178 GROWTH HORMONES IN PLANTS 



During these pendulum-like movements, the average rate of 

 growth appeared to be unchanged (Fig. 53). 



The course of the geotropic curvature of the Avena coleoptile 

 was investigated by Dolk (19296), who determined the magnitude 

 of curvature in different zones of the organ during response. 

 The seedlings were geotropically stimulated in a horizontal 

 position for 30 minutes, after which they were placed upon the 

 intermittent clinostat in a horizontal position so that the curva- 

 tures continued unaffected by the further opposing action of 

 gravity (Fig. 54). Comparison of these curvatures wdth those 

 resulting from phototropic stimulation (Fig. 46) brings out the 

 following: Curvatures begin in both cases in the tip region, but 

 the geotropic curvature sets in much earlier than the phototropic ; 

 in both cases, the curvature proceeds from the tip to the base. 



The Growth-substance Explanation. Growth Substance and 

 Geotropic Sensitivity. — The suggestion of growth-substance activ- 

 ity in geotropic curvature of the Avena coleoptile goes back to the 

 investigations of Boysen Jensen (1911), w^ho showed that the 

 geotropic stimulus from an excised and replaced tip can be trans- 

 mitted into the stump. These experiments were confirmed and 

 expanded by Stark (1924), who found that a geotropically stimu- 

 lated tip, which was transferred to a nonstimulated stump, could 

 cause the latter to curve. These studies showed that a growth 

 substance is concerned in the geotropic curvature of the Avena 

 coleoptile (see Brauner, 1923). 



The same problem now arises that was mentioned in the 

 discussion of growth substance in relation to phototropic curva- 

 ture, i.e., whether only materials involved in normal growth are 

 concerned in geotropic curvature or a special geotropic hormone is 

 formed. The constancy of the rate of growth during geotropic 

 curvature seems to warrant disregarding the latter possibility for 

 explaining negatively geotropic curvature in the Avena coleoptile. 

 By direct measurement, Dolk (1929a) demonstrated that the 

 amount of growth substance given off during the curvature is not 

 changed : some coleoptile tips were removed before and some after 

 a geotropic stimulation; both kinds were placed upon agar blocks, 

 and the amount of grow^th substance given off into the agar 

 blocks was determined in the usual way. The results of the 

 experiments are given in Table 12. Although the figures are not 

 always consistent, it may be concluded that the amount of growth 



