Extraction and Measurement Techniques 39 



fact that accessory substances can be used without extreme sensitivity 

 to small pH changes. The disadvantages of the straight-growth tests 

 are likewise the same as those listed for the slit pea test, except that 

 the Avena straight-growth test is not as sensitive to metallic ions as is 

 the pea test. 



E 

 E I 



O ' 

 K- 



4 

 UJ 

 if) 



1 3 



If) 



/\ 



w 



■^ 



Red Light 



XS==S> 



\. 



'" n a_fij_ 



q: 

 a. 2 



or 

 o 



Dark 



<^=T=^ 



3X 



3:^ 



\ 



s,.<^. 



4 8 12 16 CM 



DISTANCE FROM APEX 



Fig. 19. The comparative growth of pea stem sections from various parts of pea 

 plants grown in the presence of intermittent red light and in complete darkness 

 (Leopold and Guernsey, unpublished). 



Pea Root Test 



The physiological basis for the pea root test is essentially the same 

 as for the other straight-growth tests. It is a measure of simple growth 

 and does not depend upon the polar transport of auxins. The meri- 

 stematic tip of the root is included in the test material and conse- 

 quently the growth obtained is a result of cellular elongation and cell 

 division. That the root is an organ of extreme sensitivity to auxins has 



