( 82 ) 



when one causes light of 1255 Melercandles to react (.luiiiig — sec. 



55 



or light of 0.025 Metercandles during 15 minutes. Now, as appears 



from the great length of the latent time required for plants to render 



the reaction visible (I.e. p. 18) sometimes an hour, ,? is exceedingly 



small with experiments on plants, so that we can put ^ = and 



then the formula, even for reactions of long duration, becomes: 



y =z a I T 



which is exactly the mathematical representation of the fact found 

 by Blaauw and Fröschkl that the product /and T remains always 

 constant. 



At about the same time as Blaauw Miss C. J. Pekllharing made 

 in the Botanical Laboratory of Utrecht experiments about the curvation 

 of roots under the influence of gravity and centrifugal force. Hereby 

 it appeared that the irritation-effect was proportional to the pressure 

 which, under the influence of these forces, the celluhir fluid exercises 

 on the protoplasm or on the cellular wall. If one represents this 

 pressure by D di/nes then the formula (5) becomes: 



T 



.Bj, 



which, on account of the diminutive value of /?, changes again into : 



y = aDT 



This is the result found by Miss Pekelharing, that, in order to 

 obtain the same curvation the pressure multiplied by the time of 

 presentation must remain constant. Consequently the formula (5) 

 holds likewise for the geotropy of plants. 



At the same time it appears that Froschel and Bla.\uw's assertion 

 that every perception and irritation is proportional to the energy 

 consumed, is not correct. F'or in the case of geotropy />> 7' represents 

 tlie product of a force multiplied by a time, and consequently, at 

 least there, the irritation-effect is by no means proportional to the 

 energy. 



On the other hand Blaauw's and Miss Pfkelharing's experiments 

 are agaiu proofs of the correctness of the formula (9). 



