without fear of oeutrifiigal force. Tii the series of experiments now 

 published, tlic rotation velocity was always 5 sec. The source of 

 light was a Nkknst projectionhinip fed by a curi-cnt maintained 

 constant. Tlie light from the hxmp, which was placed outside the 

 dark room, passed through a cooling apparatus with rnnning water 

 and then through a diaphragm into the dark room. By interposing 

 plates of frosted and of milk glass the intensity of the light could 

 be changed in a few seconds. A greater intensity than 450 candles- 

 metre was not obtainable with this lamp at the distance at w^hich 

 the multiclinostat was placed. The experiments described below, 

 with unilateral illumination at greater intensities w^re carried out 

 with the aid of a projection arc lamp which gave at 1 metre an 

 intensity of 4600 candle-metre power. The numbers referring to the 

 latter illuminations have no claim to great accuracy. 



The experiments were carried out in a small dark room in the 

 experimental hothouse of the laboratory. This small space could be 

 maintained at 23^ C. by means of an electric heating apparatus and 

 regulator. 



§ 3. OmnUaternl fore-iUumination folloired by 

 unUatevdl after-ilium ination 



In order to determine the state of sensitiveness of a plant at a 

 given moment, the plant must be exposed to unilateral illumination 

 at this moment and the resultant reaction must be observed. In the 

 course of the investigation it was found desirable to make a rule 

 of following the process of curvature, for the first two hours. A 

 longer period was not required for after two hours no further photo- 

 tropic phenomena became visible. The investigation aimed at observing 

 how a plant behaves towards unilateral illumination of various 

 intensities, after previous exposures of varying duration and intensity. 

 In order to determine the state of sensitiveness exactly at the end 

 of the preliminary illumination it is necessary to supply the quantity 

 of energy of the unilateral afler-illuniination in as short a time as 

 possible. How desirable this is will be seen especially from a con- 

 sideration of the processes discussed below, affecting the returu of 

 sensitiveness. In contradistinction to Pringsheim and to Clark, I did 

 therefore not always use the same intensity for the unilateral after- 

 illumination as had been employed when the plants were rotating. 



On the contrary, an attempt was made to supply the plants in 

 as short a time as possible with a definite amount of energy, which 

 attempt was only limited at the higher amounts by the available 



