192 



GROWTH HORMONES IN PLANTS 



which growth-promoting substances are transported must of 

 necessity be capable of growth response in order to accomphsh 

 geotropic bending. In the Bryophyllum half stems, no growth 

 curvature can be produced when the woody tissue and pith are 

 oriented downward. Many variables enter into all these experi- 

 ments with spHt stems which possibly vitiate the results, e.g., the 

 errors in halving the stem, the unequal degree of drying of the 



surfaces, etc. 



Growth- substance Displace- 

 ment. — Dijkmann (1934) inves- 

 tigated the geotropic curvature 

 of the Lupinus hypocotyl and 

 demonstrated the presence of 

 growth substance there. It was 

 present over the whole growing 

 zone; the growth rate was pro- 

 portional to its concentration, 

 within certain limits. Growth- 

 substance displacement toward 

 the lower side of a stem placed 

 -Bryophyllum stem split horizontally was also studied. 



longitudinally, each half possessing a . , j ■! j. xu 



leaf at its apex. The split halves were 1 he results mdlCated that the 



suspended horizontally in moist air. \q^q^ ^{^q contains morC growth 

 A, with the cortex below. B, cortex . 



above. Geotropic curvature occurred SUbstance than the upper Side, 



only in the half stem with the cortex ^ut the results of these experi- 

 below. {After Loeb, 1917.) , ^ i xi, 



ments are not so clear as those 

 on hypocotyls. If a hypocotyl cyhnder placed in a horizontal 

 position is completely covered v/ith growth substance on the 

 apical end, it is possible after a time to extract more growth sub- 

 stance from the lower than from the upper half of the morpho- 

 logically basal end (Fig. 2) (Dijkmann). From these 

 experiments, it can be concluded that the geotropic curvature 

 of the Lupinus hypocotyl results from displacement of growth- 

 substance to the under side and that the behavior of this organ 

 fits in with the growth-substance explanation. 



Van der Laan (1934) investigated the same problem, using the 

 epicotyl of Vicia Faba. When it was oriented horizontally, more 

 growth substance could be extracted from the under than from 

 the upper side, an indication that displacement had occurred 

 (Fig. 59). When the seedlings were treated with ethylene, the 



Fig. 68. 



