692 



PHYSIOLOGICAL BASIS OF POLARITY. II 



nutrition and growth and that Haberkmdt's hypothesis of an otolith 

 function of starch granules as the cause of geotropic curvature is 

 neither needed nor warranted. The hypothesis of geotropic "stimu- 

 lation" had better be replaced by the application of the law of mass 

 action.^ 



Fig. 3. I and la, II and II a, etc., halves of the same stem split longitudi- 

 nally and put with their cut side on moist filter paper. Half stems 1 </,II </, etc., 

 on right, having each a leaf at apex, form considerable masses of roots on stem, 

 while half stems I, II, etc., on left, having no apical leaf, form practically no 

 roots in the same time. Duration of experiment 26 days. 



///. The Ifijlucjicc of LigJit U pon Rool Formatioii in the Stem. 



Six stems were split longitudinally into two pieces, each with 

 an apical leaf as described. One-half of each stem was suspended 

 horizontally, with the cut surface above, in a dark aquarium, the 

 other in an aquarium exposed to light. The temperature was the 

 same in both aquaria. After 22 days photographs (Figs. 4 and 5) 

 were taken. The stems exposed to the light (Fig. 4) had formed a 



^ Loeb, J., Forced movements, tropisms, and animal conduct. Monographs on 

 Experimental Biology, Philadelphia, 1918, 120. 



