704 



PHYSIOLOGICAL BASIS OF POLARITY. II 



Reed and Halma^ have shown that shoots grow out only on the 

 upper side of horizontally suspended branches of citron trees. Our 

 experiments published in a previous article show that shoots can grow 

 out from the under side of a piece of stem of Bryophyllum., suspended 

 horizontally, and that they invariably do so when the upper half 



Fig. 15. Stem bent passively, concave side below. Roots form on lower, 

 concave side. Duration of experiment 35 days. 



^1 





■; Fig. 16. Geotropic curvature prevented in stem. Roots are formed neverthe- 

 less on lower side. Duration of experiment 41 days. 



of such a stem is removed. This suggests the possibility that if the 

 upper half of a citron bough, suspended horizontally, is removed the 

 shoots might grow out from the under side, and that in Reed and 



« Reed, H. S., and Halma, F. F., Univ. Cal. Pub., Agric. Sc, 1919, iv, 99. 



