Ill THE MEASUREMENT OF GROWTH 757 



the reduced growth rate of the isolated organ enables the tissue tensions to be 

 partly released. 



Now if these slit organs are placed in auxin solution instead of in water the out- 

 ward curvature in the more apical parts gradually becomes reversed and presently 

 the two halves are curving inward. The extent of inward curvature reaches a 

 maximum in abovit 28 h. with etiolated pea stems (Thimann and Schneider, 1938) 

 and this maximum is roughly proportional to the logarithm of the auxin concentra- 

 tion. The inward curvature is greatly decreased by peeling the epidermis from the 

 tissue, and it is increased by cutting the stems or coleoptiles into quarters instead 

 of halves. It is deduced that the inward curvature is caused by greater elongation 

 of the outer layers than of the inner, that is, greater response to a given auxin 

 concentration (see section Via, p. 775). 



This type of curvature has one advantage over that due to placing auxin in agar 

 on one side of a coleoptile, namely that it is not dependent on transport of the 

 auxin down from a small cut surface because the whole organ is in the solution. 

 Consequently many synthetic auxins which are almost inactive in the standard 

 Avena test cause good curvatures on slit pea stems. The same is true for straight 

 growth of coleoptile, mesocotyl or stem sections floating on solutions, although 

 here there is (for reasons unknown) more specificity in Avena coleoptiles than in 

 Pisum stems, and transport from the cut surface still plays some part. 



Straight growth of stem sections from etiolated pea seedlings is a little more 

 limited but otherwise similar to the growth of coleoptiles. Sections must be taken 

 from near the growing apex since the elongating zone is quite limited. In Alaska 

 peas, the procedure is to use the 3rd internode when it is about two-thirds grown. 

 This occurs at an age of 7 days at 25", provided occasional weak red light is given; 

 in complete darkness the internodes elongate more in situ and are less satisfactory 

 as sections. If the sections are cut 20 mm long (as close as possible to the terminal 

 "hook"), sucrose is not needed and they elongate in optimal concentrations of 

 auxin solution about 60° o? reaching their maximum growth in 16 h. at 25° (Chris- 

 tiansen and Thimann, 1950a, and literature there cited). Cobalt increases the 

 growth a further 10-25%, providing that sucrose is added (Miller, 1954; Thimann, 

 1956 b). Gibberellin also increases the growth about 20%, irrespective of the auxin 

 concentration (Kato, 1957). 



Bioassays of auxins have also been based on other fvuictions of these substances, 

 such as the promotion of root development on stems, or of the growth of fruits 

 without pollination (see section VI Ic, p. 807), or even in some cases on the inhibition 

 of growth. 



One test based on inhibition has been widely used. If elongating roots are 

 placed in auxin solutions their growth rate is greatly decreased, and the extent of 

 decrease is roughly proportional, over a limited range of concentrations, to the 

 logarithm of the concentration. Oat, wheat and flax roots are the most used 

 (Wilske and Burstrom, 1950; Aberg, 1950). In many cases very low concentrations 

 of auxins, about 10"'^ A/, slightly promote the elongation, but the effect is often 

 elusive; this is most noticeable and reproducible with isolated sections of roots 

 (Leopold and Guernsey, 1953 ; Audus and Das, 1955). Even without making use of 

 this promotion, the root inhibition test is the most delicate of all tests, showing 



Literature p. 8i6 



