Plant Qrowth'Suhstances 



"In the experiments reported by other writers (with the 



exception of Amlong [1936]) only the toxic phase of the drug 



action was recorded because of the concentrated solutions 



employed and the longer periods in which such plants were 



exposed to their action." 



A fair size for the diameter of a plant cell is 0*02 mm. 

 Supposing the cell to be a cube of 0-02 mm. side, a seedling 

 of total volume of o-8 ml. will have 100,000,000 cells. If all 

 the indole-acetic acid contained in i-o ml. of a solution of 

 o-i gm. of the acid per litre is uniformly absorbed by that 

 seedling, each cell will contain 3 -5 thousand million molecules 

 of active substance. The number is roughly one-fiftieth of 

 the number of molecules in a volume of air at N.T.P. equal 

 to the volume of a cell. 



Caution in dosage is still more necessary in working with 

 excised root-tips having an initial total volume of the order of 

 1 cubic millimetre. As the excised tips are usually surrounded 

 with many thousand times their weight of solution, the weight 

 of active substance in the solution will have to be infinitesimal 

 if harm is not to result. 



Kogl et al. (1934) have shown that auxin-a can exert an 

 inhibition of root-elongation in young oat seedlings. The 

 efi"ect was perceptible when tap water bathing the roots con- 

 tained 0-0 1 mg. of auxin per litre. Indole-acetic acid was 

 found to exert a similar, but less powerful, inhibition. 



Marmer (1937) has made observations on the inhibition of 

 primary root-elongation in wheat seedlings, testing the three 

 homologous acids, indole-acetic, -propionic, and -butyric, 

 with the refinement of working at two hydrogen-ion concen- 

 trations in phosphate-buffered solutions. At pVL 4-6 the con- 

 centrations causing a 50 per cent, reduction in root-growth 

 were respectively 0-012, 0-250, and 0-055 mg. per litre, while 

 at pW 7-5, the same results required respectively I7'58, 

 4'57, and 0-56 mg. per litre. 



The magnitudes of such figures must depend very much 

 upon the conditions of experiment. 



The apparent paradox that synthetic growth-substances, 

 capable of promoting growth of apical cells of seedlings, and 



78 



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