616 



A change in adjustment is therefore made evident by a change in 

 the reaction to a stimulus of the same intensity. Thus it has been 

 known for a long time that plants grown in tlie light do not show 

 the same sensitiveness to unilateral illumination as etiolated ones. 

 Pringsheim ^), in a series of investigations, has attempted to obtain 

 a more detailed knowledge of these processes and quite recently 

 there appeared a paper by Clark ^) which, as an extension of 

 Pkingsheim's work, possesses in many ways points of contact with 

 the results about to be described. Clark's conclusions and my own 

 differ on a fundamental point, namely the validity of the energy 

 law for negative reactions. 



There are also striking differences with regard to our observations 

 on the influence of omnilateral preliminary and after-illumination. 

 Since Clark's paper fortunately appeared before the close of the 

 present investigation, I have been able to test his results by control 

 experiments, which, at least with regard to the influence of omnilateral 

 after-illumination, have sufficiently explained the divergence in our 

 results. For a further explanation and for theoretical considerations 

 I must, however, refer to the detailed account of my investigations, 

 about to be published elsewhere. 



§ 2. Method. 



My method is in principle the same as that of Pringsheim and of 

 Clark. These investigators obtained the omnilateral illumination by 

 causing pots with seedlings to rotate on a clinostat round a vertical 

 axis in front of the source of light. The objections to this method 

 are that owing to the excentric position of most of the plants, the 

 latter do not receive equal quantities of light on all sides, while 

 moreover, on account of the large numbers of plants in each pot, 

 they are continually getting into each other's shadow. Owing to the 

 kindness of Prof. Went I was able to use an apparatus specially built 

 for these experiments. It is a kind of multiclinostat, in which 20 

 pots can rotate simultaneously each on its own axis. The arrangement 

 is such, that when the source of light is one metre from the instrument, 

 the possibility is excluded of the plants getting into each other's 

 shadow. The time for a revolution varies from 4 seconds to 4 minutes, 

 whilst a brake with an electrical contact makes it possible to 

 illuminate during an integral number of revolutions. Since the plants 

 rotate round their own axis, it is possible to use fairly large velocities 



1) Gohn's Beitrage Bd. 9. 1909. Bd. 10. 1910. 



2) Zeitschr. f. Bot. Bd. 5. H. 10. 1913. 



