AUXIN AND GROWTH 81 



beyond the normal. Went (1935) found that in Aveiia appli- 

 cation of strong auxin paste to the extreme tip of the 

 coleoptiles made them reach a length of 66-72 mm., while 

 controls reached only 45-54 mm. Schlenker and Mittmann 

 (1936) and van Overbeek (m) obtained an increase both in 

 growth rate and in final length by treating dwarf plants 

 with auxin (cf. also Loehwing and Bauguess, 1936). Since 

 in some plants auxin is not the limiting factor in growth, 

 while in others the application of high auxin concentrations 

 to the growing region causes swellings which — secondarily — 

 inhibit further growth in length (see Chapter XIII), it is 

 not always possible to increase the final length by auxin 

 application. But this in no way invalidates the thesis that 

 the ultimate size of many plants is limited by the auxin 

 which they form, and not by any inherent '"morphological" 

 factor. 



D. Applications of the Two-Factor Scheme 



It is clear that the two-factor scheme of growth, developed 

 above, may be applied to a number of different types of 

 growth. Thus Bunning (1927) found in Secale coleoptiles 

 that the growing zone was limited to onl}'- 4 mm. in the 

 center of the 24 mm. length. Incision above this growing 

 zone gave positive curvatures below the incision, which 

 may be taken as showing that the auxin supply from above 

 was being intercepted. Incision below this zone gave positive 

 curvatures above the incision, showing that the supply of 

 the food factor from below was now being intercepted. 

 The existence of this narrow growing zone must be ascribed 

 to the rate of supply of these two factors being relativel}^ 

 slow compared to their rate of combination. We have been 

 unable to confirm Biinning's findings on Secale coleoptiles 

 (u), but the same considerations apply to mesocotyls of 

 Zea Mays grown in darkness (see Figure 35, a). The op- 

 posite case is given by the coleoptile of light-grown Zea 

 Mays, in which the growing zone is a very long region (Fig- 

 ure 35, c). In this part of the plant the rate of supply of the 



