38 Fundamentals of Auxin Action 



regulator activity of compounds without interference from transport 



characteristics. 



Buffers and salts can be used as variables in the straight-growth 

 tests. The same cannot be said for the slit pea test. It will be recalled 

 that very small changes in pH can considerably alter the initial nega- 

 tive curvature of the slit pea test. Consequently, for tests in which the 

 pH is to be varied the straight-growth test is more desirable. Studies 

 requiring the addition of accessory substances affecting growth, such 

 as inhibitors or organic nutrients, are most often carried out with 

 straight-growth tests. 



The addition of sugar generally inhibits growth in the pea 

 straight-growth test. Apparently a sufficient supply of sugars and fats 

 is contained within the pea stem so that increases in growth are not 

 obtained by adding more sugars. This is not true of Avena coleoptile 

 sections, nor apparently is it true of pea sections taken from com- 

 pletely etiolated plants. Galston and Hand (1949) have shown quanti- 

 tative growth increases of almost completely etiolated pea sections 

 upon the addition of sugar. Christiansen and Thimann (1950), using 

 peas which were exposed to occasional red light, found no promotion 

 of growth by the addition of sugar. Such differences in response to the 

 addition of sugar can be brought about either by differences in light 

 treatment or in temperature experience (Leopold, unpublished). 



It should be pointed out that completely etiolated peas cannot 

 be used in the slit pea test, but they are perfectly acceptable material 

 for the pea straight-growth test. Large differences in growth patterns 

 over the length of the pea stems result from the presence or absence of 

 small quantities of red light during the growth of the seedlings. The 

 growth pattern for various parts of a completely etiolated pea shows 

 a faster growth rate in the younger sections and continued growth in 

 the older sections of the stem. The growth pattern of peas exposed to 

 red light, however, shows a rapid decline in growth as the distance 

 from the leaf node increases, indicating clearly the importance of 

 uniformity of the place from which the sections are taken. Such growth 

 patterns are shown in figure 19. 



A modification of the Avena straight-growth test has been em- 

 ployed by Hancock and Barlow (1952) in assaying auxins which have 

 been separated by paper chromatography. The technique is very 

 effective. The concentration by paper chromatography of the auxins 

 in plant extracts makes the straight-growth test sensitivity adequate for 

 measuring the auxin present. 



The advantages of these straight-growth tests are essentially the 

 same as those of the slit pea test. An additional advantage is the 



