AUXIN AND GROWTH 



77 



less aging (Skoog, 1937). Went has shown (1935) that if a 

 sufficient auxin supply be maintained no aging occurs for 

 a long time. Indeed, the opposite effect, namely rejuvena- 

 tion, is observed if high concentrations of auxin are apphed 

 to greatly aged regions of the coleoptile; the growth rate 

 increases slightly at first and then more in the next period, 

 so that the sensitivity is actually increasing with time (see 

 Figure 32). 



Since the auxin content decreases steadily from tip to 

 base in the coleoptile, it follows that each zone has a dif- 

 ferent auxin content and a 

 different physiological age. 

 It is therefore essential to 

 consider each zone sepa- 

 rately, before an integral 

 picture of the growth of the 

 whole can be obtained. 

 Measurements of the growth 

 of marked zones were first 

 carried out by Rothert 

 (1894) who found that in 

 young coleoptiles, up to 

 15 mm. long, the growth 

 rate was greatest at the 

 base, while in older coleop- 

 tiles there was a maximum 

 growth rate in the middle, 

 some 6-9 mm. below the tip. 

 His observations have been confirmed by Went (see below). 

 The type of growth exemplified by coleoptiles 20 mm. or 

 more long, with a maximum growth rate near the middle, 

 is of common occurrence in plants. Hence any explanation 

 arrived at for Avena should be capable of application to the 

 many other cases known. Thus, Figure 33 illustrates the 

 growth rates of zones of successive internodes of Polygonum 

 (Van Burkom, 1913); it may be seen that the distribution 

 of growth rates in each internode, and also the distribution 



Base -, — 



Fig. 33. Distribution of growth in 

 the internodes of a rapidly elongating 

 shoot of Polygonum sachalinense. Ini- 

 tial length of all internodes taken as 

 equal. The numbers give the number 

 of the internode from the base; vertical 

 lines represent nodes. Ordinate, final 

 length of each zone. (From van Bur- 

 kom, Diss., Utrecht, 1913.) 



