Newcombe, Gravitation sensitiveness not confined to apex of root. 97 



of the perceptive organ to the part excised; for it has been thought 

 that the wounding miglit destroy the sensitiveness of the elonga- 

 ting zone posterior to the wound. By the ingenious employment 

 of glass caps bent at a right angle into which the roots were 

 forced to grow, Czapek was able to cause the apical one and 

 half millimeters of the root to take an angle of 90 ° with the 

 rest of the root; and thus one region of the root could be placed in 

 its position of equilibrium with regard to gravitation, while the other 

 region was 90° removed from this position. 



If the seedling with the glass cap over the apex of the root 

 was set up as shown in Fig. 1, in a short time it had changed 

 to the relations shown in Fig. 2. If the seedling was laid hori- 



Fig. 1. 



Seedling of Lupinus albus with glass 

 cap over root-tip. After Czapek. 



Fig. 2. 

 Seedling of Lupinus albus after the root 

 has taken its position of equilibrium. 



zontally as in Fig. 3, it continued to grow withont changing the 

 position of tip or elongating zone. 



These results were interpreted to mean that only the apical 

 one or two millimeters of the root was sensitive to gravitation. A 

 moment of reflection will show that the results accord with an 

 entirely different hypothesis. Suppose merely that in Fig. 1 the 

 horizontal 2 mm of the root-tip is more sensitive to gravitation 

 than the elongating zone above it; then the root will swing into 

 or toward the position shown in Fig. 2. If the apical 2 mm are 

 much more sensitive than the adjacent older part, the tip may 

 attain the vertical position as shown in Fig. 2, wholly overcoming 

 the effect of the sensitiveness of the elongating zone. Such a 

 distribution of sensitiveness to light has been made out by Roth er t *) 



*) Über die Fortpflanzung des heliotropischen Reizes. (Ber. d. d. bot. 

 G eselisch. X. 1892. 374.) 



Beihefte Bot. Centralbl. Bd. XXIV. Abt. I. Heft 1. 7 



