WILLIAM S. HILLMAN 075 



(e.g., 11 hours) with low energies of red light can completely inhibit 

 its effect. One such dark period is sufficient to induce a measurable 

 deuree ol flowering. 



This conversion from daylcngth-intliffercnce to typical short-day 

 behavior by high levels of chelating agents can also be viewed as a 

 specific inhibition of flowering in long days but not in short, and 

 occurs with little effect on vegetative growth. A short-day require- 

 ment is also found at about 29°C even in the medium lacking high 

 levels of chelating agents; at or above 31.5°C flowering is completely 

 inhibited irrespective of daylength, but vegetative growth continues 

 normally. The effect of high temperature appears to be exerted 

 primarily on dark-period processes. 



These few papers represent the work published to date on photo- 

 periodism and flowering in the Lemnaceae. Further details will be 

 considered later, as more recent results on both L. gibba and L. 

 perpusilla are described. 



Experimental 



L. gibba, strains Gl and G3, obtained from Dr. R. Kandeler (10, 

 11) and L. perpusilla 6746 (6, 7, 8) were grown in aseptic culture. 

 For details of the methods employed, see (7, 8) . Two basic media 

 ^vere used, with or without 1 % sucrose. For Hutner's medium, con- 

 taining a high level of EDTA and used here at 8/10 strength, see 

 (9) . The other basic medium, a modified Hoagland's designated M, 

 is the same as the L medium described elsewhere (8) except that 

 the initial pU is 4.2-4.3 unless otherwise noted, and that ferric tartrate 

 was replaced by 2 X l^-^ M of both FeCls and tartaric acid. All 

 media were autoclaved. Media, light sources and intensities, and tem- 

 peratures will be specified for all data presented. 



Vegetative growth is recorded as frond multiplication rate (MR) , 

 which is 1000 times the increase per day in the logarithm of frond 

 number. Thus an MR of 301 represents a doubling of frond number 

 each day, and an MR of 150 a doubling every two days, and so forth. 

 The notation MR (4-6) represents the MR calculated by counting 

 fronds on the 4th and Gth days of the experiment. Flowering is 

 evaluated as flowering percent {FL%) , which is the percentage of 

 fronds in ;\hich flower primordia are detectable under a 20-power 

 dissecting microscope. FL% values are obtained by dissecting all 

 fronds in a culture unless there are more than 100, in which case 100 

 taken at random are dissected. Usually the frond number is 60 to 

 80, but further details will be given as necessary. 



