I08 PLANT GROWTH SUBSTANCES 



are reported by Kelly and Avery (13) for 2,4-D in concentrations ranging 

 from somewhat less than i to approximately 100 mg. per liter. In the 

 presence of malic acid the stimulation of 2,4-D is much enhanced. In 

 similar investigations on young pea stem tissue it was found that the 

 concentration of 2,4-D required to give a 20 per cent stimulation was of 

 the order of i/iooo that needed for Avena. Growth measurements, 

 unfortunately, were not included in the study. 



At least brief mention ought to be made of several other types of 

 plant tissue which have also been investigated for their respiratory 

 response to 2,4-D. For example, the carbon dioxide output of ripening 

 pears was shown by Hansen (12) to increase as a result of dipping in 

 2,4-D at concentrations of from 50 to 100 mg. per liter of solution. Bean 

 and morning-glory plants sprayed with 2,4-D (1,000 mg. per liter) 

 showed up to 80 per cent greater carbon dioxide output than did un- 

 sprayed plants (9), Rhizomes and roots of bindweed showed an average 

 of 70 per cent increase in the uptake of oxygen as a result of spraying 

 intact plants with 2,4-D at 1,000 mg. per liter (16); and Southwick (17), 

 employing various 2,4-D treatments for several varieties of peaches and 

 one apple variety, hastened ripening and increased carbon dioxide pro- 

 duction by 10 to 30 per cent. 



One of the most recent studies (in press) involving stimulation of 

 respiration in relation to growth on tissue other than Avena coleoptiles 

 is that of Louis Nickell (14). He has kindly given me his. permission 

 to tell you about it. The tissue used was the Rumex virus tumor of L. M. 

 Black, grown in culture on a fully known substrate containing no 

 hormones. For the respiration experiments various naturally occurring 

 and synthetic hormones were added to the Warburg vessels, among them, 

 indoleacetic acid and 2,4-D. 



Nickell observed that oxygen uptake was stimulated 40 to 60 per cent 

 when indoleacetic acid (concentration range 0.00 1 to i.o mg. per liter) 

 was added to the solution in the Warburg vessels. When this same 

 hormone was added to the tissue culture substrate greatest growth 

 (about 20 per cent over the control) was obtained at 0.0 1 mg. per liter. 

 With 2,4-D the oxygen uptake was stimulated approximately 20 per 

 cent over the control in a wide concentration range, and greatest growth 

 in culture (about 20 per cent over the control) occurred when the syn- 

 thetic hormone was added to the substrate at a concentration of o.i mg. 

 per liter. It might be pointed out that although the tissue culture experi- 



