Botany. — ,,D(trk groivtk-responses" . By D. Tom.enaar. (Commu- 

 nicated by Prof. A. H. Blaauw). 



(Communicated at the meeting of April 28, 1923). 



In QUI' previons report on the light- and dark-adaptation of Pliy- 

 comifces nitens (Proc. Vol. XXIV Nos. 1, 2 and 3, 1921) the existence 

 of tiie so-called ,, dark-grow th-response" was already proved in a 

 great number of e-xperimenls. By dark-growlli-response we under- 

 stand the occurrence of a growth-response, when a sporangiophore 

 of Phycomyces nitens adapted to light (by a four-sided illumination 

 for hours at a stretch) is placed in the dark. It seemed worth while 

 considering in how far this dark-growth-response of Phycomyces-nitens 

 (the negative after-images of the human eye probably being in reality 

 comparable) occurs in other organs. 



In this communication the results are mentioned concerning the 

 dark-growtli-responses of the sporangiophore of Phycomyces nitens, 

 the hypocotyledons of Helianthus glohosus, the coleoptiles of Avena 

 sativa, the roots of Avena sativa and the roots of Sinapis alba. 



If possible the results have t)een compared with the light-growth- 

 responses hitherto known. 



Method and accuracy of the results. 



In all experiments the preceding illumination was four-sided; the 

 temperature being kept constant by means of tlie oil-thermostat, 

 described in ,,Liclit- und Wachstum 1". In this way the temperature 

 could be kept constant to 0.02°^ C. with moderate illuminations. It 

 should be particularly kept in view, that the growth was as a rule 

 only considered sufficiently constant, when it did not oscillate above 

 lO'/o. i- o. w. with an average rate of growth of 100 no rates 

 higher than 105 or lower than 95 occurred. This enables us to 

 ascertain responses of growth more than 57„ above or below the 

 average; responses of growth therefore of an acceleration or retar- 

 dation of 107, we can ascertain with some certainty. 



We mention this in order to give the illustrations and reviews 

 the value due to them, which could not be judged of without the 

 full data — which we omit here with a view to space, but all of 

 which will appear in the "Mededeelingeii der Landbouw-hoogeschool" 

 this year. 



