Fio. 35. ^Geotropic curvature of stems of Bryophyllum calycinum. These stems were 

 originally straight and suspended in a horizontal position. In about ten days they bent, 

 becoming concave on the upper side. The black rings, made with India ink, which were 

 originally parallel, remain unaltered on the upper side of the stems, while their distance 

 increases on the lower side, indicating that the curvature is due to an increase in growth 

 on the lower side (of the cortex) of the stem. 



