100 PLANT GROWTH SUBSTANCES 



TABLE 1 

 Organic acid content oi Avena coleoptiles as a function of age, and oi Pisum 

 stems. Plants grown in darkness at 25°. All figures are micro-equivalents per 



gram dry weight 



Age of tissue Total Acids Malic Acid Citric Acid 



(hours) (/X-EQUIV.) ()Li-EQUIV.) (jU'EQUIV.) 



concentration as high as 6 X io~^ M is needed for 50 per cent growth 

 inhibition. 



The question now arises as to whether this disappearance of organic 

 acids is a result simply of time or whether they are actually used up in 

 growth. To answer this question sections were cut and allowed to grow 

 in the usual way in auxin; after measurement they were analyzed for 

 organic acids as before, with an additional test for pyruvic. This work is 

 tedious, involving large numbers of stem sections. A number of experi- 

 ments were also carried out in which growth was inhibited by iodoacetate 

 or arsenite. Arsenite differs from iodoacetate in that its action is not 

 prevented by organic acids, but it probably reacts with the same 

 sulfhydryl enzyme (i i). Some of the results are given in Table 2. Malate, 

 but not citrate or pyruvate, is evidently used up when the sections are 



TABLE 2 



Organic acid content of pea stems before and after growth. 



All values expressed as micro-equivalents per grain dry weight 



^Inclole-3-acetic acid, I mg. per liter. 



