520 



RHYTHMS IN PLANTS AND ANIMALS 



r^l ^1 ^1 ^\ 



a:control 

 b,c:urethane untiLl 



2A hours 



*0 



Fig. 15. Phaseohis multiflorus. Decreased length of periods after replac- 

 ing urethane (ca. 0.02%) by water. = midnight. (After Biinning, 

 1957a,b.) 



chilling. The delay observed at first is later on replaced by an accelera- 

 tion; this means that the periods become even shorter than they are 

 normally. This compensation becomes especially clear after removing 

 the poison Fig. 15, Table VIII; Biinning 1957a; cf. Addendum). 



Table VHI. Influence of Urethane on Leaf 

 Movements of P/iascoIits (Biinning, 1957a) 



ROLE OF NUCLEI 



Within different kinds of plant tissues we have observed endogenous 

 diurnal fluctuations in the volume of the nuclei (Fig. 16; Biinning 

 and Schone-Schneiderhohn, 1957). Poisons like urethane, which dis- 

 turb the physiological clock, are also effective in this case. 



We should check whether the structural changes within the nuclei, 

 which are responsible for those volume changes, are connected with 

 the mechanism of the physiological clock. According to this working 



