GROWTH SUBSTANCES FOR GEOTROPISM 203 



"are essentially the same as those involved in the upward curva- 

 ture of the shoot axis." "During the curvature, a decrease 

 in the rate of elongation appears in the growing axis; while the 

 convex side grows more rapidly, the concave side grows less than 

 would be the case in undisturbed growth in a vertical direction." 

 That seedling roots show no noticeable change in growth rate 

 when they are placed upon the clinostat was shown at about the 

 same time by Schwarz (1881) and Elfving (1883). Later, Lux- 

 burg (1905) investigated the same question and found that the 

 individual difference in the experimental plants was too great to 

 make possible a definite decision. It seemed highly probable 

 that the growth increase in the median zone cannot be large. A 

 retardation of growth in relation to the normal took place in 

 9 out of 13 roots, while in the remaining ones an increase in 

 growth occurred. 



More recently Keeble, Nelson, and Snow (1931a) have studied 

 the growth rate of maize roots placed in both horizontal and 

 vertical positions. In all experiments, they found that hori- 

 zontally oriented roots grew considerably more rapidly than 

 vertical ones. These results, however, could not be confirmed by 

 Cholodny (1932a). Navez (1933a) repeated these experiments 

 and interpreted the results as follows: When excess water was 

 present so that drops were allowed to form at the tips of roots 

 grown in a vertical position, the rate of growth of these roots was 

 retarded. On the other hand, if surplus water was not present, a 

 difference in the rate of growth could not be shown. 



The evidence does not permit a definite conclusion regarding 

 the rate of root elongation during the geotropic response. 



The Growth-substance Explanation of Root Curvature. — In a 

 series of experiments with Vicia Faba roots. Snow (1923) demon- 

 strated that a geotropic curvature can be produced after decapita- 

 tion and replacement of the tip. This shows that the stimulus 

 can be transmitted over a wound, and it can be concluded that 

 transmission of the stimulus in the root is bound up with the 

 transport of a substance. The geotropic responses of roots have 

 been studied by Cholodny (1924, 1926, 1927, 1928a, 1929a, 1931e, 

 19336), who made (1924) the interesting discovery that decapi- 

 tated roots of Lupinus angustifolius produced a positively geotropic 

 curvature when coleoptile tips of Zea mays were placed on them. 

 The growth rate of the decapitated roots, so treated, was retarded 



