Developmental Effects of Auxins 99 



only one or possibly two of the four functions known to be responsible 

 for phototropisni. 



Lateral transport of auxin has been shown to be caused by ex- 

 posure to lateral gravitational forces (Dolk, 1929). Geotropic stimu- 

 lation produces no overall change in growth rate, but brings about a 

 lateral redistribution of auxin such that approximately 62 per cent 

 of the auxin appears on the lower side of the coleoptile, root tip, or 

 stem. This asymmetrical distribution results in asymmetrical growth, 

 with coleoptiles and stems bending away from the gravitational force 

 and roots bending toward it. The shift in auxin distribution is not the 

 result of a unilateral production by the apex, since newly decapitated 

 sections exposed to a geotropic stimulus show the same bending and 

 the same auxin redistribution pattern. After the removal of the geo- 

 tropic stimulus, coleoptiles recover symmetrical growth much more 

 rapidly than after phototropic stimulation. For example, the Avena 

 coleoptile recovers normal growth one hour after termination of geo- 

 tropic stimulation (Dolk, 1929), whereas pea stems recover normal 

 growth 24 hours after exposme to light (Galston and Baker, 1953). 

 Recovery data for Avena coleoptiles after light exposure are not avail- 

 able. This may be taken as indirect evidence that sensitivity of the 

 cells to auxin after geotropic stimvdus does not change as it does after 

 exposure to light. As in phototropism, changes in electric potentials 

 occiu' following geotropic stimulation (Bose, 1907). The lower side 

 is positively charged in relation to the upper side by as much as 5 to 

 10 millivolts. Again, the participation of this electric potential dif- 

 ference in auxin movement has been postidated. 



A second factor which sometimes is involved in geotropism is the 

 formation of auxin. In studying the geotropic responses of sugarcane 

 stems, van Overbeek ct al (1945) found that meristems located just 

 above each node were sites of renewed auxin formation after geotropic 

 stimulation. When the stems were placed in a horizontal position, auxin 

 was synthesized on the lower sides, and growth of the stem meristems 

 were consequently activated. In non-meristematic tissues such as the 

 Avena coleoptile, the formation of auxin is unaffected by geotropic 

 stimuli (Dijkman, 1933). 



Root Tropisms 



Tropisms in roots are apparently controlled by the same mecha- 

 nisms as are tropisms of coleoptiles and shoots, except that the direction 

 of response is reversed. Roots have an extremely low optimum auxin 

 concentration for growth, as can be seen in figure 46. In fact, the auxin 



