OCCURRENCE AND FORMA TION OF GROWTH SUBSTANCES 57 



and geotropic stimuli can be transmitted from the excised and 

 replaced tip into the basal region. Conduction occurred not only 

 when the same tip and stump were recombined but also in 

 interspecific and intergeneric combinations of tip and coleoptile 

 stump. Such experiments were carried out on Avena, Hordeum, 

 Secale, and Triticum. 



Zollikofer (1928), investigating the growth-substance content 

 in various species of Panicum, found that the effect upon subse- 

 quent growth in such recombinations of coleoptile tips and stumps 

 depended upon the systematic relationship existing between the 

 two parts concerned. On the basis of these experimental 

 results, Stark concluded that growth substances are to a certain 

 extent specific. This conclusion is hardly justified, in view of 

 the many factors that might modify the rates of growth in these 

 cross implantations. 



Stark (1921&) found that when a ring of coleoptile tissue was 

 placed unilaterally upon a decapitated Avena stump, the result- 

 ing curvature was toward the side with the ring, i.e., growth was 

 inhibited. This observation was confirmed by Nielsen (1924). 



Moissejewa (1928) and Soding (1929) studied the distribution 

 of growth substances in the coleoptiles of Zea and Avena. Both 

 investigators came to the same conclusions, summarized by 

 Soding as follows: 



Hormones present in the coleoptile decrease from tip to base. They 

 are most abundant in the apical millimeter, fairly abundant below this, 

 still present but in smaller amounts in the region 2.5 to 5 mm. below the 

 tip, and absent from the base of the coleoptile. The uppermost milli- 

 meter of the tip is the principal region of hormone formation. Varying 

 amounts are present in the lower regions, but whether hormone forma- 

 tion takes place there is not certain. 



Went (1928a) did not obtain tests for the presence of growth 

 substance in basal parts of the Avena coleoptile below 0.7 mm. 

 from the tip, using the agar-block diffusion technique. The 

 growth of the lower parts of the coleoptile suggests that growth 

 substance must be present there, as pointed out by Soding (1929). 

 Thimann (1934), with the chloroform-extraction method, showed 

 that growth hormone was present in decreasing amounts from the 

 tip to the base of the coleoptile. It was suggested (see also 

 Bonner, 1934a) that the substance was present in the lower zones 



