experiments took place in the dark room (cf. 

 § 6) of the Botanical Laboratory at Utrecht, in an electrical 

 thermostat, constructed specially for this purpose, so that 

 the experiments were uniform with regard to this point. 

 The plants were always stimulated by means of gravity 

 at an angle of 90° ^by placing the vessels with plants (cf. 

 § 5) vertically with the aid of the axis in the thermostat. 

 In order to détermine the presentation-time in a given 

 case, I proceeded as follows. A set of plants were stimu- 

 lated during a time, which I supposed to be nearly equal 

 to the presentation-time. Then I concUided from the per- 

 centage of curved plants, whether the stimulation must be 

 lengthened or shortened to reach the presentation-time. The 

 séries of déterminations was then increased, if possible, 

 until it extended from a stimulation-time with o/q curved 

 plants to one with 100 o/q. From this séries the presenta- 

 tion-time itself (50o/o curved) could be determined very 

 accurately. 



At first I took a large number of plants in each expe- 

 riment, mostly 50 or 100 at each détermination, hoping 

 in this way to be independent of the individual variability 

 (cf. § 7). Later I had to restrict myself because of the 



Recueil des trav. bot. Néerl. Vol. IX. 1912. 2 



