H63 



Botany. — W. H. Arisz: ''Positive and negative phototrop}/ of 

 the apex and base in oat-seedlings {Avena sativa)." (Commu- 

 nicated by Prof. F. A. F. C. Wknt.) 



(Communicated in the meeting of September 27, 1913). 



(Preliminary communication). 



Oltmanns^) has been able to show very convincingly that in the 

 sporangiophores of Flujcomyces a weak illumination produces posi- 

 tive curvatures and a strong illumination negative ones. Blaauw') 

 contirmed these results and determined the quantity of light-energy 

 necessary to produce these phenomena. In the case of Phanerogams 

 also in the' course of time negative curvatures have been seen, but 

 except in the case of some roots, the comnmnications are very 

 vague and doubtful. Oltmanns and before him N. J. C. Muller') 

 saw negative curvatures in seedlings of Lepidium sativum, while 

 Pringsheim^) saw faint inclinations of the apex in those of Avena. 



Nevertheless very clear negative phototropic curvatures can easily 

 be obtained in seedlings of Avena sativa when they are illuminated 

 with only just that quantity of energy necessary to produce the pheno- 

 menon. I have endeavoured to show in an earlier paper *) that the 

 reaction which occurs after the application of a certain quantity of 

 light (this is true for 1 — 100 C. M. S. ") has a constant strength. The 

 reaction will increase in strength in proportion as the quantity of 

 energy, used for stimulation, is greater. 



With a short duration of stimulus the stronger reaction after 

 more powerful stimuli is shown by the earlier occurrence of the 

 curvature : that is to say, the reaction-time ') is a function of the 



1) Flora Bd. 83 1897. 



2) Recueil d. Trav. Bot. Néerl. Vol. 5 1909. 



3) Botanische Unters. 1872. 



4) Gohn's Beitr. Bd. 9 1909. 



5) Proceedings Roy. Acad. Sc. Amsterdam, March 1911. 

 ^) The usual abbreviation for candle-metre seconds. 



7) By reaction-time must here be understood the time which elapses from the 

 beginning of stimulation to the moment at which the curvature becomes macros- 

 copically visible. 



The lengthening of the reaction-time after weaker stimuli, when the duration of 

 stimulus (0 is very short and constant (energy ^X^ not constant) is by no 

 means comparable to the lengthening of the reaction-time wliich Bach (Jahrb. f. 

 wiss. Bot. Bd. 44 1907) and Mrs Rutten-Pekelharing (Rec. Tr. Bot. Néerl. 

 Vol. 7 1910) found by stimulation with weak centrifugal forces during the pre- 

 sentation time (iX.t constant). Here the lengthening of the reaction-time is 



